*592 MR. F. E. SMITH, MR. T. MATHER, AND DR. T. M. LOWRY 



equivalent is, however, unimportant, possibly because a sufficient weight is not 

 retained by the 15 per cent, solution. 



The above results go far to justify the view that the majority of silver salts which 

 are insoluble in water dissolve to a slight extent in concentrated silver nitrate solu- 

 tions, and that the increase in the electrochemical equivalent which usually results 

 may be related to this difference in solubility, which probably acts by causing a 

 precipitation of the sparingly soluble salt from the impoverished solution at the 



kathode. 



I. Influence of Sulphide. 



In view ot the readiness with which metallic silver blackens on exposure to air, it 

 was evident that silver sulphide was likely to be a frequent impurity in the nitrate 

 solutions. The blackening of the bottles in which silver nitrate solutions are kept is 

 a universal experience in the laboratory, and in voltameter work the blackening of 

 the porous pots has constantly proved a source of trouble. The sediment from the 

 silver nitrate mother-liquors undoubtedly contained sulphide, and it was therefore 

 very important to determine the influence of this substance on the electrochemical 

 equivalent. 



(1) A solution of 60 grammes of silver nitrate in an equal weight of water was 

 prepared in a wide test-tube, and a small volume of hydrogen sulphide gas was 

 delivered into the tube above the surface of the solution. The first effect of the 

 sulphuretted hydrogen was to produce on the surface of the solution a yellow film 

 which turned black where the gas was present in largest quantities. On shaking the 

 solution a granular precipitate of a canary -yellow colour became distributed throughout 

 the solution, and the black sulphide disappeared. It was evident that where the 

 nitrate was in excess the precipitate was stable in a yellow form, and became black 

 only when the proportion of sulphuretted hydrogen to nitrate was increased. When 

 a considerable quantity of the yellow precipitate had been formed the solution was 

 filtered. On diluting with distilled water to 400 cub. centims., a brown cloud 

 appeared which slowly settled to a black precipitate at the bottom of the colourless 

 solution. The behaviour of the sulphide solution was thus essentially similar to that 

 of the silver nitrate mother-liquors. The diluted solution was filtered and transferred 

 to a voltameter. It gave a normal electrochemical equivalent 



I'll 828 milligrammes per coulomb (87 6). 



Three other solutions (856, 85c, 86c), prepared by somewhat similar methods, also 



gave normal values, 



1-11829, 1-11828, 1-11828. 



We are therefore driven to the conclusion that although many of the heavy 

 deposits were obtained from solutions which were undoubtedly contaminated with 

 sulphide (mother-liquors 276, 476), the presence of this substance alone is not sufficient 

 to account for the production of these abnormally high values. 



