196 Allen, Crenshaw, Johnston, and Larsen — 



Synthesis. — When heated in hydrogen sulphide, pyrite de- 

 composes gradually into sulphur and pyrrhotite. The decom- 

 position may be first detected at about 575° (see p. 205). At 

 about 665° it proceeds rapidly ; still, even after some hours at 

 750°, the dissociation is never quite complete, and when the 

 pyrite used is rather coarse (sized between screens of 8-40 

 meshes per cm.), several per cent of it persist in the product. 

 The latter is therefore melted and cooled, and the resulting 

 material, which is now free from pyrite,* serves as a starting 

 point in the preparation of pure pyrrhotite. 



When this is reheated it loses or gains sulphur according to 

 conditions. A series of products was made by heating the sul- 

 phide in hydrogen sulphide for some hours at different meas- 

 ured temperatures, and then cooling it in nitrogen. f To 

 facilitate the formation of homogeneous products the sulphide 

 was carefully sized between screens of 16 and 40 meshes per 

 cm. These products were all similar in appearance to natural 

 pyrrhotite. They were all dense, opaque, metallic, more 

 brownish than pyrite, and only very slightly tarnished. Some 

 tests were made to prove that the quantity of oxide on the 

 surface was negligible. 



Weighed samples were heated in dry hydrogen to a red heat 

 and the water formed was absorbed by passing through a 

 calcium chloride tube. The surface of the grains became 

 bright in a few minutes. The tube was cooled in hydrogen, 

 which was then displaced from the apparatus by dry air. The 

 water thus collected corresponded to less than 0*1 per cent of 

 oxygen in two different tests. Furthermore it will be noted 

 later that the preparations which were cooled in nitrogen were 

 comparable in density with those which were prepared in 

 another way and were not tarnished in the slightest degree 

 (see p. 199). ^ 



Composition of synthetic pyrrhotite. — The sulphur was 

 determined in each of the synthetic pyrrhotites by a method % 

 worked out in this laboratory, which was proved accurate 

 within at least 0*2 per cent of the sulphur present. 



Treitschke and Tammanng state that the fused sulphide of 



* Except for a slight superficial layer which decomposes in the next 

 process. 



f The nitrogen was prepared by Knorre's method (Die Chem. Ind., xxv., 

 531, 550, 1902 ; Chem. Centralb., i, 125, 1903), i. e., by dropping a saturated 

 solution of sodium nitrite from a dropping funnel into a solution of ammo- 

 nium sulphate and potassium chromate. The gas was passed through dilute 

 sulphuric acid to remove ammonia, and then successively through long col- 

 umns of chromic acid to remove oxides of nitrogen ; sulphuric acid to remove 

 moisture, and finally over-heated copper to remove oxygen, or any traces of 

 oxide of nitrogen which may have escaped. 



\ Allen and Johnston, Jour. Ind. & Eng. Chem., ii, 196, 1910; Zs. 

 anorg. Chem., lxix, 102, 1911. 



§ Z. anorg. Chem., xlix, 320, 1906. 



