YoTJNG — Pasteur Centenary Celebration. 61 



been observed, and Pasteur concluded that they must be those of the salt of 

 a hitherto unknown acid. If, then, ordinary tartaric acid turns the plane of 

 polarization in one direction — it is now said to be dextro-rotatory — it might 

 be expected that the new acid would turn the plane in the opposite 

 direction. 



In order to test the correctness of this conclusion, Pasteur, with extra- 

 ordinary care and perseverance, picked out crystals of each kind from the 

 mixture, keeping them separate. He then made aqueous solutions of each 

 kind of crystal, and was delighted to find that the two solutions did. indeed, 

 turn the plane of polarization in different directions. Pasteur had, in fact, 

 proved thatracemic acid is a mixture of equal quantities of dextro- andlaevo- 

 tartaric acid, the one tending to turn the plane to the right, the other to the 

 left to an equal extent, with the result that racemie acid does not affect 

 polarized lioht. 



Pasteur's discovery was received with scepticism by the Academy of 

 Sciences until the observations had been verified, step by step, by Biot, who, 

 doubtful at first, finally congratulated Pasteur with effusion on his brilliant 

 discovery. 



Pasteur arrived at the conclusion that the differences in optical properties 

 and in crystalline form were, in all probability, due to a different arrange- 

 ment of the atoms in the molecule, the arrangement being dissymmetric in 

 the same way that the crystals are dissymmetric. He thus laid the 

 foundation for the great branch of the science afterwards named stereo- 

 chemistry, and for the notion of the asymmetric carbon atom, and of the 

 arrangement of the atoms in space. 



Pasteur continued his researches while professor of chemistry at 

 Strasbourg, and he there succeeded in converting ordinary tartaric acid, on 

 the one hand, into inactive racemie acid, and, on the other hand, into a new 

 inactive acid which he called mesotartaric acid. He showed that this acid, 

 unlike racemie acid, cannot be resolved into isomeric active acids. 



The method of resolving racemie acid into dextro- and laevo-tartaric acid 

 first adopted by Pasteur, was extremely laborious ; but he found that if, 

 instead of preparing the sodium-ammonium salt, he neutralized the acid with 

 an optically active base, the separation of the optical isomers was greatly 

 facilitated, because their properties — solubility and so on — dilfer far more 

 widely than those of salts formed with inactive bases. 



Some little time afterwards, Pasteur happened to hear of an observation 

 by a German firm of chemical manufacturers that some solutions of 

 commercial calcium tartrate containing organic matter had undergone 

 fermentation in warm weather with loss of material. Thinking the 



