Effect of Temperature, Acidity, etc. 399 



ical tests where they are available. Synthetic sulphides, pre- 

 pared as above described, except where hydrochloric acid is 

 used, consist of crystals which are usually not sufficiently 

 developed to make microscopic diagnosis certain. Apart from 

 any other evidence, color and crystal habit in most of the prep- 

 arations make the presence of marcasite almost certain, but 

 that of pyrite doubtful. When the preparations are made in 

 double tubes (see p. 412), however, the crystals are well 

 developed, and Larsen* found pyrite in many of them ; the 

 marcasite was always better developed, in fact, the crystals 

 were often measurable, though some crystals of pyrite were 

 0*5 ram across. The pyrite crystals usually showed a combina- 

 tion of the cube and octahedron. 



2. Color. — Pyrite is distinctly yellower than marcasite, 

 though in making comparisons care should be taken that the 

 specimens are untarnished. Synthetic products which consist 

 of very small crystals can not be satisfactorily compared for 

 color differences by simply placing small portions side by side ; 

 but by grinding a few tenths of a gram to a rather fine powder 

 and then boiling in a small beaker with dilute hydrochloric 

 acid (to remove tarnish), a considerable amount can be made 

 to float on the surface of the liquid in the form of a metallic 

 mirror. When examined in this way, the products which 

 were determined by the Stokes reaction to be either pure 

 marcasite, or very near it, showed the same color as a mirror 

 of the natural marcasite obtained in the same manner, while 

 products found by the Stokes reaction to contain 28 per cent 

 and 35 per cent pyrite respectively, formed mirrors of 

 decidedly yellower color, identical with that of pyrite-marca- 

 site mixtures, as three observers all agreed. This test, while 

 purely qualitative, gave evidence which may be regarded of 

 considerable weight. 



3. Evidence concerning inclusions. — It has already been 

 admitted that the synthetic sulphides might possibly have 

 occluded ferrous sulphate, hydrogen sulphide, or in some 

 cases sulphur dioxide, and that any of these substances would 

 interfere with the Stokes test and the conclusions drawn from 

 its use. The absence of hydrogen sulphide was pretty con- 

 clusively shown by a series of experiments in which the 

 synthetic sulphides were heated in vacuo. These experiments 

 were made on products formed at 200° under conditions where 

 sulphur dioxide never formed. 



A vertical glass tube about l cra in diameter, closed at the 

 lower end, was used for heating the sulphide. About 15 cm 

 from the lower end a sifle tube 5 mm in diameter was sealed on. 

 Above the side tube the vertical tube was constricted and left 



* This Journal (4), xxxiii, 215, 1912 ; Zs. anorg. Chem., lxvi, 251, 1912. 



