176 Biological Chemistry. 



of the enantiomorphs is desired, must take place at a low 

 temperature. If the crystals are allowed to separate above 

 267 (the "transition temperature"), the two kinds of 

 crystals do not form ; the sodium ammonium salt of the 

 racemic acid separates instead, which contains no water 

 of crystallization. 



B. Resolution by Biological Methods. These are also 

 due to Pasteur. Generally speaking, living organisms 

 exert a selective action on enantiomorphs. Pasteur showed, 

 for example, that when the green mould penicillium 

 glaucum was allowed to grow in a solution of a racemic 

 acid containing a little potassium phosphate, it gradually 

 became Isevo-rotatory ; the mould used only the dextro- 

 rotatory form as food, and left the laevo variety for 

 the most part untouched. Innumerable instances of 

 selective action of this character are known, and for this 

 reason alone the study of stereoisomerism is one of great 

 biological importance, for in choosing a substance as a 

 foodstuff, due regard must be paid to its stereochemical 

 configuration. This remark applies not only to the lower 

 organisms, but even to the higher mammalia. There is, 

 furthermore, a marked difference in the pharmacological 

 and therapeutic action of the various stereoisomerides, as 

 might be expected from the differences in their food- 

 values. 



C. Resolution by Active Substances. This is a third 

 method due to Pasteur. If an acid, such as racemic acid, 

 is neutralized by an optically active base, two salts can 

 be obtained. Suppose the base laevo-rotatory, like cin- 

 chonine. The two salts formed would be the cin- 

 chonine Z-tartrate and the cinchonine cZ-tartrate. These 

 two salts are not enantiomorphs, because two acids of 

 opposite rotations are united to the same optically active 

 base; their rotations, therefore, are not equal but in 



