But supposing that the generation of alkaloids could take place in the body, or some analogous poisonous matter, we have yet to imagine a whole host of peculiar and essential conditions to effect this change, besides an atmospheric agent or agents to set in motion those compositions and decompositions, capable of bringing out these new products from the elements of blood. We are aware that in the blood, carbon and nitrogen are sufficiently abundant as well as saline compounds, to generate cyanides, and, with hydrogen also there in plenty, hydrocyanates, and thus from them many other poisonous products, but how is all this to be effected? And even if effected, it is yet a question if such compounds can in any way simulate the attacks of epidemic disease. We have {126} already shewn that the amount of most poisons necessary to destroy an individual, can be pretty clearly estimated, and their _modus operandi_ is tolerably well understood. Again, the most essential part, in which all chemical theory fails, is an explanation of the reproduction of contagious matter. The catalytic process, by which decompositions are said to be effected, and in which Liebig includes the various fermentations, is one of those chemical relations of matter to matter, considered by some as the probable cause of infection. Mr. Simon, in a late lecture, has said, "I consider the phenomena of infective diseases, to be essentially chemical, and I look to chemistry to enlighten the darkness of their pathology. Qualitative modifications, affecting the molecules of matter as to their modes of action and reaction, are such as form the subject of chemical science; and those humoral changes which arise as the result of infection clearly fall within the terms of its definitions." Further on he adds: "The phenomena of infected diseases appears then, in many respects, to be sui generis. Certainly they are chemical. _Probably_ they belong to that _class_ of chemical actions called _catalytic_."[52] {127} It is not improbable that something resembling a catalytic action may take place in the blood in those diseases of endemic and epidemic origin, but that it can be by a chemical process alone is contrary to all experience of catalytic operations, for except in the instance of fermentation proper, there is no multiplication of the fermentative matter. The action of the matter of contagion seems to stand on the confines between electro-chemical and bio-chemical manifestations, and so long as no chemical explanation can be given for the multiplication of the matter of infection, the most rational course to adopt is to assume that life under some unknown form is, as we every where find it, the sole reproductive agent. * * * * * {128} SECTION II. THE ANIMALCULAR THEORY OF EPIDEMICS UNTENABLE.