Cadmium, and Mercury. 



373 



As. evidence of the above reaction we give the following 

 data : 



Table X. 

 Action of HgCl 2 on a large excess of Na 2 S 2 3 . 





HgCU 

 taken 



Na a S„O s . 

 5H 2 



taken 



NaCl 

 taken 



H 2 



taken 



Na 2 S 2 3 . 

 5H 2 

 used 



Na,S,0,. 

 5H 2 

 Cal. 



HgS+S 

 found 



HgS + S 

 Cal. 



HgS 

 fouud 



1. 



2. 



1 -OOOg 

 l-000g 



5 -OOOg 

 5 -OOOg 



2'0g 

 2 0g 



75 cc 

 75" 



366 

 3-56 



366 

 3-66 



1-32 

 1-29 



1-32 

 1-33 



•86 



•82 



HgS 

 Cal. 



■85 

 •85 



The data were obtained in the manner described on p. 352, 

 i. e., the excess of thiosulphate was obtained by titrating an 

 aliquot part of the filtrate with iodine ; the total weight of the 

 precipitate, washed with alcohol and dried, was then taken, 

 and finally the weight of the sulphide was found after extract- 

 ing the precipitate with carbon disulphide, washing out the 

 excess with ether, and drying. 



Metacinnabar, a'-HgS. The metacinnabar of nature, when 

 pure, is entirely black and crystalline and is generally re- 

 garded as an isometric mineral, though crystallographic data are 

 meager. We have obtained black crystalline mercuric sulphide 

 by only one method, viz., by the action of an excess of sodium 

 thiosulphate on sodium mercuric chloride in dilute solution. 

 By reference to the previous pages (pp. 371-372) it will be seen 

 that moderately concentrated solutions (10^-25$) of total salts 

 containing the substances in the ratio HgCl 2 :4Na 2 S 2 3 give 

 first a red precipitate, which continues to form until the con- 

 centration of the mercuric chloride is approximately 1 per cent, 

 when the black crystalline sulphide separates with the red. 

 The solutions a, b, c, d, e (fig. 5) precipitated a mixture of the 

 two forms from the outset. The formation of the red modifi- 

 cation may be entirely inhibited, however, by the addition of 

 sulphuric acid to the solution before boiling. In six different 

 experiments where the concentration of the mercuric chloride 

 remained constant, lg. : 700 cc H 2 0, and the thiosulphate varied 

 from 2g. to 20 g., the addition of 4 drops 30% H 2 S0 4 entirely 

 prevented the formation of the transparent red sulphide. This 

 was proved by microscopic examination of the products. On 

 the other hand, if the acid was omitted, all the other conditions 

 remaining constant, a large quantity of the red form (/3'-HgS) 

 was intermixed. For the preparation of the black a'-HgS in 



Am. Jour. Sci.— Fourth Series, Vol. XXXIV, No. 202. -October, 1912. 

 25 



