CHAPTER IV. 



STEREOISOMEBISM. 



MENTION has been already made of the fact that certain 

 substances, such as amyl alcohol (p. 76), lactic (p. 116) 

 and tartaric acids, exist in the form of isomerides, the 

 relationship of which one to another cannot be explained 

 in a satisfactory manner by the ordinary structural 

 formulae. Certain of these isomerides are also optically 

 active. It is proposed to devote the present chapter to 

 the discussion of isomerides of this character and to the 

 question of optical activity, both of which subjects are 

 matters of considerable biological importance. 



Lactic acid was discovered by Scheele in 1780 in sour 

 milk, and a similar acid was isolated in 1807 by Berzelius 

 from extract of muscular tissue. The acids were con- 

 sidered to be identical by Liebig, who reinvestigated them 

 in 1847; but in the following year Engelhardt pointed out 

 that there was a difference in the calcium and the zinc 

 salts of the acids derived from the two sources. The acid 

 from muscular tissue was designated " sarcolactic " acid by 

 Strecker. Between 1863 and 1873 the acids were sub- 

 jected to a detailed investigation by Wislicenus, who 

 succeeded in synthesizing a third lactic acid in the follow- 

 ing way. Ethylene chlorhydrin was obtained by treating 

 ethylene with hypochlorous acid 



CH 3 CH.OH 



|| + HC10 = | 

 CH, CH 2 C1 



