By the once general use of the term zymotic, there is suggested a line of thought which has been quite prevalent since the prominence of Liebig's teachings in chemical physiology, until recently. That great chemist did not imagine that a true zymosis or fermentation occurs under the action of a virus upon the human economy. His thought was more clearly expressed, in the phraseology of the late Dr. Snow of London, as the theory of continuous molecular change. Its most striking physical instance or analogue is the extension of flame from a burning body to combustible matter within its reach. Sugar formation from starch by diastase, and the change of albumen into peptone by pepsin, are familiar examples, in organic materials, of the propagation of molecular movement in special directions and with characteristic results.[8] It does not seem to be more than a short step from these to the processes which we study in fermentation, putrefaction, septicæmia, and the multiplication of small-pox contagion, from the smallest inoculation, in the human body.[9] [Footnote 8: In anticipation of the argument concerning the necessity of the action of minute living organisms to produce fermentation, putrefaction, and specific diseases, emphasis may be here laid upon the fact that the above named changes, and many others like them, are produced, in the absence of such organisms, by chemical agents formed in the body, or even (as when sulphuric acid changes starch to sugar) by inorganic substances. Pasteur considers that the yeast-cell secretes a sort of diastase which changes starch or cane-sugar into glucose, on which the cell then lives, decomposing the glucose into alcohol, carbonic acid, etc. Koch and others now assert that a bacillus produces the souring of milk, and another the butyric acid fermentation.] [Footnote 9: The assertion of some advocates of the "germ theory of disease," that only living organisms reproduce their kind, loses weight as an argument in view of the natural history of small-pox and analogous diseases; unless it be proved that every particle of contagious matter is (at one time at least) a living organism.] But here comes in a new hypothetical factor, introduced by the aid of {138} the microscope, although anticipated conjecturally before actual discoveries in this field were made certain. So prominent is this subject in the discussions of the present time, under the expression "the germ theory of disease," that we are justified in giving attention to it here somewhat at length.