As here used, this term is intended to apply to substances which prevent, destroy, or render inoperative all microscopic living things which are hostile to human health. There are two distinct sets of these existences; one, the individuals of which make their home in some special structure of the body, generally on the surface, there propagating, and acting as causes of diseases more or less chronic, such as the itch insect and the fungus of scald-head; the other, operating by setting on foot series of changes in the matter around them, called fermentations, which either generate without the system peculiar morbific influences, or taking place in the system, produce specific diseases by a direct action upon it. Now it is obvious, from the origin of the term antizymotic (earn, against, and Cu/i>j, ferment), that strictly speaking it is applicable only to the latter of these two categories; but, as the particular articles of the class have an equal influence on the former, and are indeed the very means employed for their destruction, and the cure of the diseases they produce, there is no good practical reason for making distinct classes of them; the learner being guarded against error by the preliminary explanation here given. in fact, it was from their known efficiency in the treatment of the invisible cutaneous parasites, that the particular members of the class suggested themselves as proper remedies against zymotic causes of disease. This is especially the case with sulphurous acid and the sulphites, which were first employed, at the suggestion of Prof. Graham, of London, for the destruction of the human parasites, from their known deleterious influence on the lower forms of organization out of the body. The subject, however, of fermentation itself requires a brief development, before we can enter understandingly on the consideration of the influences opposed to it.

Chemists have now for many years been familiar with the fact, that there are various substances which, when in contact with certain other bodies, have the power, without undergoing themselves any appreciable change, of setting on foot in the latter chemical reactions, which result in their decomposition, and the formation of altogether new products. Thus, to cite an illustrative fact, already well known to the reader, the emulsin of bitter almonds, in contact with the amygdalin of the same fruit and water superadded, causes a reaction between these two substances, resulting in the generation of the oil of bitter almonds containing hydrocyanic acid, while the emulsin, so far as is known, remains unaltered. There are many other examples of a similar character. The agent here, in the absence of any real explanation, is said to operate by its presence; and the process is designated as the action of presence. The term catalysis, of Greek origin, signifying from its etymology (xazakuffiff) dissolution or destruction, has also been applied to the process, and the action is denominated catalytic; but it is obvious that this explains nothing. Now fermentation was considered as belonging to the same category; the ferment acting by its presence, or catalytically, in producing change in other bodies; as in the example of the common vinous fermentation, in which yeast, introduced into a mixture of sugar and water, at a certain temperature, gives rise to a series of changes in the sugar, resulting in the production of alcohol and carbonic acid; the yeast, as was supposed, remaining unchanged, or at least not being concerned in the changes referred to except as their cause. Liebig attempted an explanation of the phenomena by considering the yeast or ferment as a nitrogenous body in the course of decomposition, and the change in the sugar as being produced merely by a sort of sympathy with that going on in the ferment; the action of decomposition in one body giving rise to a similar action of decomposition in another, by simple contact. But this is in fact no explanation, and expresses little more than the influence of presence. At length, however, it was found that in vinous fermentation there is always present an organized living being, a microscopic fungus, which increases with the continuance of the process, and may fairly be deemed an essential accompaniment. it is now genei*ally admitted that the yeast plant is the real agent of all the changes; that, for the necessities of its growth and propagation, it decomposes the sugar, and appropriating a portion, leaves the residue to assume other forms. But the alcoholic fermentation is not the only one. Numerous others are known, each producing a special result; and the researches of Pasteur leave little room for doubt, that each of these fermentations is attended with a peculiar organized living microscopic being, vegetable or animal, upon which in fact it depends, and for the support and propagation of which it seems to be specially designed. Even the changes which take place in putrefaction have been placed in the same category of processes; and this, like all other fermentations, is thought to depend on a living cause. The degradation of pus, either in the body or out of it, probably belongs to the same set of actions, and, like all other fermentations, is a vital process. Now whether we consider this living cause as a separate organized being derived from a distinct germ, and producing the germs of a similar growth elsewhere, or whether we adopt the hypothesis of Dr. Lionell Beale that it is merely a portion of the living matter of the body, retaining life after leaving it, and undergoing a new development under different circumstances, the result for our purposes is the same. Advancing a step further, we can easily conceive that pus thus changed, if it re-enter the system, and its living portion find its way into the blood, may produce there a similar series of changes, resulting in its own increase, and various morbid phenomena consequent upon this altered state of the blood. Hence the disease named pyaemia, or more properly purulent infection of the blood; for it is not the mere presence of pus in the blood that causes the phenomena, but the fermentative process with its vital cause. As putrefying animal substances, whether liquid or solid, undergo in their putrefaction a species of fermentation depending on a peculiar living agent, the entrance of this agent into the blood may give rise to a similar fermentation with a multiplication of itself; and thus we have another series of morbid phenomena, which have been denominated septicaemia (putridity of the blood), and which consists essentially of a typhoid state of system, with a strong tendency to obstinate gastro-enteritis, and not unfrequently extensive ulceration in the bowels. Proceeding still further, it is easy to conceive that all strictly contagious diseases, and all the febrile affections, whether contagious or not, depending on special aerial contaminations, may also have as their cause each one a peculiar living microscopic being, which, generated either within the system, or through a fermentation of its own, is capable of producing, when absorbed, a peculiar disease, and only that one. Hence small-pox, scarlatina, diphtheria, typhus, erysipelas, plague, etc., and even cholera may be ranked in the same association. in each of these cases, and all others of a similar kind, the living cause, having found its way into the blood, sets in action a series of changes in the constituents of that fluid, attended with characteristic morbid phenomena, and, in the contagious diseases, with a propagation of the cause, resulting either in the death of the patient, or in recovery after a more or less protracted struggle, with a discharge from the system of the offending cause, and all its morbid products. Admitting all this, we but advance one step by supposing, that certain substances are noxious to these microscopic beings, and, without chemically acting on them, are nevertheless capable of even destroying them while engaged in the process of fermentation, whether without or within the body. These are true zymotics. Hitherto what has been said is, to a considerable extent, theoretical. But it is not without the support of experiment and observation.