BIOT AND J. HERSCHEL 7 



visible externally, since this prism can be derived from 

 a rhombohedron. Some rhombohedrons may, very 

 easily, pile themselves up in such a way that they 

 form a hexagonal prism. In the same way some tetra- 

 hedrons may so adjust themselves as to give a cube. 

 Therefore, if we admit that the crystalline net-work 

 of prismatic quartz is formed of rhombohedral mole- 

 cules, as boracite is formed of tetrahedral crystals, 

 all difficulty vanishes between Weiss and Haiiy: the 

 molecular polyhedron will express the dissymmetry 

 by its form, but this dissymmetry will not necessarily 

 appear in the external aspect of the crystal." 



This solution of the difficulty is, I repeat it, very 

 infantile. It is a pure invention and Delafosse did 

 nothing to give it a firmer foundation; nevertheless it made 

 for progress, by virtue of that which I have just pointed 

 out, for it introduced into the mind this idea that the 

 form of the integral molecule of the crystal is not as 

 closely bound up as Haiiy thought with the form of 

 the crystal itself. We shall soon see the influence 

 of this conception upon Pasteur, the pupil of Delafosse, 

 and, like him, passionately fond of questions of molecu- 

 lar structure. 



II 

 BIOT AND J. HERSCHEL 



The general law, just now stated, that a science pro- 

 gresses above all by changing its point of view, explains 

 the aid which it always derives from kindred sciences; 

 and it is especially because young minds search most 

 eagerly and are more open to these suggestions from 

 without, that youth is particularly the time when the 

 spirit of invention flourishes. In the case with which 



