236 C. B. and J. B. Bhaduri — On Double Thiosulphates [No. 1, 



it was found that the same yellow salt is invariably obtained at all 

 temperatures between 20°C and 34°C provided the green colour of the 

 mixture is not discharged by an excess of sodium thiosulphate. Even 

 with fairly dilute solution, the same salt is obtained. In one or two 

 cases, however, the sodium thiosulphate was so much in excess that the 

 liquor left after the separation of the crystals was yellow. Sometimes 

 the precipitate was allowed to remain in contact with the mother liquor 

 overnight. It may be mentioned here that the salt separates out soon 

 when prepared from concentrated solutions. If, however, dilute solu- 

 tions are used it takes a much longer time for the salt to crystallise 

 out, but it is richer in colour and more distinct in crystalline structure. 

 In contact with the mother liquor decomposition sets in generally 

 after a day or two, and a dark reddish brown precipitate, more soluble in 

 water than the yellow salt, is formed which finally changes into black 

 insoluble copper sulphide, the supernatant liquid becoming perfectly 

 clear and transparent. In the first few preparations the salt was 

 simply washed with water, in which it seemed to dissolve to a slight 

 extent, until free from sulphuric acid, pressed between filter paper, 

 powdered and dried by exposure to the air. It was found, however, 

 that the salt so treated did not keep well for a few days, decomposition 

 setting in, sometimes, even in course of a few hours. The colour 

 slowly changed to black, and sulphur dioxide was evolved. It was 

 subsequently found that a fairly stable salt could be obtained if it were 

 washed at first with water until free from sulphuric acid and finally 

 repeatedly with rectified spirit over filter pump, dried by exposure to 

 the air, powdered, and carefully sifted in a fine sieve. The salt thus 

 prepared did but lose a trifling when kept in a desiccator over calcium 

 chloride for weeks together. The resnlt of a determination of the loss 

 is given here using about (7) seven grams of the salt. 



22nd December, 1896. Platinum basin and salt =31-2436 grams. 

 23rd „ „ „ „ =31*2425 „ 



9th January, 1897. „ „ =31-2412 „ 



26th „ „ „ |t ,= 31-2412 „ 



Thus about 7 grams of the salt lost only 2'5 milligrams in three 

 weeks. This loss may easily be accounted for from a small quantity of 

 alcohol still present in the air dried sample. In connection with the 

 decomposition of this salt at ordinary temperature, it may conveniently 

 be pointed out, here, that more dense the precipitate is and the richer 

 the colour, the better it keeps. Moisture is perhaps the most important 

 factor in bringing about the decomposition, as appears from a little of 

 the above sample, kept sealed in an ordinary glass tube. While the 

 substance kept quite well in a desiccator over calcium chloride for about 



