401 



axes could not be reasonably expected in these directions. He 

 considered that the biaxial systems of rings would be found to exist 

 in thicker crystals (when discovered), having triangular replacement 

 planes upon the solid angles, either of the shorter or longer oblique 

 diameters of the right rhombic prism, and the results of these optic 

 axes would be seen in a direction perpendicular to these surfaces of 

 replacement. 



The various formulae necessary to produce different crystalline 

 forms are given in detail. 



The author enters into a re-discussion of the double refractive 

 powers, and attempts to show that Herapathite possesses a 

 principal axis having " positive " characters. He also endeavoured 

 to measure the thickness of certain very thin plates, by the differen- 

 tial tints produced by the exercise of their doubly refracting powers 

 upon the colours of the selenite stages, and found that these thin 

 plates raised the blue of the second order to the yellow and violet ; 

 the red of the second to the violet and blue-grey ; and the green of 

 the same to orange and violet of the second order. Assuming, there- 

 fore, this substance to have similar doubly refracting powers to 

 selenite, and of equal intensity, and also the same index of refraction, 

 their thickness may be estimated as equal to that of a plate of selenite 

 necessary to give the same difference in tint, viz. '0U0517 of an inch. 



These very thin plates no longer possessed the Cantharides-like 

 or brilliant metallic green colour when examined by reflected light. 

 They appeared more like portions of the elytra of the Blatta 

 germanica ; still reflecting a polarized beam, but of a brown colour, 

 and also retaining their doubly absorbent powers when examined by 

 transmitted light. 



The author considers that his recent investigations indicate that 

 whatever properties Professor Stokes may have assigned to the ray 

 reflected from the a -prisms (which alone were experimented on by 

 him) in the principal plane of the breadth, must be equally true in 

 that reflected from the primary rhombic prism in the principal plane 

 of its length, i. e. its long diameter ; and whatever properties he may 

 have assigned to the reflected ray from the principal plane of the 

 length of the a-prism, will be equally true of that reflected from 

 the primary rhombus in the principal plane of the breadth, i. e. its 

 short diameter. 



The paper concluds with some recently determined facts relating 

 to the chemical characters of this peculiar substance. 



Its sp. gr. at 60° Fahr. was found to be 1-895. 



It is very sparingly soluble in ether, turpentine or water at the ordi- 

 nary temperatures, scarcely more than -20^00^^^ part; boiling water does 

 not dissolve y^Vo^^^ part. It does not appear more soluble in ether 

 or turpentine by boiling. Chloroform does not dissolve it, but has 

 a great attraction of surface for it. 



Alcohol of '^37 dissolves g-jo"^^ P^^^ Fahr., and by boiling, 



it takes up g^^th part, which readily crystallizes on cooling in 

 /3-hexagons. 



Acetic acid, sp. gr. 1*042, dissolves j^oth. part at 60° Fahr., and 



