494 ON THE INTERDEPENDENCE OF SCIENCE 



Pasteur's acuteness as an observer and his ingenuity in experiment, as well as 

 his almost intuitive perception of truth. 



A series of other beautiful investigations followed, clearly proving that 

 all true fermentations, including putrefaction, are caused by the growth of 

 micro-organisms . 



It was natural that Pasteur should desire to know how the microbes which 

 he showed to be the essential causes of the various fermentations took their 

 origin. It was at that period a prevalent notion, even among many eminent 

 naturalists, that such humble and minute beings originated de novo in decom- 

 posing organic substances ; the doctrine of spontaneous generation, which 

 had been chased successively from various positions which it once occupied 

 among creatures visible to the naked eye, having taken its last refuge where 

 the objects of study were of such minuteness that their habits and history were 

 correspondingly difhcult to trace. Here again, Pasteur at once saw, as if by 

 instinct, on which side the truth lay ; and, perceiving its immense importance, 

 he threw himself with ardour into its demonstration. I may describe briefly 

 one class of experiments which he performed with this object. He charged 

 a series of narrow-necked glass flasks with a decoction of yeast, a liquid peculiarly 

 liable to alteration on exposure to the air. Having boiled the liquid in each 

 flask, to kill any living germs it might contain, he sealed its neck with a blow- 

 pipe during ebullition ; after which, the flask being allowed to cool, the steam 

 within it condensed, leaving a vacuum above the liquid. If, then, the neck 

 of the flask were broken in any locality, the air at that particular place would 

 rush in to fill the vacuum, carrying with it any living microbes that might be 

 floating in it. The neck of the flask having been again sealed, any germs so 

 introduced would in due time manifest their presence by developing in the clear 

 liquid. When any of such a series of flasks were opened and resealed in an 

 inhabited room, or under the trees of a forest, multitudes of minute living forms 

 made their appearance in them ; but if this was done in a cellar long unused, 

 where the suspended organisms, like other dust, might be expected to have all 

 fallen to the ground, the decoction remained perfectly clear and unaltered. The 

 oxygen and other gaseous constituents of the atmosphere were thus shown to be 

 of themselves incapable of inducing any organic development in yeast-water. 



Such is a sample of the many well-devised experiments by which he carried 

 to most minds the conviction that, as he expressed it, la generation spontane'e 

 est une chimere, and that the humblest and minutest living organisms can only 

 originate by parentage from beings like themselves. 



Pasteur pointed out the enormous importance of these humble organisms 

 in the economy of nature. It is by their agency that the dead bodies of plants 



