492 Mr. FoRSTER on the Molecular Formation of Crystals. 



molecules were united by their corners, or that the crystal was formed of tetra- 

 hedral and octahedral molecules mixed. The necessity for such an arrangement, 

 so unaccountable, and so totally at variance with the rest of his theory, was 

 evidently subversive of it. 



However easy it may be to divide a crystal into any number of molecules, 

 it is by no means so easy to build up a crystal of these molecules ; — in fact, he 

 does not seem to have considered the problem as a physical one, nor to have 

 investigated at all how the molecules could have united. To any one viewing 

 the question in such a light, the difficulties of showing how such molecules 

 could come together would appear insurmountable, if not subversive of the 

 theory which gave them birth. No such difficulties exist, however, when we 

 consider the atoms spherical, and attached to each other by means of poles, or 

 centres of force : we can then begin with the individual atom, and trace it as 

 it becomes part of a crystal, and as other atoms become attached to it. 



Such is the system of JNIr. Dana, the American mineralogist. To him be- 

 longs the merit of clearly pointing out how some hemitrope crystals occur. The 

 first idea, however, undoubtedly originated with Sir David Brewster, who 

 stated the fact without explaining it. To give a detailed account of this theory 

 is not my intention : suffice it to say, that the completeness and beauty of this 

 part of it carry with them an irresistible conviction of its truth, so far as it goes, 

 resembling in this respect the theory of decrements of Hauy, which, despite 

 the errors of his system, still remains, and applies where the particles are sup- 

 posed to be spheres as well as where they are supposed to be cubes or tetra- 

 hedrons. 



Dana considered the particles to be spheres or ellipsoids, possessing six 

 poles or centres of force on their surface : three of these poles were of one 

 name, and three opposite of another — those of a like name repelling, those of 

 an unlike attracting each other. He explained the formation of twin crystals 

 by showing how two molecules may unite at a point intermediate between two 

 or three poles. 



In Fig.' 6 is shown the arrangement which he supposed the poles to have ; 

 the intersection of the circles shows their position ; and in Fig. 7 the form into 

 which the molecules will pile themselves. This was the only primitive form 

 which he supposed to exist in the first system ; and accordingly, he left quite 



