362 Allen and Crenshaw — Sulphides of Zinc, 



therefore, tested for these impurities only, and then used for a, 

 determination of the density. No lead was found, though 

 ■02 per cent of residue remained after driving off the volatile 

 acid of the chloride with sulphuric acid. There was also a 

 trace of iron which probably came from the reagents. Nearly 

 10 g. of material were used in the gravity determination. At 

 25° the value 4 - S33 was obtained. After finer crushing, to 

 eliminate possible air bubbles, a second determination gave 

 4-s;!.\ proving that porosity was negligible — a conclusion con- 

 firmed by the microscope. From the above value we obtain 



. - . mineral at 25° . „_- „.. , „ ,, . 



the density - -75 — = 4'820. lhe value of tins con- 



J water at 4 



stant given in the text-books depends on meager data and is, 



doubtless, too high.* 



At a much lower temperature, well developed prisms (about 



l mm maximum length) were obtained by Schuler'sf method — 



i. e. by heating;}: the amorphous sulphide with a flux consisting 



of equal parts of potassium carbonate and sulphur.§ These 



prisms were also hexagonal. They probably dissolve a small 



percentage from the flux, since Merwin finds e Li = 2'447 as 



compared with 2 - 45 for greenockite prepared by heating the 



amorphous sulphide in hydrogen sulphide. 



Cadmium sulphide from acid solutions. 



Failing to get any evidence of the existence of any other 

 sulphide of cadmium than the ordinary greenockite by dry 

 methods, experiments on aqueous solutions were undertaken. 

 When acid solutions of cadmium salts are precipitated by 

 hydrogen sulphide, the product may be either amorphous or 

 crystalline — the result depending on the rapidity of precipita- 

 tion, the acidity of the solution, its cadmium concentration, tem- 

 perature, etc., as it is with zinc salts. By the use of the double 

 tube, a device already referred to (p. 355), hydrogen sulphide 

 was generated slowly, and comparatively large prisms (about 

 q. 5mm long) were obtained. The composition of the solution 

 in the inside tube was 2 g. CdS0 4 .8/3H 2 + 20 cc. 30$H 2 SO 4 ; 

 the outside tube contained 9 g. Na 2 S,0 3 .5H 2 + 20 cc. water. 

 The duration of the experiment was three daj's at 180°. The 

 crystals were undoubtedly greenockite, though the index of 

 refraction, e Li , was slightly lower than that of the purest 

 product, being 2 - 440 instead of 2'456. This is accounted for 



* Thus Dana ^Text-book of Mineralogy, 6th ed., 1906) gives 4'9 — 5-0. 



fLieb. Ann., lxxxvii, 40, 1853. 



X The temperature in one exijeriment was 365°. 



§ Much amorphous residue remained after dissolving out the flux with 

 water, perhaps from a decomposition of a compound of the cadmium and 

 potassium sulphides. 



