The more common opinion however is, that joined to the predisposing causes there must always be present a primary cause, having a distinct existence, capable of travelling from one part of the globe to another; capable of spreading over any space however extended, or of confining itself to any space however small—a district, a street, a house, a room. Footnote 10: See Baltimore case, p. 78. It is urged that though we are unacquainted with the physical form or chemical properties of this body, this is no reason why we should not understand its force as a special agent in the production of disease, just as we know the forces of other physical bodies, though not their nature. The existence of such a body being assumed, it is conceived that it exists not in a gaseous but in a liquid state. It is supposed that it cannot exist in a gaseous state because a gas is readily diffused and dissipated; because when organic matter is reduced to a gaseous state, it has passed from the organic into the inorganic kingdom, and there is no evidence that the elementary bodies belonging to this kingdom are capable of producing any form of fever; and because there is indubitable evidence that organic matter in a recent state of putrescence—the more recent the more potent—is capable of producing the most deadly forms of fever. From these considerations it is conjectured that the primary cause, whatever it be, is some subtle fluid which has not wholly lost its organic composition, and that it consists of particles of extreme minuteness, capable of attaching itself to the surfaces of other bodies, and even of increasing under favourable circumstances. It is further thought that this body is not equally diffused through the atmosphere, but is only partially distributed, and that this accounts for the local distribution of epidemics, and for their occasional absence from places which apparently present all the conditions favourable to their development. Lastly, the opinion is gaining ground, that this body acts in the manner of a ferment. It is urged in favour of this view, that a ferment being an azotized substance in a state of putrefactive alteration, the body in question must find, in the decomposing organic compounds with which impure blood is charged, precisely the materials for taking on the fermenting process. The advocates for this view think that the term “_zymotic_” is not only the appropriate name of the whole of this class of diseases, but that it also declares an interesting fact connected with them. Whatever may be the truth with respect to these points, on which at present we have no positive knowledge, one thing is certain, that practically our concern is with the known causes,—the ascertained conditions. These are palpable, definite, and capable of complete removal and prevention.