Formation of Compound or Twin Crystals. 295 



If it is supposed that the axes are fixed lines, of a determinate 

 and unalterable length, it might be necessary for the sake of the ex- 

 istence of the different forms, that this supposition respecting the 

 cube should be made. But if, as they appeared to be, they are 

 mere directions in which attraction is exerted, subject to alteration 

 by the peculiar qualities of any substance with which combination 

 takes place, the hypothesis must be considered wholly incorrect. 

 The union of two molecules of sulphur in one, may be the source of 

 one of the forms of this dimorphous substance. But if so. the pe- 

 culiarity of form is probably owing to the mutual influence of the 

 different axes. In general, however, Dimorphism appears to be 

 due to the different circumstances under which a substance crystal- 

 lizes. It is not an improbable conclusion that the nature of the sol- 

 vent, the degree of heat during crystallization, combined with some 

 other causes, may effect a change in the direction of the axes, al- 

 though generally, their only effect is a production of secondary 

 planes. The probability of this supposition will appear from the 

 following account (others similar are numerous,) by Mitscherlich,. 

 of a change produced in Sulphate of Zinc by the influence of heat. 

 (Brewster's Ed. Journal of Science, Vol. iv, p. 301.) "When a 

 Right Rhombic Prism of this salt is heated above a temperature of 

 126°, F., we may observe certain points at its surface become 

 opaque, and then bunches of crystals shoot out from these points in 

 the interior of the original specimen. In a short time, the whole is 

 converted into an aggregate of those crystals, diverging from several 

 centres, that are situated on the surface of the original crystal. No 

 water escapes during this process except what may have been ac- 

 cidentally included in the lamellae of the specimens, a circumstance 

 which proves the identity of the chemical composition of the two 

 species, and the difference merely to depend on the arrangement of 

 their particles." The small crystals formed were Oblique Rhombic 

 Prisms, which form is also produced when the solution crystallizes- 

 at a temperature above 126°, F. 



The prismatic form of Arragonite may possibly then be owing tc 

 the presence of but a small quantity of Carbonate of Strontian, whicb 

 is sufficient to change the direction of the axes, or even to the pres- 

 ence of this mineral in the solvent, or perhaps some other cause witb 

 which we are not acquainted. 



