The views hitherto expressed with reference to the causes of fermentation and putrefaction, and to the interpretations which M. Pasteur’s experiments are capable of receiving, seem to derive all the additional support that can be needed, from the results of my own experiments with boiled fluids, in sealed flasks, from which all air had been expelled. Some of the same fluid being taken and divided into three parts, each portion is placed in a separate flask, in which it is boiled for a period of ten minutes. One of the flasks (A) is provided with a long and bent neck, so that the air which re-enters is deprived of its germs and organic particles; another (B) has only a short neck, and to this, the access of germs and organic particles is freely permitted till the fluid has become cool, and then the neck of the flask is hermetically sealed; whilst the last (C) is sealed during ebullition, after all air has been expelled. Now, if Pasteur’s theory of fermentation, and the prevalent notions concerning the universal distribution of “germs” throughout the atmosphere were true, it might be expected that the fluid in B would always rapidly change; that that in A would always remain pure; and that the fluid in C would, similarly, undergo no alteration. The facts, however, are quite the reverse: if a strong turnip infusion be employed, the fluid in A will almost always remain unchanged; that in B will sometimes rapidly change, and at other times will remain quite pure; whilst that in C will almost invariably become turbid in from two to six days. But, even if it were not the case that some fluids, different from those used by M. Pasteur, will almost invariably undergo change in bent-neck vessels, M. Pasteur’s explanation of the cause of the preservation would have been altogether upset by the fact that _some of the very fluids which remain pure in the bent-neck apparatus will become fœtid if shut up in vacuo_. It is, therefore, of course useless to talk of a particular boiled fluid having been saved from putrefaction on the ground that the living atmospheric germs (whose presence is supposed to be necessary for the initiation of such a process) have been altogether filtered from the re-entering air, when some of the same fluid will putrefy, if placed under different conditions by which it is freed both from the influence of the atmosphere and from its germs--_i.e._, when, instead of filtering the re-entering air, no air is permitted to enter. Germs and atmospheric particles being equally got rid of in both sets of cases, the great difference between them is that the weight of the atmosphere is also got rid of in my experiments--the fluids being contained _in vacuo_. Now it has been ascertained by Mr. Sorby, that pressure undoubtedly influences “chemical changes taking place slowly, and therefore, probably due to weak or nearly counterbalanced affinities;” and he also states, in the Bakerian Lecture for 1863,[84] that “a