CHEMISTRY 11 



but the one form rotates the plane of polarisation 

 to the right hand, and the other rotates it to the left. 

 Thus, there is a " right-handed " quartz, and there 

 is a " left-handed" quartz: there is likewise a right- 

 handed sugar, and a left-handed sugar. Further- 

 more, it had been observed by Hatiy it may or 

 may not have been known to Pasteur that these 

 two forms of quartz-crystals, closely examined, are 

 visibly different. Haiiy had discovered, on these 

 crystals, one very minute facet, which, before him, 

 had not been recognised : and this facet was so 

 placed that he could sort the crystals into two 

 groups, according to the position of the tell-tale 

 facet on each crystal. This difference between two 

 forms of one crystalline substance is called disym- 

 metry. The two forms differ, as the reflection in 

 a mirror differs from the object in front of the 

 mirror. They correspond, but they do not 

 coincide. There is a pair of them : they go 

 together : it takes both of them to make up the 

 complete plan of the crystalline substance. And 

 this disymmetry, which is writ large, by a tell-tale 

 facet, on each crystal, is writ small on every one of 

 its molecules : for, if a crystal of right-handed sugar 

 be dissolved in water, this solution will still rotate 

 the plane of polarisation to the right. The structure 

 of the crystal itself is disymmetric, because the 

 structure of each of its molecules is disymmetric : 



