Prof. Johnston on the Constitution of Chabasie. 167 



here the form of all the layers is the same; so that this mode 

 of accounting for the appearances must also be rejected. Sir 

 David Brewster indeed, speaking of the form of the crystal 

 at the neutral line or line of no double refraction, says: " At 

 this period the form of the crystal would be a cube :" but he 

 speaks here only in reference to the absence of the doubly re- 

 fracting structure which in a single atom or group of atoms 

 of a simple substance would indicate a cube; but which, in a 

 mass, such as the smallest of these layers, is quite compatible 

 with a congeries of rhomboids. 



This transition from a positive to a negative state of double 

 refraction may be explained on the principle that the mole- 

 cules of two substances possessed of opposite optical pro- 

 perties may neutralize each other if the relative positions of 

 the optical axes and the number of molecules of each be 

 rightly adjusted. Let the i)roper material of a crystal be 

 possessed of a positive double refraction, such as the nucleus 

 in the present case indicates ; and let a second substance hav- 

 ing a negative index deposit itself during crystallization, either 

 in the form of distinct layers or mixed with the molecules of 

 the first as fluids mix, forming only part of the mass of such 

 layers ; let also the quantity so mixed increase, or the distance 

 of the layers decrease, as we depart from the central nucleus, 

 and let the optical axes of the two substances be parallel ; we 

 shall have a crystal possessed of properties analogous to those 

 observed in chabasie. The positive energy of the nucleus will 

 gradually decrease till it disappear in a neutral line, and from 

 this neutral line the refraction will again increase with a nega- 

 tive sign. 



But the second substance must not only be negatively 

 double-refractive, but also isomorphous with the first or ca- 

 pable of replacing it during crystallization without affecting 

 the form of the crystal. Now among the substances contained 

 by chabasie in large quantity, is silica ; and this substance 

 is not only negatively double-refractive, but appears also to be 

 isomorphous with chabasie. The form of this mineral as de- 

 termined by Mr. William Phillips from cleavag-e planes, is an 

 obtuse rhomboid of 94° 46'; while that of quartz is a similar 

 rhomboid of 94"^ 15'. These two forms are as nearly identical 

 as isomorphous bodies generally present themselves in nature: 

 we may therefore consider them as capable of replacing each 

 other. 



Suppose then that during the crystallization of the chabasie 

 a sli'dit excess of silica is present in tiie solution and deposits 

 itself in occasional layers or otherwise as above described, and 

 that the (juantity so deposited increases with the size of the 



