506 Sir David Brewster on the Existence of Crystals 



ing facts present themselves to our notice. The crystals of 

 the tessiilar class, which are modifications of the cube, are 

 very numerous, and have no action upon polarized light. 

 Many of them melt easily, while others refuse to yield to the 

 action of heat ; and hence there must be two different sub- 

 stances in the cavities which assume the same shape. In like 

 manner, some of the doubly refracting crystals melt readily, 

 others with very great difficulty, and others not at all ; so that 

 there must be three different substances, which belong to the 

 classes of forms that give double refraction ; a conclusion 

 which is confirmed by the different secondary forms which I 

 have already enumerated. 



I have seldom found any crystals in these cavities which 

 depolarize white light, or the highest order of colours. I have 

 found some that depolarize ybz/r orders of colours ; and when 

 the crystal which does this is a flat hexagonal plate, it is highly 

 interesting to see it pass through all the tints which these 

 orders include, while slowly melting, and again reproducing 

 them during its recrystallization. 



In a cavity which was so placed as to be entirely black from 

 the total reflexion of the light which fell upon it, I observed 

 three iiohite openings, a, b, c, of a crystalline form (see fig. 6). 

 These appeared to be fixed crystals, or rather parts of the 

 topaz, surrounded by a cavity. I found, however, that the 

 hexagonal one C depolarized white light, while the rest had 

 no action upon polarized light. Upon applying heat, the 

 crystal c melted, and took up a position at e, fig. 1 5, in a nar- 

 rower part of the cavity, where it remains of an irregular 

 form, having been repeatedly melted and recrystallized. Upon 

 turning the cavity into a position where it became transparent, 

 I found that there was no fluid whatever in the cavity ; so that 

 we have here an example of a crystal melting and recrystal- 

 lizing without having been dissolved in one of the fluids. From 

 the irregular state of the laminae close to this cavity, there is 

 every appearance of the fluids having escaped from one of its 

 extremities. 



In the course of these observations, I observed a phaeno- 

 menon, produced by heat, of the most novel and surprising 

 kind, and one which 1 feel myself utterly unable to explain. 

 It piesented itself when I was studying the very interesting 

 collection of crystals in the cavity AB, fig. 8. This cavity is 

 filled with the dense fluid, in which there is a vacuity V : the 

 fluid swells to sucli a degree with heat as to diminish very 

 perceptibly the size of this vacuity ; and as I can find no trace 

 of any portion of the volatile fluid, I have no doubt that this 

 vacuity would disappear by an increased degree of heat. The 



