412 Allen, Crenshaw, and Merwin — 



tion possible. The results in Table III do not prove that very 

 small quantities of hydrogen sulphide might not be occluded 

 by the products, and it may be that this apparent 5 per cent of 

 pyrite is due to a slight occlusion, though there is no proof 

 of it. 



III. WURTZITE. 



It has been shown in a previous paper* that crystalline zinc 

 sulphide may be obtained by the action of hydrogen sulphide 

 on acid solutions of zinc salts at sufficiently high temperatures. 

 Nearly all our work was done on sulphate solutions, since 

 chloride solutions for some unknown reason gave no crystal- 

 line products. Many of the products contained both the stable 

 form sphalerite and the unstable form wurtzite, f and the ex- 

 periments indicated that the more acid was the solution for 

 any given temperature, the greater was the proportion of 

 wurtzite ; and the higher the temperature for any given acid- 

 ity, the greater was the proportion of sphalerite. 



It was hoped that a more thorough investigation would not 

 only establish these points, but would also decide whether 

 temperature and acidity were the sole determining influences 

 on the crystalline form or whether other conditions, such as the 

 pressure of the hydrogen sulphide, the time and the concentra- 

 tion of the zinc salt had any effect. 



A. Method for preparing wurtzite. — Zinc sulphide is very 

 difficult to crystallize — a fact which complicates the process of 

 its formation and makes the conditions difficult to study. 

 First, it needs a higher temperature than marcasite, not less 

 than 250°, and secondly, it must be precipitated slowly ; con- 

 sequently a double tube was required in its preparation, for 

 when an acidified solution of a zinc salt was saturated with 

 hydrogen sulphide and heated in a single tube at 300°, only an 

 amorphous precipitate was obtained. The procedure in our 

 experiments was as follows : 20 cc of a solution containing meas- 

 ured quantities of zinc sulphate and sulphuric acid were dropped 

 into a platinum tube l'5 cm in diameter and 25 cm long, closed 

 at the lower end. This was supported by a piece of glass 

 tubing of convenient size, inside a larger glass tube l'8 cm inside 

 diameter also closed at the lower end. The outside tube, which 

 contained a solution (either sodium thiosulphate or ammonium 

 thiocyanate) yielding hydrogen sulphide when heated, was then 

 sealed, enclosed in a steel bomb with water and heated to the 

 desired temperature. After heating for the necessary time 



* Allen, Crenshaw and Merwin, this Journal (4), xxxiv, 341, 1912; Zs. 

 anorg. Chem., lxxix, 125, 1912. 



f Sphalerite undergoes a reversible transformation into wurtzite at about 

 1020° ; wurtzite is therefore unstable at lower temperatures. 



