Chemistry.

Chemistry. Although chemistry has only taken its place as an exact science based upon accurate experimental investigation within a comparatively recent period, yet its origin dates back to the earliest times of philosophical study. It will be convenient to give in the first place a short sketch of the history of chemistry, and then to state some of the principles of the science, illustrating these from the simplest facts. When possible, such illustrations will be chosen as are likely to be not altogether unfamiliar to non-scientific readers.

Historical Sketch.—The word chemistry has come to us from the Greek through the Arabic, as shown in our article ALCHEMY. With regard to the chemistry of the ancients, we know that the ancient Egyptians, Phoenicians, Greeks, and Romans were acquainted with a very considerable number of useful substances, and that their processes for preparing some of these did not differ in any essential particular from those now in use. It does not appear, however, that they have left any chemical records behind them, or that they knew anything of the science of chemistry. Several metals were known to, and employed by, these ancient peoples, who were acquainted with processes for reducing them from their ores. Amongst these metals were gold, silver, mercury, copper, tin, lead, and iron; whilst they also knew and worked with brass, although they were not aware that it was an alloy of copper and zinc. Various alloys were employed for bronzes for statues, and these usually contained copper, lead, and tin. The processes for manufacturing soap, starch, glass, leather, various mineral and vegetable pigments, stoneware, and other useful substances, were all known and carried on in very early times; and wine and beer appear likewise to have been prepared and used as beverages long before the process of distillation, which was unknown to the ancients, had been introduced. Vinegar, sulphur, and carbonate of soda were also known.

We find the application in medicine of many chemical products at a comparatively early period, and the Arabians appear to have been the first who tried to prepare new medicines by chemical methods. Geber, who lived in the 8th century A.D., is the most noted of the Arabian chemists, and he has left some writings which show us what was the state of chemistry at that early date. Geber knew, for instance, how to make and distil vinegar and nitric acid, and even sulphuric acid was made and used as a solvent by him. He knew, amongst other substances, white arsenic, borax, common salt, alum, sal-ammoniac (ammonium chloride), cupras (ferrous sulphate), nitre (potassium nitrate), and corrosive sublimate (mercuric chloride), and was acquainted with a number of their properties. He used almost all the kinds of apparatus that were commonly in use down till the 18th century, and understood the processes of distillation, filtration, sublimation, and crystallisation. In one of his works he describes the construction of furnaces for chemical purposes.

From the 8th till the 17th century but little real progress was made in chemistry as a science. The new knowledge that was gained during this period was mainly due to the assiduity of the alchemists, who, in their vain search for the philosopher's stone, necessarily made useful discoveries from time to time. Many of the alchemists so called were mere tricksters who deceived their dupes by more or less clumsy experiments, which appeared to demonstrate the production of gold from baser metal. Others, however, were really earnest and untiring in their labours, and held the fullest belief in the prospects of the ultimate success of some fortunate worker. The new substances obtained by the alchemists were frequently used in medicine, and it is to these infatuated workers, therefore, that we owe our first knowledge of many potent medicines. The writings of many of the alchemists are preserved, but numbers of them are entirely worthless from a scientific point of view, as the descriptions of processes are mixed up with so much of mystery and extravagance that they present a wholly unintelligible jargon. For more detail, however, regarding this remarkable period in the history of chemistry, see the article ALCHEMY.

As Geber has been called the patriarch of chemistry, so Robert Boyle (1627–91) has been called the father of modern chemistry, since it was Boyle who first tried to free chemistry from the trammels of alchemy and to place it upon a true scientific basis. Boyle in his Sceptical Chemist tried to discredit the salt, sulphur, and mercury of the alchemists (as well as the Aristotelian earth, air, fire, and water) as elements or ultimate constituents of substances, and he gave a scientific definition of an element. Boyle was an experimental investigator of considerable skill, and to him we owe the introduction of the air-pump and the thermometer into this country. His experiments upon the physical properties of gases led to the formulation of the law concerning the relation of the volume of a gas to the pressure, which is commonly known as Boyle's Law.

Theory in modern chemistry begins with Becher (1635–82) and Stahl (1660–1734). The latter adopted, with some modifications, a theory propounded by the former concerning elements and compounds, and formulated the phlogiston theory of combustion. The views of Becher and Stahl regarding elements were not so enlightened as those of Boyle, and must be considered as retrograde. Stahl's phlogiston theory (1697) was at once adopted almost universally by chemists, and for fifty years it was held to give the full explanation of the phenomena of combustion. According to this theory phlogiston was a constituent of all combustible substances. When a substance burned, the phlogiston made its escape, and the product of combustion was regarded as the other substance with which the phlogiston had been previously united. When a metal such as lead was heated in the air, it lost its phlogiston, and the oxide formed was looked upon as the other constituent of lead besides phlogiston. The process of reduction of lead from its oxide by means of charcoal was the transfer of phlogiston from the charcoal to the lead. It did not present itself to the adherents of the theory as an absurdity that a metal, in losing its phlogiston on oxidation, gained weight, although some of them at least were aware of the fact. The idea of gain of matter being a necessary accompaniment of gain of weight is so familiar to us that we can scarcely realise that it was not always so regarded. To this may fairly be attributed the persistence with which the phlogiston theory held its ground for so long a period.

The Dutch chemist Boerhaave (1668–1738), who did not accept Stahl's theory, published in 1732 his system of chemistry, which was a compilation of practically all that was known up till that date, collected with great labour from a large variety of alchemical and other writings.

The interval between the introduction of the phlogiston theory and its overthrow by Lavoisier in 1772–85 was one of great advance in chemical knowledge, and a number of very eminent chemists preceded and were contemporaries of Lavoisier.

In Germany, Marggraf (1709-82) studied the properties of the almost unknown alumina and magnesia, and made considerable advances in the qualitative analysis of substances in solution.

Amongst British chemists of note may be mentioned Hales (1677-1761), who was amongst the first to experiment on gases; Black (1728-99), who in 1756 published his research on Magnesia Alba, showing the nature of fixed air or carbonic acid gas, and of the difference between caustic and mild (or carbonated) alkalies; Priestley (1733-1804), who, in addition to his discovery of oxygen in 1774, investigated nitric oxide, nitrous oxide, sulphurous acid, carbonic oxide, hydrochloric acid, and ammonia gases, being specially attracted to the study of gaseous substances and their properties; and Cavendish (1731-1810), who investigated the nature and properties of hydrogen, analysed atmospheric air, and discovered the compound nature and composition of water and of nitric acid.

Lavoisier (1743-94) was one of the ablest chemists of his time, and his labours include a vast variety of subjects. His attack upon, and eventual demolition of the phlogiston theory, and his experiments in connection with his new theory of combustion, occupied him for a considerable number of years. He taught that combustion was the union of the combustible substance with atmospheric oxygen; he was the first to introduce system into chemistry and chemical research; he determined the constituents of a large number of substances, including sulphuric, phosphoric, and carbonic acids, numerous metallic oxides, and many animal and vegetable substances; and he, along with Berthollet, Fourcroy, and Morveau (1737-1816), introduced a new and consistent system of chemical nomenclature. Two contemporary Swedish chemists, Bergman (1735-84) and Scheele (1742-86), must be mentioned before leaving the phlogiston age. Bergman investigated, amongst other things, carbonic acid gas, studied the phenomena of affinity, and made advances in the processes and reagents used in qualitative analysis. Scheele was one of the most laborious chemists of his time. He discovered citric, malic, tartaric, oxalic, lactic, hydrocyanic, arsenic and other acids, and chlorine, besides investigating the nature of a large number of other bodies and independently discovering oxygen.

It was towards the end of the 18th century that the value of quantitative analysis of substances began to be generally recognised. The question as to whether the quantitative composition of a given substance was always the same gave rise to a discussion which lasted for several years, and was at length decided in favour of constant composition.

The researches of Richter (1762-1807) on the quantities of various acids neutralised by a given quantity of a base, and of various bases neutralised by a given quantity of an acid, led him to the general conclusion that the quantities of two acids, $a$ and $a\text{'}$, which form neutral salts, $a\; b$, and $a\text{'}\; b\text{'}$, with the quantities of two bases, $b$ and $b\text{'}$, are just the quantities required to form two other neutral salts, $a\; b\text{'}$ and $a\text{'}\; b$. This fundamental discovery was erroneously attributed to Wenzel by Berzelius in 1819, and the error has been carefully perpetuated in a considerable number of text-books since that time (Kopp, Entwicklung der Chemie in der neueren Zeit, p. 251).

Berthollet (1748-1822), who was one of the most active opponents of the theory of the constant composition of chemical substances, contributed valuable researches into the laws of chemical affinity, and applied chlorine to processes of bleaching. The processes of chemical analysis were improved, and large numbers of analyses, especially of minerals, were carried out by Klaproth (1743-1817), Vauquelin (1763-1829), Fourcroy (1755-1809), and others; and many quantitative observations of all kinds were made about the end of the 18th century, all preparing the way for Dalton's statement of the Atomic Theory (q.v.) in 1803-4.

The progress of chemistry during the present century has been immense, and it is not possible to do much more than mention the names of some of the most prominent workers. A stimulus was given to research by the publication of Dalton's atomic theory; and the labours of Gay-Lussac (1778-1850), who experimented with gases, of Dulong (1785-1838), and Petit (1791-1820), who pointed out the relation between specific heats and atomic weights of elements, and of others, supported and amplified Dalton's views.

Wollaston (1767-1829) discovered palladium in 1803, and rhodium in 1804. The first alkaloid (morphine) was obtained pure by Sertürner in 1816, and this led to the discovery of a number of others in a short time.

The decomposition by electricity of the bases potash and soda by Davy (1778-1829) in 1807, and the separation from these of the metals potassium and sodium, threw an entirely new light on the nature of these substances. The metals were more fully investigated by Gay-Lussac and Thénard (1777-1857). Davy is noted also as the inventor of the miners' safety-lamp, and for experiments on the respiration of nitrous oxide and other gases.

Amongst the foremost chemists of the earlier part of the 19th century was the Swede Berzelius (1779-1848), whose careful and exact analyses of mineral substances contributed a good deal to the confirmation of the law of constant proportions and to the fixing of the atomic weights (see ATOMIC THEORY) of the elements. Berzelius was very conservative with regard to new theories, which he declined to accept without putting them to the strictest experimental tests. He formulated the electro-chemical theory of the constitution of salts, introduced great improvements into the methods of quantitative analysis, increased the value of the blowpipe as an aid in mineral analysis, discovered many new substances, and further examined and elucidated points concerning many already known, both inorganic and organic.

The artificial production of urea in 1828 by Wöhler (1800-82) marks the beginning of a new era in the branch of organic chemistry, and enormous strides have been made in this department since that time by Dumas (1800-84), Liebig (1803-73), Laurent (1807-53), Gerhardt (1816-56), Wurtz (1817-84), Kolbe (1818-84), Baeyer, Cannizzaro, Frankland, Hofmann, Kekulé, Williamson, and many others. Advances in general inorganic chemistry and analysis have been made by Leopold Gmelin (1788-1853), H. Rose (1795-1864), Sainte-Claire Deville (1818-81), and Bunsen; whilst in connection with advances in chemical physics may be mentioned Faraday (1791-1867), Mitscherlich (1794-1863), Graham (1805-69), Regnault (1810-78), Andrews (1813-85), and Berthelot. These lists do not include all of even the most prominent names that might be mentioned in connection with each department.

The most striking feature of modern chemistry is the extraordinary development of organic chemistry, the account of one branch of it—the chemistry of the coal-tar products—constituting of itself quite a literature which receives additions every day.

Amongst the most recent triumphs of chemical research may be mentioned the artificial production of indigo and grape-sugar, and the isolation, in sufficient quantities to study its properties, of the hitherto all but unknown element fluorine.

Of the greatest possible interest from a theoretical point of view is the fact that since 1870 three new elements have been discovered—gallium, scandium, and germanium—the existence of all of which had been predicted, and the properties of which had to a certain extent been described beforehand by Mendeleëff. (See periodic law in article ATOMIC THEORY.)

Of late much attention has been given to measurements of the quantity of heat produced in various chemical changes, notably by Berthelot and Thomsen.

Elementary Principles of Chemistry.—The science of chemistry deals with a certain class of changes which matter undergoes when subjected to particular conditions. Similar treatment may produce very different effects upon different substances, as, for instance, the effect of strong heat upon a piece of quartz, a piece of limestone, and a piece of sugar. The quartz does not suffer any permanent change, that is, it has the same properties after it is cold again as it had before the action of heat. The limestone, although not necessarily much altered in appearance, has its properties entirely changed, and what remains is a new kind of matter—quicklime. The sugar melts, darkens, and chars, and becomes quite manifestly changed into more than one new kind of matter, for gaseous products, having the smell characteristic of 'burnt sugar,' go off, whilst a black coaly mass remains.

The first of the above changes is merely a physical change, from cold to hot; the other two are chemical changes, which result in the production of new kinds of matter having properties entirely different from those of the kinds of matter from which they were obtained. The existence of chemistry depends upon the existence of different kinds of matter, and it is with such different kinds of matter and the change from one kind to another that chemistry has to do.

When the properties of matter are studied, it is found that for chemical purposes all kinds of matter may be divided into two great classes, which are called respectively elements and compounds. The name element is applied to any kind of matter that has not been proved to be composed of more than one simpler kind of matter. This conception of an elementary substance we owe to Boyle, and it will be noted that some of those substances which are now looked upon as elements (see article ATOMIC THEORY for a list of the 68 known elements) may hereafter be proved to be compounds, or kinds of matter composed of more than one simpler kind, just as some substances which were at one time rightly classed as elements (according to Boyle's definition) are now known to be compounds of two or more elements.

The compound nature of a specimen of matter may be proved in one or other (or both) of two ways. One of these methods is called Synthesis (q.v.), and consists in building up the compound from the component simpler kinds; the other is called Analysis (q.v.), and consists in separating more than one simpler kind from the compound kind.

The distinction between chemical compounds and mere mechanical mixtures is a fundamental one, and must be fully understood. The substance gunpowder, for instance, is an intimate mixture of finely powdered sulphur, charcoal, and saltpetre (potassium nitrate), certain precautions being observed during the mixing in order to avoid explosion. These substances are not combined together chemically in gunpowder, but are only mixed, a fact as to which we can easily satisfy ourselves in various ways. We may examine the gunpowder under the microscope and identify the separate particles of the ingredients; or, by the use of appropriate solvents, we may dissolve out first the saltpetre and then the sulphur, and thus recover all three ingredients separately. The explosion of gunpowder when heated to a sufficiently high temperature is due to the occurrence of a series of changes of the kind we call chemical, for these changes result in the production of new kinds of matter, gaseous and solid, which possess properties in no way resembling those of sulphur, charcoal, or saltpetre, and from which these substances cannot now be dissolved out.

A mixture possesses to a greater or less extent the properties of its respective ingredients; a compound, on the other hand, has not as a rule any properties resembling those of its constituents. A piece of magnesium wire heated in the air to a sufficiently high temperature takes fire and burns. This is a chemical change in which the metal magnesium combines with the oxygen of the air to form a white, brittle, solid compound called magnesia or magnesium oxide. This magnesia does not in the least resemble either magnesium or oxygen in its properties, and the most powerful microscope fails to reveal particles of either of these substances to our vision.

The Atomic Theory (q.v.) is based upon the assumption that matter of every kind is made up of extremely minute indivisible particles called atoms. The atoms which exist in a substance may be all of the same kind, as in elements, or of different kinds, as in compounds. Chemists believe that the element hydrogen consists of molecules or aggregates of atoms—each molecule consisting of two atoms; further, that the compound substance water consists of molecules, each composed of two atoms of hydrogen and an atom of oxygen united to each other by that force which is called Chemical Affinity (q.v.); and that similarly every other compound substance is composed of molecules, each molecule consisting of two or more different kinds of atoms united by chemical affinity. The weight of a new compound formed by the union of two or more substances is in every case equal to the sum of the weights of its constituents. In chemical actions it is only the kind of matter which is changed, whilst, as in every physical change, the quantity of matter concerned remains constant and unalterable.

It has already been seen that one of the characteristics of the chemical combination of two substances is that the properties of both disappear and are not observable in the compound. Another and a most important characteristic is the evolution of heat, which is a very frequent although not an invariable accompaniment of chemical action. The best examples of this may be seen in the ordinary phenomena of Combustion (q.v.). All combustion, whether it be of magnesium wire, coal, phosphorus, paraffin oil, or a candle, is nothing more than a chemical action accompanied by the evolution of heat and light, oxygen gas of the atmosphere being almost invariably one of the substances taking part in such action.

The conditions under which substances act chemically upon each other are very various for different substances. In the first place, certain substances cannot be got to act upon each other at all. Such substances may have little affinity for each other, as chlorine and oxygen, or no affinity, as fluorine and oxygen. Other substances, again, only act upon each other with difficulty. The main conditions upon which action of one substance upon another depends are the state of physical aggregation and the temperature. Certain chemical actions take place at ordinary temperatures, as, for instance, the combination of chlorine with metallic antimony or copper, or the spontaneous ignition of one of the compounds of phosphorus and hydrogen when brought into contact with oxygen. Other actions only take place when the temperature of the substances which are to take part in them has been sufficiently raised. Thus magnesium requires to be strongly heated in air before it takes fire; once the action is started, however, the heat given out by the combustion of one part of the magnesium is sufficient to raise another part to the temperature necessary for combustion to go on, and so the change is propagated. Coal-gas only burns in air when it is raised to a bright-red heat. A jet of coal-gas escaping into the air may be easily ignited by applying a brightly red-hot poker, but when the poker cools to dull redness it will no longer ignite the jet. A bar of metallic iron does not undergo any chemical change on exposure to dry air at ordinary temperature, but if iron in the state of very fine powder (a form in which it can easily be obtained by appropriate methods) be thrown into the air, combination at once takes place with the evolution of heat and light. When a piece of iron (say a moderately fine iron wire) is heated to redness in air, combination with the oxygen of the air takes place with the formation of a scale composed of a black oxide of iron, but the quantity of heat given out during the combination is not sufficient to propagate the combustion from particle to particle of the iron after removal of the source of heat. If, however, iron wire be raised to a red heat in an atmosphere of oxygen, it takes fire and burns with great brilliancy. The difference noticed here is due to the presence in the one case, and the absence in the other, of the diluting nitrogen which forms nearly four-fifths of the air by volume.

There are certain chemical actions which in taking place are accompanied, not with evolution, but with absorption of heat. In such cases heat has to be supplied throughout the action, and not merely to start it. This is frequently noticed in the combination of substances which have feeble affinity for each other, and the compounds produced are less stable, or more readily break up into their constituents, than those which are produced with the evolution of heat. In general terms it may be stated that the quantity of heat given out in the formation of a compound is a measure of the stability of the compound. When a given weight of magnesium unites with oxygen to form magnesia, a quite definite and measurable quantity of heat is given out. In order to separate the magnesium from the oxygen again, exactly the same quantity of heat must be supplied. In the case of those substances in the formation of which heat is absorbed, we find, as we should expect, that heat is given out during their decomposition, and that its quantity is exactly that which was absorbed during their formation.

Chemical Notation.—For the purpose of shortly expressing the composition of chemical substances, and for representing chemical changes, chemists employ a system of notation which is in extremely common use. In the table of Atomic Weights (see ATOMIC THEORY) it will be noticed that after the name of each element is placed its symbol, which usually consists of the first, or of the first and another letter of the Latin name of the element. Each symbol distinctly indicates the element which it is intended to represent, but it must always be borne in mind that the symbol for an element is not merely a contracted form of its name, but that it stands for a definite quantity of that element, this quantity being the atomic weight expressed in terms of the unit of weight employed. The unit of weight almost universally employed by chemists and scientific men in general is the gramme (see METRIC SYSTEM), and that unit will be adopted for illustrations throughout this article. With the gramme as unit, H stands for 1 gramme of hydrogen, Cl for 35.4 grammes of chlorine, O for 16 grammes of oxygen, Mg for 24 grammes of magnesium, and so on. In order to represent the composition of a compound, the symbols of the various elements which occur in the compound are written side by side, and this collection of symbols is called a formula. Thus, MgO represents 40 (= 24 + 16) grammes of magnesium oxide, and HCl is 36.4 (= 1 + 35.4) grammes of hydrogen chloride. When a compound contains more than one atom of the same element the symbol for that element is not repeated, but the number of atoms is indicated by a subscripted numeral. Thus the formula for water is written H₂O, which indicates that the molecule of water contains two atoms of hydrogen and one of oxygen; and the formula for sulphuric acid is written H₂SO₄, which indicates that the molecule of sulphuric acid contains two atoms of hydrogen, one of sulphur, and four of oxygen (besides the quantitative signification of these formulæ already mentioned). A number subscribed to a portion of a formula inclosed in brackets multiplies the portion so inclosed. Thus the formula Ba(NO₃)₂ represents one atom of barium united to twice the quantity of the group NO₃, which is represented as united to one atom of potassium in the formula KNO₃. A number prefixed to a formula multiplies the whole of the formula that follows. Thus 2H₂O represents twice the quantity of water represented by H₂O.

Chemical symbols and formulæ are used to represent shortly chemical changes. A simple illustration of the method of using them may be given to represent the case of the burning of magnesium. The symbols for the magnesium and the oxygen entering into combination (connected by the sign +) are written on one side of what is called a chemical equation, whilst the product is written on the other side, thus :

The formula for free (or uncombined) oxygen is written O₂, because a molecule of oxygen is believed to consist of two atoms (see ATOMIC THEORY). In order to represent the element magnesium, the simplest possible formula (Mg) is employed because there is no evidence for writing a more complicated one. 2Mg simply represents twice as much magnesium as Mg does.

The above equation when fully interpreted gives a great deal of information about the change which it is intended to represent. It shows that magnesium and oxygen unite with each other (under conditions which are not expressed) to form an oxide of magnesium, and that these elements are united in the compound in the proportions by weight of 24 of magnesium to 16 of oxygen; and, further, it enables us, by applying a simple and easily remembered rule, to calculate the volume of oxygen taking part in the action as well as its weight. This rule for ascertaining the volume may be conveniently stated here. From certain theoretical considerations, as well as for convenience in calculations concerning the volumes of gases, chemists write the formulæ of gaseous substances in such a way that the quantity of a gas represented by its formula, in terms of any unit of weight, shall occupy, under similar conditions of temperature and pressure, the same volume as two units weight of hydrogen. Thus, the unit being the gramme, H₂ represents 2 grammes of hydrogen, and 2 grammes of hydrogen at standard temperature (0° C.) and pressure (760 millimetres of mercury) occupy a volume of 22.33 litres (see METRIC SYSTEM). Similarly, the quantities in grammes of oxygen, carbonic anhydride, and nitrous oxide, represented by their respective formulæ, O₂ (16 × 2 = 32 grammes), CO₂ (12 + 32 = 44 grammes), and N₂O (28 + 16 = 44 grammes), each occupy, when measured at $0^\{\backslash circ\}$ C. and 760 mm. pressure, 22.33 litres. This rule holds for other gases, and also, with a certain qualification, for the vapours of volatile liquids. In the case of the latter, of course, conditions of temperature and pressure must be chosen such that the substance is in the state of vapour; and the quantity in grammes which, as a vapour, occupies the same volume as 2 grammes of hydrogen under the same conditions, is the quantity which the formula is chosen to represent. Thus, the formula $H\_2O$ informs us that 18 ($=\; 2\; +\; 16$) grammes of water occupy, in the form of steam, the same volume as 2 grammes of hydrogen when both are measured at the same temperature and pressure. It must, of course, be understood that the formula for a substance is chosen so as to represent the observed facts. The formula of a volatile liquid is deduced from the determination of the vapour density of the liquid; this determination is made by ascertaining the weight of that quantity of the liquid which, when converted into the state of vapour, occupies the same volume as a given weight of hydrogen, both being measured at the same temperature and pressure.

Returning to the equation already given, it will be seen that from it we learn that 48 ($=\; 2\; \backslash times\; 24$) grammes of magnesium unite to form magnesium oxide with a quantity of oxygen (32 grammes) which at $0^\{\backslash circ\}$ C. and 760 mm. occupies 22.33 litres. What volume this quantity of oxygen would occupy under other conditions of temperature and pressure can be calculated from formulæ deduced from the laws of Charles (relation of the volume of a gas to the temperature) and Boyle (relation of the volume of a gas to the pressure). See further in article GASES.

As there are certain conditions under which chemical combination takes place, so there are definite laws which regulate combination. The first of these has been called the law of constant proportions, and it states that any chemical compound always contains the same constituents and in the same proportions. Thus magnesium oxide, $MgO$, always consists of magnesium and oxygen in the proportions by weight of 24 to 16—one atom of magnesium weighing 24, being combined with one atom of oxygen weighing 16. No compound of magnesium and oxygen containing these elements in any other proportion has ever been obtained. If in preparing magnesium oxide quantities of magnesium and oxygen were employed differing from this proportion, then some either of the magnesium or of the oxygen would remain over after the action, according as the former or the latter had been employed in excess of the right quantity.

Intimately connected with the foregoing law is the law of multiple proportions. Whilst certain elements combine with each other in only one proportion by weight, others combine in two, and sometimes more than two different proportions. The law of multiple proportions states that when elements combine in two or more proportions these various proportions can be expressed by simple multiples of the atomic weights of the elements concerned. Thus carbon and oxygen unite with each other to form two different compounds: 12 parts by weight of carbon unite with 16 parts by weight of oxygen to form carbonic oxide, $CO$; 12 parts by weight of carbon unite with 32 parts by weight of oxygen to form carbonic anhydride, $CO\_2$. Here the relation is of the simplest kind, for the one compound contains exactly twice as much oxygen for the same quantity of carbon as the other. Again, iron and oxygen unite with each other to form three different compounds: 56 parts by weight of iron unite with 16 parts by weight of oxygen to form ferrous oxide, $FeO$; 112 parts by weight of iron unite with 48 parts by weight of oxygen to form ferric oxide, $Fe\_2O\_3$; 168 parts by weight of iron unite with 64 parts by weight of oxygen to form ferroso-ferric oxide, $Fe\_3O\_4$. This case is not quite so simple as that of the oxides of carbon, for here it is necessary to employ multiples of the atomic weights of both elements concerned in order to see the simplicity of the quantitative relations existing amongst these oxides of iron. The law of multiple proportions is, however, fully illustrated by both series of oxides.

It may be useful to call attention here to the simple explanation furnished by the Atomic Theory (q.v.) for the occurrence of compounds illustrating this law of multiple proportions. There is no compound intermediate in composition between carbonic oxide and carbonic anhydride. The atomic theory explains this very simply. Under one set of conditions we can obtain a compound of one atom of carbon with one atom of oxygen, whilst under other conditions we obtain a compound of one atom of carbon with two atoms of oxygen, or exactly twice as much. This is why we find such marked intervals in composition between two or more compounds of the same elements. The molecule of one compound cannot differ from that of the other by less than an atom, and the addition of an atom to a molecule necessarily forms a new molecule differing in weight from the old one by the weight of the added atom.

The last law of combination has been called the law of volumes. It states that when gases combine to form new compounds, the volumes taking part in the action bear a very simple relation to each other and to the volume of the product if gaseous when all the volumes are measured at the same temperature and pressure. Thus, one volume of hydrogen combines with one volume of chlorine to form two volumes of hydrochloric acid gas; two volumes of hydrogen combine with one volume of oxygen to form two volumes of water vapour; two volumes of carbonic oxide combine with one volume of oxygen to form two volumes of carbonic anhydride, and so forth. The very simple relations of the volumes concerned in these examples are sufficiently manifest, and much greater complexity is not frequently met with.

Chemists divide the elements into two great classes, the typical members of which are very different in their physical and chemical characters. These are metals and non-metals, and as representative of each class may be mentioned copper and sulphur. The more prominent physical characteristics of metals are the metallic lustre, malleability, ductility, and the property of conducting heat and electricity, all of which are possessed to a more or less marked degree; whilst non-metallic elements as a rule possess these properties to a very limited extent, if at all. Differences in chemical behaviour are also very striking in typical representatives of each group. It must be borne in mind, however, that all the members of each group are not typical, but that there is a gradual transition from one group to the other, and certain of the transition elements possess some of the properties of both groups, as in the cases, for instance, of arsenic and antimony.

With the exception of bromine and fluorine, all the elements enter into combination directly or indirectly with oxygen to form oxides. The oxides produced from metallic elements are quite different in chemical character from those produced from non-metallic elements. We shall look first at the oxides of the metals. Every metal forms one or more oxides, and at least one oxide of every metal is a basic oxide—i.e. an oxide which has the properties of a Base (q.v.). A distinction is made between what are called anhydrous bases and hydrated bases or hydroxides. The oxide of lead, PbO, is an anhydrous base (or basic oxide), whilst the compound obtained by the action of water upon calcium oxide, CaO (a basic oxide, and the only compound of calcium and oxygen known), is called a hydrated base (or hydroxide). The formation of the latter is represented by the equation

The oxides produced from non-metallic elements are very frequently acid oxides—i.e. oxides which unite with water to form the class of bodies called Acids (q.v.). The oxides themselves are often called acid anhydrides, whilst the compounds produced by the action of water upon them are called acids, or hydrogen salts. When phosphorus burns in air, phosphoric anhydride, P₂O₅, is obtained. This is a white solid substance which unites with water with the evolution of much heat to form a solution of metaphosphoric acid, or hydrogen metaphosphate:

There are a few acids known which do not contain oxygen, and are not obtainable by the combination of an oxide with water. Examples are hydrochloric acid, HCl, hydrobromic acid, HBr, and hydrocyanic acid, HCN. These are also called hydrogen chloride, bromide, and cyanide respectively.

The two classes of substances, bases and acids, are nearly related to the very large class of salts. A salt is a compound which can be obtained, amongst other ways, by the action of an acid upon a base, water being almost invariably eliminated at the same time; and just as we saw that the properties of two elements are totally different from those of the compound formed by their combination, so we find that in the formation of a salt the properties of both acid and base to a great extent or altogether become neutralised and disappear.

If to a solution in water of potassium hydroxide, KHO (which is a powerful base), we add a sufficient quantity of nitric acid, HNO₃, that is until the liquid on thoroughly mixing does not possess either the acid or the alkaline reaction, we obtain a solution in water of potassium nitrate (saltpetre), and nothing else—the water eliminated in the action simply mixing with that which is already present:

Acids have already been mentioned as hydrogen salts. The above equation shows how hydrogen nitrate is exactly comparable with potassium nitrate—an atom of potassium taking the place of an atom of hydrogen—and a characteristic of all hydrogen salts, or acids, is that they contain hydrogen, which is capable of removal and of having its place thus taken by an equivalent quantity of another metal. In the example above mentioned every 1 part by weight of hydrogen has its place taken by 39 parts by weight of potassium. These quantities of hydrogen and of potassium are equivalent, both being capable of uniting with the group NO₃. This group is an example of what is called a compound radical—i.e. a group of elements which is capable of going as a whole through a series of changes. Acids which contain in their molecule one atom of hydrogen replaceable by another metal are called monobasic acids. Nitric acid is thus a monobasic acid, whilst sulphuric acid, H₂SO₄, is dibasic, orthophosphoric acid, H₃PO₄, is tribasic, and so on.

Bases, likewise, are sometimes spoken of as monacid, diacid, triacid, and so on, according as one molecule of the base requires one, two, three, &c. molecules of a monobasic acid (as nitric acid) to form what is called a normal salt, that is, a salt in which all the replaceable hydrogen has been replaced by another metal. Thus potassium hydroxide, KHO, is a monacid base; calcium hydroxide, or slaked lime, Ca(HO)₂, is diacid; bismuth hydroxide, Bi(HO)₃, is triacid, and so on. Equations may make this clearer (see the equation above for a monacid base):

Salts are formed in many cases by the replacement of only a part of the replaceable hydrogen of a hydrogen salt by another metal. Such are called acid salts, and KHSO₄ is an example. This salt, KHSO₄, may be looked upon as intermediate between the acid, H₂SO₄, and the normal salt, K₂SO₄.

Many salts are known which may be looked upon as bases which have their basic character only partially neutralised by an acid. Such salts are called basic salts, and as examples may be mentioned BiONO₃ and Pb(OH)NO₃. The former is intermediate between the normal nitrate, Bi(NO₃)₃, and the oxide, Bi₂O₃, the latter between the normal nitrate, Pb(NO₃)₂, and the hydrate, Pb(OH)₂. Such basic salts are often produced by the action of water upon the normal salts, as, for instance, in the case of the basic bismuth nitrate:

Salts are looked upon as being composed of metal and salt radical, the latter name being given to all of the salt that is not metal. Thus SO₄ is the salt radical of the sulphates, NO₃ the salt radical of the nitrates, &c. This way of looking at salts arises from the phenomena observed when salts are decomposed by Electrolysis (q.v.), metal and salt radical being the primary products of decomposition.

Chemical Nomenclature.—Chemists endeavour to make the nomenclature of compound substances as systematic as possible, and a certain amount of system has even been introduced into the nomenclature of the elements themselves. The oxides of the metals are named after the metal which they contain, as magnesium oxide, MgO; aluminium oxide, Al₂O₃; and the series of salts derivable from these oxides are similarly named after the metal. Thus MgCl₂ is magnesium chloride, and Al₂(SO₄)₃ is aluminium sulphate. When a metal forms more than one basic oxide, adjectival terminations are employed to distinguish these; thus the two basic oxides of iron are named ferrous and ferric oxides (FeO and Fe₂O₃) respectively, and correspondingly there are ferrous and ferric salts. FeSO₄ is ferrous sulphate; Fe₂Cl₆ is ferric chloride. Acid salts and in general salts which contain more than one metal are named after the metals which they contain, the compound radical NH₄ (ammonium; see AMMONIA) being regarded as a metal for purposes of nomenclature. Thus, KHSO₄ is potassium hydrogen sulphate, whilst HNaNH₄PO₄ is hydrogen sodium ammonium orthophosphate.

The nomenclature of non-basic metallic oxides has been rendered systematic by the use of names descriptive of the number of atoms of metal and of oxygen contained in the oxide, as, for instance, trimanganic tetroxide for Mn₃O₄. A considerable number of non-basic oxides, as BaO₂, PbO₂, MnO₂, &c., are somewhat less systematically called peroxides.

The acid anhydrides, which, as has already been stated, are oxygen compounds or oxides of the non-metallic elements, are named after the elements of which they are oxides. As there are frequently two or more such acid anhydrides derived from one element, different terminations and, where necessary, other devices of nomenclature are employed to distinguish amongst these. Thus there are two acid anhydrides derived from sulphur—sulphurous anhydride, $\backslash text\{SO\}\_2$, and sulphuric anhydride, $\backslash text\{SO\}\_3$. The latter unites with water to form sulphuric acid, $\backslash text\{H\}\_2\backslash text\{SO\}\_4$, and it is believed by some chemists that the solution in water of sulphurous anhydride (a gaseous substance) contains at least some of the corresponding sulphurous acid, $\backslash text\{H\}\_2\backslash text\{SO\}\_3$. From sulphuric acid there is derived the series of salts called sulphates, from sulphurous acid the series called sulphites. It sometimes happens that an acid and series of salts are known of which the corresponding anhydride is unknown, just as the existence of certain acids is doubtful although the corresponding anhydride is known. In other cases series of salts are known, although both the corresponding anhydride and acid are unknown. Certain of these peculiarities, as well as some further forms of nomenclature, are illustrated by the table given below of the compounds corresponding to known or unknown oxides of chlorine:

It has recently been proved that the substance described in most text-books as chlorous anhydride, $\backslash text\{Cl\}\_2\backslash text\{O\}\_3$, is really a mixture, and that as yet $\backslash text\{Cl\}\_2\backslash text\{O\}\_3$ has not been prepared. The hypothetical chloric and perchloric anhydrides would have the composition $\backslash text\{Cl\}\_2\backslash text\{O\}\_5$ and $\backslash text\{Cl\}\_2\backslash text\{O\}\_7$ respectively.

A very large number of salts and other chemical compounds are commonly known by popular names, the latter being frequently of extremely ancient origin. The popular name as a rule conveys no information as to the composition of the substance. For instance, copperas (ferrous sulphate, $\backslash text\{FeSO\}\_4$) is not recognised by its name as an iron compound, nor calomel (mercurous chloride, $\backslash text\{HgCl\}$) as a mercury compound, nor litharge (lead oxide, $\backslash text\{PbO\}$) as a lead compound. It is the aim to convey, by the systematic name of a substance, the greatest possible amount of information as to its composition. It is not possible to attain to a perfect system of nomenclature, as new discoveries render changes necessary from time to time.

Graphic Formulae.—In addition to representing the composition of a substance by means of formulae, chemists endeavour to express certain ideas as to the constitution, or arrangement of the atoms in the molecule of substances by means of graphic formulae. It must not be supposed (as has sometimes erroneously been done) that graphic formulae are intended to represent the shape of molecules or the arrangement in space of the atoms constituting such molecules, but simply as a short method of expressing on paper certain facts. No one supposes that a printed word in any modern language is an attempt to draw the object spoken of, or that it is more than a method of representing on paper a given series of sounds, and yet criticism based upon assumption scarcely less absurd, has been directed against graphic formulae. In a graphic formula we have the symbols for the different elements grouped in a particular way, so as (1) to indicate the valency (see the article ATOMIC THEORY) of each element, and (2) to express ideas based upon observed facts as to the most likely arrangement of the atoms in a molecule, when various arrangements are conceivable.

The following may be given as simple illustrations of (1): $\backslash text\{H\}-\backslash text\{Cl\}$, hydrochloric acid; $\backslash text\{H\}-\backslash text\{S\}-\backslash text\{H\}$, sulphuretted hydrogen; $\backslash text\{Mg\}=\backslash text\{O\}$, magnesian oxide; $\backslash text\{N\}\; \backslash begin\{array\}\{l\}\; \backslash text\{H\}\; \backslash \backslash \; \backslash text\{H\}\; \backslash end\{array\}$, ammonia;

$\backslash text\{O\}=\backslash text\{C\}\; \backslash begin\{array\}\{l\}\; \backslash text\{Cl\}\; \backslash \backslash \; \backslash text\{Cl\}\; \backslash end\{array\}$, phosgene; $\backslash text\{O\}=\backslash text\{C\}=\backslash text\{O\}$, carbonic anhydride; $\backslash text\{S\}=\backslash text\{C\}=\backslash text\{S\}$, carbon bisulphide, &c. The letters representing monovalent atoms are written with one stroke proceeding from them, those representing divalent, trivalent, and tetravalent atoms being written with two, three, and four such strokes respectively.

Illustrations of (2) are:

These two substances illustrate two other points of importance. One of these is the occurrence of the nitrogen atom sometimes trivalent, as in ammonia, $\backslash text\{NH\}\_3$, sometimes pentavalent, as in the ammonium salts—e.g. ammonium chloride, $\backslash text\{NH\}\_4\backslash text\{Cl\}$. In ammonium cyanate one atom of nitrogen is represented as trivalent and the other as pentavalent. The two substances, moreover, illustrate Isomerism (q.v.), or the existence of two or more compounds containing exactly the same elements and in the same proportions, and yet differing from one another in chemical and physical properties.

Chemical Changes.—There are several kinds of chemical changes which are of very frequent occurrence, and may conveniently be classified. The simple union of one element with another has already been mentioned, and closely related to this kind of change is the union of a compound with an element or with another compound. Along with these changes may be classed those in which a compound breaks up into two or more elements or simpler compounds, or into one or more of each. All these variations are illustrated by the following equations:

One of the most important kinds of chemical change is that called double decomposition. This occurs perhaps most frequently when solutions of salts are mixed with each other, and it is characterised by a mutual exchange of metal and salt radical. If an aqueous solution of sodium chloride be mixed with one of potassium bromide, although no visible change takes place, we have reason to believe that double decomposition goes on to a certain extent, with formation of some sodium bromide and some potassium chloride, whilst some of each of the original salts also remains, a state of equilibrium being eventually established amongst the four salts. If, however, one of the new products formed by double decomposition be insoluble or practically insoluble in water, as soon as any of it is formed it will appear as a precipitate, and be thus removed from solution, so that no condition of equilibrium can be established until formation of a precipitate no longer occurs—i.e. until the double decomposition is complete. Thus, if solutions of sodium chloride and silver nitrate be mixed in the proper proportions, the extremely insoluble silver chloride will be precipitated, and only sodium nitrate will remain in solution. The action may be represented by an equation :

The action of sulphuretted hydrogen on many metallic solutions illustrates double decompositions in which the action is complete, as,

where the mercuric sulphide formed is insoluble in water, and is consequently obtained as a precipitate.

In connection with the subject of double decomposition the bearing of the law of Richter (already mentioned in the historical sketch) may be illustrated. Looking at the quantitative signification of the following equations,

we see that the quantity of chlorine which was united with 39 parts by weight of potassium or 23 of sodium to form a salt is exactly the quantity required to form a salt with 108 parts by weight of silver, whilst, similarly, the quantity of the group $\backslash text\{NO\}\_3$ which was united to these 108 parts by weight of silver is exactly the quantity required to form a salt with 39 parts by weight of potassium or 23 of sodium. The same holds good generally for double decompositions.

Another very important kind of chemical change is the displacement of one element in a compound by another. Chlorine, for instance, displaces the iodine in potassium iodide and takes its place :

The greater affinity of potassium for chlorine than for iodine is the explanation given of this displacement. Displacement of one metal by another is a familiar phenomenon, although the chemistry of what is taking place may not be familiar to all who have seen it. When a piece of bright iron or steel, as a key or the blade of a knife, is dipped into an acidulated solution of cupric sulphate (blue vitriol), a reddish deposit of metallic copper is formed almost immediately upon the surface of the metal. This copper is derived from the cupric sulphate solution; but what is not manifest from observation alone, is that at the same time an equivalent quantity of iron is dissolved away and goes into solution as ferrous sulphate. The action is,

The whole of the copper would eventually be separated from the solution in the metallic state if enough iron were present, and for every 63 parts of copper precipitated 56 parts of iron would go into solution.

Inorganic and Organic Chemistry.—The whole subject of chemistry has been divided into two great divisions, named respectively inorganic and organic. Made originally to separate from each other the chemistry of purely mineral substances, and that of substances of animal or vegetable origin, which were at the time supposed to be capable of formation only as products of vital processes, this subdivision is retained still mainly as a matter of convenience. The division of organic chemistry is sometimes spoken of now as the chemistry of the compounds of carbon; but this is not a very strict definition, as many carbon compounds occur in nature as purely mineral substances, and having really no connection with organic chemistry, such as numerous mineral carbonates. As has been already stated, it is mainly for convenience that the consideration of the majority of the compounds of carbon is taken as a separate branch, not because of any difference in the chemical principles involved, but really on account of the very great number of these compounds, and of the great complexity of many of them.

It is in the domain of organic chemistry that the study of the constitution of substances has been most diligently prosecuted, and with the greatest amount of apparent success. The graphic formula which chemists assign to acetic acid (to take a simple example) is,

This formula is adopted in order

to express a number of ideas concerning the supposed mode of arrangement of the atoms in acetic acid, deduced from the study of its formation, its decompositions, and the action upon it of various substances. The known facts find suitable expression in the formula, and there is no observation yet made as to the chemical relations of acetic acid which is at variance with the constitution indicated by it. It would not be possible here to quote evidence in favour of a particular constitution for any substance, but it may be stated generally that chemists endeavour to fix the constitution of the simplest compounds on the firmest possible basis, and, in passing from the simple to the more complex, to make secure every step.

The tetravalent character of the carbon atom, and the great facility with which carbon atoms enter into combination with other carbon atoms and with the atoms of other elements, give their impress to the whole of organic chemistry. The graphic formulae of organic substances amply illustrate the former, whilst the syntheses of a long array of simple and complex organic compounds as amply illustrate the latter.

A certain amount of knowledge of chemistry is eminently useful in almost every walk of life. An intelligent knowledge of the chemistry involved in the processes of the kitchen, the dairy, the dye-house, the farm, or the manufactory, places the possessor engaged in any of these processes on a different level from the rule-of-thumb worker, who is as ignorant of the reason for adopting a particular method as he is of the properties of the materials he employs. Technical chemistry deals especially with the application of the principles and processes of chemistry to the arts and manufactures, and it is to those who are engaged in manufactures of almost every kind that a knowledge of chemistry is a particular advantage. It is not a question of expediency alone, but one of absolute necessity that a technical education, including chemistry as one of its principal subjects, should form not the least important part of the equipment for his work of any artisan who is to excel in his employment in intelligence and skill.

In connection with this article should be read the articles ATOMIC THEORY, which is to a certain extent supplementary to this, ANIMAL CHEMISTRY, and VEGETABLE PHYSIOLOGY. See also separate articles on the several elements, those on the various acids, those on the great chemists, and the following as amongst the most important of the many chemical articles throughout this work :

For further information readers may consult : Professor Crum Brown's Chemistry (Chambers's Science Manuals); Roscoe and Schorlemmer, Treatise on Chemistry (1878-

89); Watts, Inorganic Chemistry, and Organic Chemistry; Wilson, Inorganic Chemistry (new ed. 1885); Watts, Dictionary of Chemistry (new ed. 4 vols., 1890-94); Thomson, History of Chemistry (1830-31); French works by Hoefler, Chevreul, Berthelot; Ladenburg, Handwörterbuch der Chemie; Kopp, Geschichte der Chemie; Von Meyer, A History of Chemistry (trans. by McGowan, 1892); Mendeléef, The Principles of Chemistry (1892); Perkin and Kipping, Organic Chemistry (1894); Thorpe, Dictionary of Applied Chemistry (3 vols. 1890-93).