Disinfectants are, strictly speaking, agents which can prevent infectious diseases from spreading, by destroying their specific poisons. The term is, however, often applied to all substances which destroy or neutralise bad odours, though not all such have the power of counteracting infection. Many infectious diseases have now been proved, and all are believed, like Putrefaction (q.v.), to be due to special micro-organisms which are found in different parts of the body, and are communicable in different ways in different diseases (see GERM THEORY). The action of disinfectants is therefore exactly analogous to that of Antiseptics (q.v.), and consists in the destruction of low forms of life. But the two classes do not necessarily correspond, as the same substance may have unequal poisonous effects on different forms. It is of the utmost importance to discover the conditions which are most deadly to each disease-poison, and to apply them, if possible, within as well as without the diseased body. But little has yet been done in this direction.
Carbolic acid, which probably stands highest in popular esteem as a disinfectant, is undoubtedly one in the strict sense. It is not, however, in the very dilute state that it can act thus, and it is necessary to use it comparatively concentrated before good can result. Thus a 2 per cent. solution, mixed with vaccine lymph, completely destroys it, but a more dilute solution has almost no action on it. As a deodoriser, carbolic acid is not so energetic as chlorine and permanganate of potash, but there is this great difference, that while the acid destroys the organic substances which give rise to the offensive odour, the others mainly attack the odour itself, and therefore require to be applied frequently if perfect sweetness is desired. Thus a piece of putrid flesh is not rendered odourless so quickly by carbolic acid as by the other substances named, but one thorough application of it will prevent the recurrence of decay, a property not possessed by the others.
The vapour of carbolic acid is not a disinfectant at ordinary temperatures, as bacilli are not destroyed, even when exposed to it for six weeks. It is therefore evident that the mere exposure of that substance in vessels is of no service in disinfecting a room. It is curious that carbolic acid dissolved in oil or alcohol has no antiseptic action whatever, but that if water be present, as in the case of a wound, it acts powerfully.
Sulphurous acid has long been in repute, both in the form of solution and in the gaseous state. Recent experiments on cultivated bacilli seem to prove that, while in the liquid state it is a powerful disinfectant, it has little action either as dry gas or along with water vapour.
Mercury salts, such as the perchloride (corrosive sublimate) and biniodide, are powerful disinfectants, and are much used at present as antiseptics. For general domestic use, however, they have great disadvantages. The former attacks metals, and therefore ruins many pipes, while it is very rapidly neutralised by the presence of organic matter, especially where sulphuretted hydrogen has been developed. The biniodide is preferable, but neither of them is completely satisfactory. Of all the long list of popular disinfectants, chlorine, bromine, iodine, osmic acid, potassium permanganate (Condy's fluid), and corrosive sublimate seem to be the most certain and rapid in their action, but all of these are more or less open to objections. The employment of fumigating pastilles, burning brown paper, camphor, benzoin, mastic, amber, lavender, and other odoriferous substances, is merely serviceable in cloaking over the offensive, fetid, and hurtful gases, and should never be resorted to unless in conjunction with the use of other agents possessing the properties of true disinfectants.
It will be seen from the foregoing that a general, satisfactory disinfectant is still a desideratum, and that even those in most use require favourable conditions, and a more lavish application than is generally forthcoming. See, for Condy's fluid, the article MANGANESSE.