We now, then, turn to the remarkable experiments and discoveries of M. Pasteur, which have gained for him a world-wide reputation, and the bearing of which on the science of preventive medicine is commanding the attention and admiration of the whole scientific world, and indeed we may say of mankind at large. M. Pasteur is not a medical man, nor, indeed, a physiologist. He is simply a French chemist, a modest, retiring labourer in the field of science whose sole object has been the discovery of truth, and whose chemico-physical researches gained for him the Rumford Medal of the Royal Society in 1856. Having devoted himself specially to the chemistry of organic substances, he was naturally attracted by the discovery of Cagniard de la Tour, that yeast is really a plant, a species of fungus, whose vegetative action in fermentable liquids is the true cause of their fermentation. This was so opposed to the theories of all the chemists of the day, among whom may particularly be mentioned the celebrated Liebig, that it met with their warm opposition. When, however, Helmholtz and others succeeded in showing that by preventing the passage of the minute organisms constituting the yeast plant into fermentable liquids, no fermentation took place, the doctrine soon became established that the first step in the process of alcoholic fermentation is due, not to ordinary chemical changes, but to the presence of living organisms. In like manner the putrefaction and decomposition of various liquids containing organic matter was found to be due, not to the simple action of the oxygen of the atmosphere, but to the introduction from without of microscopic germs which found material for their development in such liquids. So that if by mechanical filtration of the air the entrance of such germs can be prevented, or if by heat or other means they can be destroyed, any fluid, however readily it may undergo putrefaction in ordinary circumstances, will remain perfectly sweet, though freely exposed to the air. And the same fluid will undergo a different kind of fermentation according as it is subjected to the action of different species of germs. These and other facts of scarcely less importance, which cannot here be detailed, induced Pasteur to test the application of the doctrines deduced from them to the study of disease in living animals.