352 MOLECULAR GROUPING 



and an axial angle not very far from 90 : 

 /3 = 9433'- 



Finally the silicate, H 2 Mg 19 Si 8 34 F 4 , is found as humite 

 (rhombic) : 



a : b : c = 1-0803 : i : 4-4013, 



as chondrodite (monoclinic) : 



a : b : c = 1-0803 : i : 3*1438 /3 = 90, 



and as clinohumite (monoclinic) : 



a : b : c = 1-0803 : i : 5-6588 /3 = 90. 



This equality of form on change of structure would 

 perhaps come out more strikingly if the comparison of 

 angles and lengths were made, not at the ordinary tempera- 

 ture, but at the transition point, since then the two forms 

 have just the same stability, a fact which may be due to 

 mechanical causes. In any case it is to be remarked that 

 this relation between the two forms is associated with the 

 possibility of a transition without upsetting the internal 

 connexions of the substance. Thus, e. g., boracite passes at 

 266 into the regular form without any loss of transparency, 

 but only with a sudden change of optical properties, 

 becoming singly refracting. 



Isorneric change constitutes a deeper modification in the 

 molecule. Accordingly, the accompanying change mostly 

 complete in the symmetry of the molecule corresponds to 

 a complete change in molecular structure, without any 

 relations that have so far been traced. In one case, 

 however, the circumstances are different, viz. in that of 

 stereo-isomers with an asymmetric carbon atom. The two 

 isomers here present, as we have seen, a difference of con- 

 figuration shown in Figs. 16 and 17 (p. 105), and known as 

 enantiomorphic. A completely analogous enantiomorphic 

 difference is found in the crystalline form, as is shown in 

 Fig. 1 8 and 19 (p. 106), for the oppositely active ammonium 

 bimalates. 



