268 HISTORY OF CHEMISTRY. [LECTURE XIII. 



the case of the former the salt of left tartaric acid crystallised 

 first ; in that of the latter, the salt of right tartaric acid 

 crystallised first. 



3. By treating a solution of acid ammonium racemate with 

 spores of Penicillium Glaucum, by which means the salt of left 

 tartaric acid alone remains in the solution after the develop- 

 ment of the fungus. 



Pasteur prepared inactive tartaric acid by heating cinchonine 

 tartrate ; and Dessaignes was able to show that this acid was 

 partially reconverted into racemic acid by heating it to 2oo. T1 



Facts of a similar kind have been observed in the cases of 

 various other substances, such, for example, as the glucoses, 

 the terpenes, amyl alcohol, aspartic acid, etc. Exactly the 

 same relations that are observed in the case of tartaric acid 

 occur also in that of mandelic acid, according to experiments 

 by Lewkowitsch. 72 



In all these cases of isomerism it is in their physical 

 properties that the corresponding substances differ from each 

 other ; and on this account Carius 73 introduced for such cases 

 the designation physical isomerism. 



In 1874 Le Bel 74 and, shortly after him, van 't Hoff 75 en- 

 deavoured to explain 'these facts also upon the theory of 

 atomicity, having first adopted the view that a substance only 

 possesses optical activity when its molecule contains an 

 asymmetric carbon atom ; that is, when a carbon atom is 

 present in it which has its four valencies satisfied by union with 

 four atoms or groups all different from one another. This view 

 is warranted by the facts, in so far that all optically active 

 substances known up to the present contain at least one 

 asymmetric carbon atom. It must be stated, however, that by 

 no means all substances which contain asymmetric carbon 

 atoms possess rotating power, and that the view stated above 

 cannot, therefore, be turned about and generalised as if this 



71 Annalen. 136, 212. 7 - Berichte. 16, 1565 and 2721. 73 Annalen. 

 126, 214 ; 133, 130. 74 Bull. Soc. Chim. [2] 22, 337. 75 Ibid. 



[2] 23, 295 ; compare also La chimie dans 1'espace, Rotterdam, 1875 ; 

 Chemistry in Space, translated and edited by Marsh. Oxford 1891. 



