CHEMISTRY 13 



Two forms of tartaric acid were known at this 

 time. One was the ordinary commercial acid : the 

 other was a very rare form, which had been found, 

 by chance, in 1820, by Kestner, in the course of 

 making the ordinary acid. Gay Lussac and 

 Berzelius had seen and studied this rare form, and 

 had named it paratartaric, or racemic, acid. Then, 

 in 1844, came Mitscherlich's discovery, that the 

 ordinary commercial acid rotated the plane of 

 polarisation to the right : but racemic acid did 

 not rotate it either to the right or to the left. 



Thus, the problem which Pasteur had to answer 

 may be stated as follows. " We know that a 

 crystalline substance may be disymmetric : that it 

 may have two forms of crystals, the one right- 

 handed, the other left-handed. But here is a 

 crystalline substance, having two forms of crystals : 

 and the one of them is right-handed, but the other 

 is neither right-handed nor left-handed. What is 

 the meaning of that ?" By careful microscopic study 

 of the crystals of the commercial acid, he found on 

 them a minute facet, not described either by 

 Mitscherlich or by de la Provostaye : and this facet 

 was so placed that he was able to feel sure that 

 these crystals were disymmetric, one of a pair 

 of forms : and that there must be, somewhere in 

 Nature, the other form, the unknown acid, with a 

 facet so placed that the two forms together would 



