46 PASTEUR: THE HISTORY OF A MIND 



species of Penidllium. Since then Pfeffer has found a 

 bacterium which acts like the species studied by Pasteur. 

 On the contrary, a bacterium that developed spontane- 

 ously in the laboratory, in a solution of left-handed tar- 

 trate of soda and ammonia, consumes by preference 

 the left tartrate from a solution of paratartrate, although 

 it is quite able to attack the right also. Other living 

 species consume indifferently the two salts, and all 

 cases are possible. But we are none the less in possession 

 of this fact: that the nutritive character of a tartrate 

 may bear a relation to its molecular dissymmetry. 



This fact merits special attention. The right- and 

 left-handed tartrates of ammonia are composed of ex- 

 actly the same elements, carbon, hydrogen, oxygen, 

 nitrogen, in the same quantity. The only difference is in 

 the arrangements of the atoms. Still this difference is 

 not great since the two arrangements are the image of 

 each other in the mirror, and nevertheless it is great 

 enough so that a living organism can respect one of the 

 two salts while it entirely destroys the other. 



To understand this fact it is evidently necessary to 

 relate it to what we know on the subject of the difference 

 in chemical properties brought about by combination 

 of tartaric acid with an active substance. In the pres- 

 ence of potash and soda the right and left tartaric 

 acids behave exactly alike and have the same stability. 

 This is no longer the case when they unite with sub- 

 stances having rotary power. Now it is just these which 

 they encounter in the living tissues where there are 



