378 LOUIS PASTEUR 



We may here add a few words on the non-transformation 

 of yeast into penicillium glaucum. 



If at any time during fermentation we pour off the fer- 

 menting liquid, the deposit of yeast remaining in the ves- 

 sel may continue there, in contact with air, without our 

 ever being able to discover the least formation of 

 penicillium glaucum in it. We may keep a current of pure 

 air constantly passing through the flask; the experiment will 

 give the same result. Nevertheless, this is a medium 

 peculiarly adapted to the development of this mould, in- 

 asmuch as if we were to introduce merely a few spores of 

 penicillium an abundant vegetation of that growth will 

 afterwards appear on the deposit. The descriptions of 

 Messrs. Turpin, Hoffmann, and Trecul have, therefore, 

 been based on one of these illusions which we meet with so 

 frequently in microscopical observations. 



When we laid these facts before the Academy, 1 * M. 

 Trecul professed his inability to comprehend them : u " Ac- 

 cording to M. Pasteur," he said, " the yeast of beer is an~ 

 aerobian, that is to say, it lives in a liquid deprived of free 

 oxygen; and to become mycoderma or penicillium it is above 

 all things necessary that it should be placed in air, since, 

 without this, as the name signifies, an aerobian being can- 

 not exist. To bring about the transformation of the yeast 

 of beer ii*to mycoderma cerevisiae or into penicillium 

 ghiucum we must accept the conditions under which these 

 two forms are obtained. If M. Pasteur will persist in keep- 

 ing his yeast in media which are incompatible with the de- 

 sired modification, it is clear that the results which he ob- 

 tains must always be negative." 



Contrary to this perfectly gratuitous assertion of M. 

 TrecuFs we do not keep our yeast in media which are cal- 

 culated to prevent its transformation into penicillium. As 

 we have just seen, the principal aim and object of our ex- 

 periment was to bring this minute plant into contact with 

 air, and under conditions that would allow the penicillium 

 to develop with perfect freedom. We conducted our ex- 

 periments exactly as Turpin and Hoffmann conducted theirs, 



10 PASTEUR, Comptes rendus de I'Academie, vol. Ixxviii., pp. 213-216. 

 "TRECUL, Comptes rendus de I' Academic, vol. Ixxviii., pp. 217, 218. 



