FIRST DISCOVERIES. 17 



The tartrates of soda and ammonia presented, as did 

 all the other tartrates, the dissymmetry manifested by 

 the absence of any plane of symmetry ; that is to say, 

 the crystals of this salt placed before a mirror pro- 

 duced an image which was not superposable upon the 

 crystal. It was like a right hand having its left for 

 an image. With regard to the paratartrates of soda 

 and ammonia, one circumstance struck Pasteur in a 

 quite unexpected manner. Far from establishing in 

 the crystals of this salt the absence of all dissymmetry, 

 he found that they all manifestly possessed it. But, 

 strange to say, certain crystals possessed it in one 

 sense and other crystals in a sense opposite. Some 

 of these crystals, when placed before a mirror, produced 

 the image of the others, and one of the two kinds of 

 crystals corresponded rigorously in form with the tar- 

 trate prepared by means of the tartaric acid of the 

 grape. Pasteur continued his reasoning thus : Since 

 there is no difference between the form of the tartrate 

 derived from the tartaric acid of the grape and one of 

 the two kinds of crystals deposited at the moment of 

 crystallisation of the paratartrate, the simple observa- 

 tion of the dissymmetry proper to each will enable me 

 to separate, by hand, all the crystals of the paratartrate 

 which are identical with those of the tartrate. By 

 ordinary chemical processes I ought to be able to ex- 

 tract a tartaric acid identical with that of the grape, 

 possessing all its physical, mineralogical, and chemical 



