Even this however,--as the preceding experiments fully show--cannot be truly affirmed to be a general rule. Some infusions do undergo change, notwithstanding this treatment and deprivation, whilst others do not: that is to say, some still preserve a first degree of fermentability even after boiling, whilst others are reduced by this process to the second degree of fermentability. These latter are unable to initiate changes by virtue of their own inherent instability; molecular re-arrangements require to be set on foot in them by contact with some more unstable substance, which is itself undergoing change. That such is the correct explanation of the reason why _some_ fluids do not ferment in bent-neck flasks, seems obvious from the discordant results obtained in many other experiments, after the free admission of uncalcined air to the fluids which had been boiled. The fluids were deprived of their virtues in some cases by the heat to which they had been subjected, so that whether they underwent change or not, may have depended upon the accidental presence or absence, in the air which was subsequently admitted to the fluid, of some unheated organic fragments, capable of initiating fermentative changes. If germs were as omnipresent as they have been represented to be, such fluids ought always to have undergone fermentation. Whilst I have found that any given fluid, whose strength is about equal on different occasions, acts in a definite manner when the flask is hermetically sealed after expulsion of all its air and during the continuance of ebullition; and, whilst a like definite result can generally be obtained, when calcined air is admitted to the boiled fluid before the vessel is hermetically sealed; it is found, on the other hand, that the result is in no way predicable when uncalcined air is admitted. Sometimes fermentation takes place, and sometimes in other flasks--sealed at the same time, and subsequently placed under the same conditions--no change whatever occurs. My own experience in this respect accords perfectly with that of M. Pasteur.[74] He, however, at once came to the conclusion that the only inference from such facts was that “germs” are not so universally distributed as they had been supposed to be by Bonnet and Spallanzani.[75] The unprejudiced inquirer, however, will perceive that M. Pasteur was entitled to come to no such conclusion concerning germs which was not equally applicable to minute fragments or débris of organic matter floating in the air. And, similarly, the evidence which he adduces with regard to the diminution in the number of the fertile flasks when they were filled with some of the still air of the caves of the observatory, or else with some from the peaks of the Jura,[76] far away from the haunts of men, had no bearing upon the distribution of germs which was not equally applicable to that of dead organic particles. Such evidence, therefore, was valueless for settling between the rival