LECTURE xin.] HISTORY OF CHEMISTRY. 267 



of the methyl anilines into homologues of aniline 6S and 

 Demole's examination of the spontaneous oxidation of ethylene 

 derivatives, 69 may be mentioned here. 



Those phenomena of isomerism, however, which cannot 

 be expressed or represented by means of the ordinary formulae, 

 are of still greater importance. Examples of this nature have 

 long been known, and some of them were minutely studied at 

 an early period. More recently (after the recognition of the 

 importance of the matter) an attempt has been made to 

 introduce a special method of explaining them, which is, no 

 doubt, connected with the theory of valency, but is also a 

 further development and an extension of that theory. We 

 must here enter upon a more particular account of this matter. 



The discovery of racemic acid, isomeric with tartaric acid, 

 has already been mentioned in Lecture VII. (p. 118). The 

 recognition of the relations between these two acids forms the 

 subject of an investigation by Pasteur which is of fundamental 

 importance for the subject now to be considered. 70 Pasteur 

 showed that there are four isomeric tartaric acids, viz. : 

 racemic acid, inactive tartaric acid, and right and left rotating 

 tartaric acids. He showed, moreover, that the two latter acids 

 crystallise in similar, but in oppositely built-up (enantiomorph) 

 forms j that they both deviate a ray of polarised light through 

 equal angles, but in opposite senses ; and that when mixed in 

 equal quantities they yield optically inactive racemic acid. 

 Further, he succeeded in decomposing racemic acid again into 

 the two optically active tartaric acids, by three different 

 methods : 



1. By preparing and crystallising the sodium-ammonium 

 salt ; when two enantiomorph varieties were obtained. These, 

 after having been separated and decomposed, yielded the two 

 tartaric acids. 



2. By preparing the cinchonicine and quinicine salts. In 



68 Berichte. 4, 742 ; 5, 704, etc. 69 Ibid. II, 315, 1302 and 1307. 

 70 Ann. Chim. [3] 24, 442 ; 28, 56 ; 38, 437 ; compare also Pasteur, 

 Recherches sur la dissymetrie moleculaire des produits organiques natu- 

 rels. Le9ons de chimie, Paris 1861 ; Alembic Club Reprints, No. 14. 



