118 PASTEUR: THE HISTORY OF A MIND 



to the experiment entire security, and consequently all 

 its significance. The practice of heating to 120 C. all 

 liquids which are to be sterilized dates from this time. 

 It was the advent of the autoclave into the laboratory. 



Air is often another important factor in the revivi- 

 fication of germs, and here it is that we find again the 

 experiment, cited above, of Pouchet, Joly, and Musset. 

 They worked, as I have said, with a hay infusion, ob- 

 tained by macerating hay in tepid or hot water, which 

 was then filtered and boiled. But this hay contains 

 ordinarily, as Cohn has since shown, an elongated ba- 

 cillus forming a pellicle on the surface of the infusion 

 when it develops in it, and changing into very resistant 

 spores. It is the famous Bacillus subtilis which is every- 

 where widespread, and owes its ubiquity solely to the 

 fact that it is admirably equipped for the strife, being 

 one of the most resistant of known organisms. Its 

 spores, particularly, can withstand several hours of 

 boiling without being killed, but they are the more 

 difficult to rejuvenate the more maltreated they are. 

 If we seal, in a flame, the neck of a flask which contains 

 them, at the time, when the liquid in which they are 

 submerged is boiling, they are not killed, but they do 

 not develop in the liquid when it has been cooled and put 

 in a thermostat, because air is lacking. If we allow 

 air to enter, the infusion becomes populated and this 

 is also the case if we allow only heated air to enter; for 

 the air does not act by introducing germs, as Pasteur 

 believed at the time of the debate before the Academic 

 commission on spontaneous generations: it is its oxygen 

 alone which comes into play. 



We see here how necessary ingenuity and discernment 

 are in these matters. Here we have an experiment in 

 which air coming into contact with an infusion brings 

 to it fertility. This was performed by Gay-Lussac with 



