THE BEGINNINGS OF LIFE. 



419 



this liberation or 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 absence 

 of free oxygen, an utter absence of light, and also 

 an absence of that diminished pressure which my ex- 

 periments seem to show 1 is favourable to the pro- 

 motion of many kinds of fermentative change. It 

 would seem that fluids whose fermentation or putre- 

 faction is hindered by increased pressure, and favoured 

 by diminution of pressure, may be placed under con- 

 ditions which are successively more favourable than 

 the last for the occurrence of such changes, by putting 

 a gradually smaller and smaller quantity of fluid into 

 a flask, to which calcined air is admitted 2 . Whilst, 

 if the stimulus of free oxygen is not absolutely needed 

 in order to incite fermentation in the fluid employed, 

 the conditions may often be still further improved by 

 only half filling the flask, and procuring a more and 

 more perfect vacuum before it is hermetically sealed. 



If any one wishes, therefore, to understand why I 

 have been enabled to bring about putrefaction and to 

 obtain living organisms in my flasks, whilst preserved 

 meats do not usually change in vacuo, let him repeat 



' conditions ' to which the fermentable substances are exposed), as to 

 cause no appreciable detriment to the provisions. In other rare cases, 

 the change does proceed, and the contents of the tin become more 

 or less putrid. 



* See Appendix C, Exps. ix. and xv., Exps. xxxiii. and xxxvi., etc. 



8 See p. 348. 



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