SUGAR 423 



the complicated nature of this field of enquiry it is necessary to 

 remember that the majority of these substances rotate the plane 

 of the polarised ray, and that this property is traced to the 

 presence in each molecule of several asymmetric atoms of carbon. 

 The general nature of this conception has been already explained, 

 but the case of the sugars requires a little closer examination. 



Imagine a single atom of carbon united with four other atoms 

 or groups of atoms all different. These may follow one another, 

 projected on the plane of the paper, in either of the two ways 

 shown below, that is in the direction in which the hands of a 

 clock move or the reverse. 



a C c c C a 



I I 



d d 



It is obvious that these two arrangements are such that the 

 one is a mirror-image of the other. If but one atom of carbon in 

 this condition is present in the compound there may be two 

 varieties of the same which agree together in all chemical char- 

 acteristics, and, generally speaking, in all physical characters 

 except one, and that is in their action on the polarised ray. One 

 will rotate the plane to the right, the other to the left through 

 the same number of degrees. But if a pair of compounds, both 

 being alcohols or acids or aldehyds or ketones, etc., contain two 

 asymmetric atoms, then there will be a pair of stereoisomerides, 

 one dextro- and one Isevo-rotatory, while there will be a third 

 substance which will be optically inactive. Now as glucose con- 

 tains four asymmetric carbon atoms, there must be, according 

 to the hypothesis, eight stereoisomeric forms with right- 

 handed rotation and eight others with equal left-handed rotation. 

 Most of these compounds are actually known, though only a small 

 proportion of them occur in nature. 



Though practically identical in respect to chemical reactions 

 a very important difference is observed in their physiological 

 relations, for they are acted upon very differently by yeast and 

 other ferments. Thus the two components of invert sugar, 

 glucose, and fructose are fermented into alcohol and carbonic 

 acid at different rates, and while d glucose is fermentable / glucose, 

 its optical isomeride remains unaffected. In fact, it is in general 



