1 06 ORIGIN OF LO WES T OR GA NISMS. 



residual gases may be emitted. When this emission 

 (which is almost always one of the accompaniments 

 of a fermentative change) has taken place to a slight 

 extent, the meats are in the very best condition for 

 preservation. There is an utter absence of light, an 

 absence of free oxygen, and also an absence of that 

 diminished pressure which my experiments seem to 

 show is favourable to the promotion of many kinds of 

 fermentative change. So that if fermentation does 

 not take place in a closed flask which is full of a 

 boiled infusion of hay,* it may be owing to the fact 

 that there is no space for the residual gases, and that 

 undue pressure retards many fermentative changes. 

 This is also perfectly compatible with the other fact 

 that the same kind of fluid will undergo change when 

 a small quantity of it is contained in a comparatively 

 large flask owing to there being, in such a case, 

 plenty of room for residual gases to be effused, before 

 that undue amount of pressure is brought about, in 

 the presence of which such a fluid will no longer 

 ferment or putrefy. Fluids, therefore, whose putre- 



It has, however, been ascertained by M. Pouchet, that beer- 

 yeast, even after prolonged ebullition, will undergo change in a 

 flask which is full and hermetically sealed : and the manufac- 

 turers of preserved meats also find that occasionally, in some of 

 their best prepared tins, the meats become putrid, and that the 

 putridity is accompanied by the presence of organisms. Some 

 fermentations are doubtless attended by a less copious emission 

 of waste gases than that which characterizes other fermenta- 

 tions ; and some fermentations will progress in spite of a mo- 

 derate amount of pressure. 



