Allotropy is the peculiarity, which certain elements exhibit, of existing in two or more distinct modifications, which, although chemically identical, usually differ very much in their physical properties, such as colour, density, hardness, and so forth. The element carbon, which is a constituent of the very numerous chemical compounds commonly called organic bodies, is known in several allotropic modifications. Of these, two crystalline forms occur in nature—the first, the widely distributed but comparatively rare diamond; and the second, graphite or black-lead, familiar to all as the substance used in the manufacture of the so-called lead pencils. These two forms of carbon occur, as has been said above, crystallised—the diamond, in forms related to the cube, while graphite crystallises in thin hexagonal plates; and they thus present an example of what chemists call dimorphism, or the occurrence of the same substance in two totally distinct crystalline forms, which are not geometrically related to each other.
Besides the two crystalline forms of carbon found in nature in an almost pure state (graphite usually contains at least a small proportion of impurities), the various kinds of wood and animal charcoal, and coke, consist of carbon in a more or less pure state. These latter kinds of carbon are called amorphous, because they have not any definite crystalline form.
It is almost needless to point out the entire difference from each other in regard to colour, transparency, hardness, and other physical properties of the diamond and of graphite, diamond being, when quite pure, colourless and transparent, and the hardest of known substances; while graphite is black and opaque, and sufficiently soft to mark paper easily. That the various forms of carbon are chemically identical, is proved by burning any of them in oxygen, when in each case it is found that carbonic acid gas is produced and nothing else, and that exactly the same quantity of this gas is produced from equal weights of the various kinds.
In the case of many allotropic substances it is possible easily to convert one modification into another, but this is not markedly so in the case of carbon. When diamond is heated in the electric arc, out of contact with the air, it blackens and swells up into a somewhat coke-like mass. Many attempts have been made to prepare artificial diamonds from the more common kinds of carbon; but as yet the measure of success attained has not been great, although minute crystals have been obtained, which under the microscope exhibited the crystalline form of true diamonds.
Phosphorus presents another very good instance of allotropy. Several forms of this element have been described, but only two are commonly known. These are ordinary phosphorus, which is a pale yellow, semi-transparent, waxy solid, soluble in carbon bisulphide, and crystallising from this solution in octahedra; and red or amorphous phosphorus, usually seen in irregular lumps or in powder as a dark, reddish-brown, non-crystallisable solid, which is not soluble in carbon bisulphide. Ordinary phosphorus is very readily oxidised in the air, and must be preserved under water to prevent its taking fire, and it is extremely poisonous. The amorphous variety does not undergo any change in the air at ordinary temperature, and it is not poisonous. When ordinary phosphorus is melted in close vessels, and kept for a long time at a temperature near its boiling-point, a certain proportion of it is converted into red phosphorus. This change is accompanied by the evolution of heat, as can be easily demonstrated by suitable experiment. From the mixture obtained, the red phosphorus can be separated by dissolving out the unchanged ordinary kind by means of carbon bisulphide. When heated to a temperature somewhat higher than that at which it was prepared, amorphous phosphorus changes back again into the ordinary kind. Most persons are familiar with the appearance of ordinary phosphorus as such. The brownish surface provided on the boxes of safety matches for igniting these consists mainly of amorphous phosphorus.
Sulphur, again, is known in several allotropic forms, some crystalline and some amorphous, which differ greatly in colour, melting-point, solubility in solvents, &c. Most of these forms are, however, unstable, and quickly begin to change back again into ordinary sulphur. One of the most remarkable varieties is the so-called plastic sulphur, familiar to many as the clear yellow or brownish, pliable and elastic mass obtained by pouring melted sulphur, at or near its boiling-point, into water. This soft condition does not continue long, as in a few days the substance becomes opaque and hard, and passes, in great part, into the ordinary form of sulphur again. This change may be hastened by heating the plastic variety to near the melting-point of ordinary sulphur, when it suddenly becomes solid, giving out in doing so sufficient heat to raise its own temperature several degrees.
Another instance of allotropy, and one of great interest, is the existence of oxygen in two forms—as ordinary oxygen and as ozone. Both forms are gases—oxygen odourless, while ozone possesses a peculiar and powerful odour. The manner of formation in the atmosphere of the ozone, which is a constant constituent, in small quantity, of country and sea air, is not with certainty known. Ozone is formed when an electrical machine is worked, and also during many chemical processes, as in the slow oxidation of phosphorus in moist air. It is best prepared by passing what is known as the silent discharge of electricity through oxygen, by which means a considerable proportion of the oxygen is converted into ozone. Ozone is a much more active oxidising agent than oxygen, and the property is attributed to it of destroying deleterious organic impurities in the atmosphere.
The nature of the difference between allotropic forms of the same substance is, to some extent at least, understood in the case of oxygen; ozone being a condensed form of oxygen, and having a density half as great again as oxygen. Whilst the molecule of oxygen is represented by the formula , that of ozone is represented by . It is probable that an analogous explanation may be found to account for the formation of allotropic modifications of carbon, sulphur, phosphorus, &c.
Allotropy is nearly related to ISOMERISM, which see.