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



of the solutions used had been prepared from silver nitrate recovered from previous 

 electrolyses, purified by adding nitric acid, boiling down to dryness, and fusing in a 

 platinum basin ; the fused mass was dissolved in water, filtered from the black residue 

 which was always left after fusion, and was often used for electrolyses without further 

 purification. On a few occasions silver nitrate was crystallised from the filtrate and 

 a 15 per cent, solution made from this recovered salt. The values for the electro- 

 chemical equivalent of silver varied from T11832 to 1'11886 milligrammes per 

 coulomb (mean of 14 determinations = 1 "11857) when the Rayleigh voltameter was 

 used, and from T11786 to T11854 (mean of 21 determinations = T11825) when the 

 porous pot voltameter was used. 



Before a definite figure could be established for either form of voltameter it was 

 necessary first to demonstrate the possibility of preparing again and again from silver 

 nitrate of different origins solutions which should give identical weights of silver 

 when electrolysed under identical conditions. The following experiments were there- 

 fore made in order to test the constancy of the electrochemical equivalent of a range 

 of silver nitrate samples of different origins. 



I. Silver Nitrate from Electrolytic Silver. 185 grammes of electrolytic silver 

 recovered from previous electrolyses were dissolved in a mixture of equal volumes 

 of " commercial pure " nitric acid and water. The resulting solution was filtered, by 

 means of a small Gooch crucible, from a small residue of insoluble matter,* and 

 evaporated on a water-bath (since it was not thought to be desirable to fuse the 

 product) during 50 hours, water being added from time to time. A crop of crystals 

 was then drained off and dried overnight in the oven. When recrystallised the 

 product was found to be neutral, but yellow in colour. After three further recrystal- 

 lisations the electrochemical equivalent of the sample was tested. The solution for 

 electrolysis was prepared by dissolving 90 grammes of the purified salt in 600 cub. 

 centims. of water of low conductivity, and when electrolysed in a Kayleigh voltameter 

 gave a deposit of I'll 825 milligrammes per coulomb (246). 



II. Prom Recovered Silver Nitrate. 850 grammes of strongly acid crystals, 

 recovered from previous electrolyses by acidifying and concentrating the solutions, 

 were purified by repeated crystallisation only, without any attempt to remove the 

 acid by evaporating on the water-bath, by drying in an oven, or by fusing. After 

 four crystallisations the nitrate was found to be neutral to litmus, and after one 

 further crystallisation it was dissolved in water (113 grammes AgNO 3 in 750 cub. 

 centims. of water of low conductivity) and electrolysed. The electrochemical 

 equivalent of the sample was found to be 1 '11827 milligrammes per coulomb (24a). 



h The insoluble residue referred to was suspended in very minute particles in the liquid, to which it 

 imparted a red colour suggestive of a colloidal metal. It was found to be platinum, and 0-0136 gramme 

 was obtained from 185 grammes of electrolytic silver. The impurity, therefore, was probably not a 

 product of electrolysis, but may have been derived from the platinum bowls or from the platinum spatula 

 which was used to remove the electrolytic silver. 



