6 PASTEUR: THE HISTORY OF A MIXD 



One finds analogous facts in the other systems of crystals. 

 Whence come these apparent exceptions to that regular- 

 ity which, really without knowing just why, we attribute 

 to the laws of nature? 



Haiiy was very familiar with these phenomena of the 

 hemihedron and if he did not attribute to them any 

 great importance it is because his theory led him to 

 a somewhat distorted view of them, as I have just said. 

 According to his conception the form of the integral 

 molecule was, first of all, that which cleavage, the natural 

 division of the crystal, gave to it. A cubical crystal of 

 marine salt produces cubes by cleavage; a rhombohedral 

 crystal of Iceland spar gives in the same way rhombo- 

 hedrons. The rhombohedron was, therefore, for Haiiy 

 a primitive form. When we intersect the six lateral 

 angles by planes having the same angle of inclination 

 to the faces of the rhombohedron, we obtain by a 

 perfectly regular process of derivation, the hexagonal 

 prism of quartz. And so for the other cases. This 

 conception formed a logical and coherent whole, but left 

 Haiiy indifferent to the questions of the hemihedron. 



In order to imderstand the hemihedral character 

 of the rhombohedron, it is necessary to reverse the 

 order and take the hexagonal prism as the primitive 

 form. Then the rhombohedron can be derived from 

 it only by way of the hemihedron. The same is true 

 in the other systems. Weiss, the mineralogist, did 

 this and straightway the hemihedron appeared to 

 be a phenomenon more frequent than was supposed, 

 and there arose a problem requiring solution. Why 

 this deviation from the law of symmetry? 



This is what Delafosse tried to explain in 1840, 

 by the aid of a deceptive hypothesis which to-day 

 seems very childish. **In the prismatic quartz," he 

 said, ''the hemihedral constitution exists without being 



