CHAPTEE IV 



SPACE-ISOMERISM AND THE CHEMISTRY OP 

 THE SUGARS 



Towards the end of the preceding chapter reference waa 

 made to certain varieties of lactic acid which could not be 

 distinguished by their chemical reactions and yet whose 

 physical properties were not identical. It is found, e.g., 

 that if ordinary lactic acid produced by fermentation is com- 

 bined with strychnine, which has the properties of a base and 

 thus forms salts with acids, and if the strychnine compound 

 is allowed to crystallise slowly from solution, the first portions 

 of salt which crystallise out will differ in physical properties 

 from those which are obtained later ; the most important 

 difference is in regard to the action of their solutions upon 

 polarised light. 



This property of affecting polarised light is one of very 

 great importance, and its study has led to great extensions 

 in our conceptions of molecular structure. Moreover, the 

 effect of certain solutions upon polarised light affords a 

 means of determining the quantity of dissolved substances 

 present in solution. For all these reasons it is important 

 that the fermentation chemist should possess some elementary 

 knowledge at any rate of the subject of the polarisation 

 of light, and of the practical use of the polarimeter, and 

 at this point, therefore, it will be well to make a digression 

 and devote some space to the theory and use of the polari- 

 meter. 



