388 THE BEGINNINGS OF LIFE. 



drops of strong acetic acid were added, so as to make 

 the whole yield a faintly acid reaction to test paper. 

 The two flasks were then exposed side by side to a 

 temperature of 75 to 80 F during the day. In twenty- 

 four hours the neutral solution was clouded, and 

 more or less opaque, whilst the portion which was 

 acid appeared perfectly unchanged. It was as clear as 

 ever; and so it continued even to the end of forty-eight 

 hours, although by this time the neutral solution was 

 quite opaque and muddy-looking, with a pellicle on its 

 surface and also some flocculent deposit at the bottom 

 of the flask. A microscopical examination of two or 

 three drops of this fluid showed that it was teeming 

 with plastide-particles, and most actively moving Bac- 

 teria and Vttriones $ whilst a similar examination of the 

 acid fluid, showed not a trace of these or of any other 

 kind of organisms 1 . 



The difference between the results in these two sets 

 of cases was thus extremely well marked, and the 

 results themselves are well worth our serious attention. 

 We had to do with equal bulks of fluid, placed under 

 similar conditions and similarly constituted, with the 

 exception that in each set a few drops of acid had been 

 added to the one fluid, whilst the other was allowed to 

 remain neutral. And it must be acknowledged that the 

 difference encountered was very similar in kind to that 

 which was observed by M. Pasteur when he made use 



1 The reverse results, which may be produced by neutralising the 

 acidity of a naturally acid fluid, will be exemplified farther on. 



