Cadmium, and Mercury. 383 



It may also well be that tlie acidity or alkalinity of solutions 

 will be found to determine the composition of minerals in cer- 

 tain cases. Thus chemical experience shows that the mineral 

 chalcopyrite CuFeS, can be prepared synthetically out of alka- 

 line solntions, while it is a matter of common knowledge that 

 copper is separated from iron by hydrogen sulphide in acid 

 solutions. 



Geologists regard chalcopyrite in certain occurrences as a 

 secondary mineral. . It would be premature to enter upon 

 a chemical discussion of this question at present ; the case suf- 

 fices to illustrate our meaning. Enough work has already been 

 done to show that the difference in chemical character between 

 acid and alkaline solutions, therefore in general between deep- 

 seated and surface solutions, is of vital importance in geochem- 

 istry. 



IV. Microscopic Study. 



Zinc Sidphide. 



Amorphous. — If examined immediately after being rapidly 

 precipitated in a pulverulent form, the particles of zinc sul- 

 phide are spherical, and have diameters of -0002 to -0005 mm 

 (2 to 5fi). If the precipitation is slow and the solution is 

 agitated, the particles increase in size, and after standing in 

 contact in the solution may aggregate into clusters or crusts, 

 indicating that they are semi-fluid. In fact, the aggregates 

 when compressed under a cover-glass flatten and break open 

 like a stiff jelly. Though jelly-like, the globules contain very 

 little water. On several occasions such masses after being 

 pressed have been observed to become distinctly doubly 

 refracting (probably crystalline). When formed very slowly 

 in acid solutions without agitation — as in the double tube 

 method — the precipitate is : (1) partly in the form of compact, 

 stratified crusts having a decided double refraction with o par- 

 allel to the surface, (2) partly in globules resembling spher- 

 ulites, and having a parallel to the surface. These crusts and 

 spherules are hardened to the point of being gritty aud brittle, 

 yet their refractive indices* are far lower than those of the 

 crystalline forms of zinc sulphide, sphalerite and wurtzite. 

 The results of a large number of measurements are tabulated 

 below. 



Pure sphalerite n u = 2-34 



-d . .. ( e Li = 2-35 



Jrure wurtzite { n oo 



( V>u = 2-33 



Amorphous : 



Stiff jelly n u = 2-1 S — 2"25 



Doubly refracting globules n u = 2 - 18 — 2 - 24- 



* Measured tinder the microscope in mixtures of amorphous sulphur and 

 selenium. See this Journal, xxxiv, 42, 1912. 



