The idea that diseased actions may possibly be accounted for by supposing the existence of special _fermentations_ in the blood, is by no means a new one. Inscrutable as these diseased actions themselves may be, yet, we are enabled to recognise processes of a nature analogous to fermentation as going on in that fluid in health. Of such a kind probably are, the change of albumen into fibrine; the changes which take place in the starchy matters of the food before they can be oxidized into carbonic acid; and the changes that must occur in nitrogenous substances before absorption, as well as those that accompany afterwards the downward progress of the same materials, from living tissue into Urea and Uric acid, to be finally excreted from the system. It is a curious fact that nearly every known product of organic fermentation has been discovered in the human body in health or in disease. Lactic, butyric, and acetic acids, have been frequently found there. Dr. Heintz has lately added to these succinic acid, discovered in a hydatid cyst of the liver. The production of many disorders by the access of a known morbid material, the working of that material on the particles of the blood after a special fashion peculiar to itself alone, and the gradual elimination of certain products, also peculiar to this one operation, are circumstances in which diseases bear an obvious analogy to processes of fermentation. The same remark applies to the working of Catalytic medicines. It is then just possible that one of these medicines might produce in the blood a fermentation which should meet and neutralize the morbid fermentation; or that it might simply determine the latter in a different direction, and thus bring it to a desirable end. Acting in health so as to produce a morbid change, it might operate in disease by means of diverting into a right direction a change that is already going on in a wrong one. But let us leave generalities, and descend more into particulars. Are we in a position to be able to indicate the actual nature of the changes which we thus assume to be probable? The term Fermentation is used to express a change or series of changes of a special character, caused among the particles of a compound body, by the presence of a certain other body called a Ferment. There are two kinds of fermentation. In one the ferment itself is undergoing change, and impresses a similar change upon a substance which is analogous in nature to itself.[39] The process caused by a contagious disorder would probably be of this kind. Just as the changing yeast forms and increases itself out of the fermenting dough, so does the virus of such a disease renew and propagate itself out of the particles of the blood.