:>7ti Allen and Crenshaw — Sulphides of Zinc, 



constant mixing, to the thiosulphate solution. The mixed solu- 

 tion should not contain a trace of precipitate. The product is 

 now brought as rapidly as possible to boiling. After about 12 

 minutes the precipitate is rapidly filtered on a hot water filter 

 and washed thoroughly with hot water. The temperature of 

 the solution should not be allowed to fall materially during- the 

 process. In that case a further precipitate containing some 

 black ELgS is formed, as may be seen by the color. When the 

 filtrate has reached room temperature, the further precipitation 

 becomes very slow. If now the supernatant liquid is raised again 

 to boiling, the /3'-IIgS is again formed. After the precipitate 

 has been dried by alcohol and ether, the free sulphur is removed 

 by carbon disulphide. That the substance is really a sulphide 

 of' mercury will be seen from the following analysis : 



HgS = 98-49 



CI = -it) 



Na 2 S0 4 = -23 



H.,0 = 114 



99-96 



A weighed portion of the substance was dissolved in caustic 

 alkali, and the solution treated for a short time with hydrogen 

 sulphide. A slight excess of dilute sulphuric acid was then 

 added, and finally a slight excess of ammonia. The mercuric 

 sulphide was then collected, freed from sulphur, and weighed 

 according to Treadwell.* 



The water and sodium sulphate were determined by the 

 methods described for the a '-com pound. The chlorine was 

 doubtless present as sodium chloride, and the remainder of the 

 sodium was probably in the form of sodium sulphate. If we 

 calculate the chlorine and the sodium in this way it would 

 change the above results only a trifle. The greater quantity 

 of impurity found here than in the a'-HgS is due, no doubt, 

 to the higher concentration of salts in the solution which one 

 must use in order to obtain the /3'-forin. The determinations of 

 density and refractive indices were made on preparations which 

 had heen heated to 250° in vacuo for some hours. Determi- 

 nations of the water and sodium as sulphate in two of them 

 were as follows : 



I II 



H s O. Na 2 S0 4 H 2 0. Na 2 SO< 

 0-37 & 0-46 0-49 0-33$ 



These powders were apparently homogeneous as. observed by 

 the microscope, but they were quite fine and somewhat vari- 



* Quantitative Analysis, translation by Hall, p. 135. 



