42 Mr. J. F. Daniell on Crystallization, 



the nature of their form, be exerted with greater force in the 

 direction of their shorter axis than in that of their longer, 

 taking for granted the two fundamental laws of attraction, 

 first, that all the particles of matter attract one another directly 

 as their masses, and inversely as the squares of their distances : 

 secondly, that a body of any shape will attract a particle of 

 matter anywhere with the same force, and in the same direc- 

 tion, as if all the matter of the body were collected in its 

 centre of gravity, it is clear that their repulsive atmospheres 

 will not be distributed in equal layers over their surfaces ; but 

 will collect in greater depth above the shorter axis than the 

 longer ; and the atom, with its atmosphere, will assume more 

 pf the spheric form. 



A solid crystal, therefore, thus constructed, must change the 

 measure of its angles with every change of temperature. This 

 is precisely what Professor Mitscherlich has ascertained to 

 happen with crystals of carbonate of lime and other substances 

 not crystallizing in the octohedral series. 



He found that a rhombohedron of calcareous spar changed 

 the inclination of its planes to the amount of 8'. 5 in the inter- 

 val between 32 and 212. As the temperature augmented, the 

 obtuse angles diminished ; that is, the smaller axis of the 

 rhombohedron dilated more than its other diagonals, so as to 

 cause an approach to the cubic form. 



In substances crystallizing in the octohedral series, he found 

 that the expansion was equal in all directions. 



The mere inspection of the rhombohedron, made to represent 

 the primitive form of carbonate of lime, upon the spheroidical 

 hypothesis, is almost enough to produce a conviction that it 

 must expand and contract differently in the directions of its two 

 axes ; and the theory might certainly have had the advantage 

 of anticipating an observation which tends so powerfully to its 

 support. 



A new and interesting field of research has thus been opened 

 by this third method of disturbing crystalline cohesion, which 



