ON THE OPTICAL PECULIARITIES OF THAT MINERAL. 333 



the spectrum. The ray of compensation, in place of being 

 yellow, may have any position in the spectrum, and those on 

 each side of it will afford positive or negative tints, according 

 as the positive or the negative axis exercises over them a pre- 

 dominating influence. 



This view of the polarising structure of Apophyllite, affords 

 a complete explanation of the singular tints which surround 

 its resultant axis. Each order of colours is as it were a resi- 

 dual spectrum *, arising from the opposite actions of the ne- 

 gative and the positive axis, and the tints of which these orders 

 are composed, will consequently vary, according to the loca- 

 lity of the ray of compensation f . 



Having thus described the structure and properties of the 

 tesselated Apophyllite, it becomes interesting to inquire how far 

 such a combination of structures is compatible with the admit- 

 ted laws of crystallography. The growth of a crystal, in vir- 

 tue of the aggregation of minute particles endowed with pola- 

 rity, and possessing certain primitive forms, is easily compre- 

 hended, whether we suppose the particles to exist in a state of 

 igneous fluidity or aqueous solution. But it is a necessary 

 consequence of this process, that the same law presides at the 

 formation of every part of it, and that the crystal is homoge- 

 neous throughout, possessing the same mechanical and physi- 

 cal properties in all parallel directions. 



The 



* Among the various residual spectra which I have examined in the course 

 of my experiments " on the Action of Transparent Bodies upon the differently 

 cdoured Rays of Light," there are many among the polarised rings which have 

 exactly the same tint ; and there are some which resemble as nearly as possibly 

 those in Apophyllite. See Edinburgh Transactions, vol. viii. p. 1. 



+ The preceding explanation may be presented in a more brief form, by stating 

 that the specimen of Apophyllite in question had three positive rectangular axes, 

 which were in equilibrium only for the yellow rays of the spectrum. See Phil 

 Trans. 1818, p. 256. 



