OF ARTS AND SCIENCES. 49 



which escaped from the porcelain tube. It was evident from the first 

 that, as ah-eady stated, the reduction could be at least nearly completed 

 below visible redness ; and, as we now know, argentic sulphide can be 

 perfectly reduced at this low temperature, but in all the earlier deter- 

 minations we found it necessary to raise the temperature at the end of 

 the process to a full red heat, and continue the heating for several 

 hours before the untarnished lead paper indicated that the evolution of 

 hydric sulphide had ceased. We subsequently discovered that this 

 effect was due to the presence of the finely divided silver, determining 

 a reducing action of the hydrogen on certain impurities which the gas 

 had contracted from the crude sulphuric acid, and which had escaped all 

 the purifiers. These impurities contained both nitrogen and sulphur, 

 and, when reduced by hydrogen in presence of the metallic silver, 

 yielded both ammonia — to which we have already referred — and also 

 the hydric sulphide which had misled us in regard to the completion of 

 the reduction. On passing the gas through a glass tube containing 

 platinum sponge, heated to low redness, the effect was still more 

 marked ; and, on placing this tube after the first alkaline purifier, the 

 products just named were evolved in abundance, although previously, 

 even at this point, the gas produced no effect on " lead paper." We thus 

 traced the impurity back to the hydrogen generator, and were able to re- 

 move it by placing in the line of the purifiers a glass tube filled with plati- 

 num sponge, and heated by a combustion furnace. The tube was placed, 

 as above described, after the alkaline purifier ; and from this the gas was 

 passed through several purifiers containing a solution of nitrate of 

 silver, and through driers containing in part sodic hydrate and in part 

 calcic chloride. Afterwards, the nitrate of silver in the purifiers was 

 replaced by alkaline and acid solutions of potassic permanganate, as 

 recommended by Schobig,* which were, at least, equally efficient. The 

 crude acid used in the generator was found to contain an unusual 

 amount of nitric acid, and the impurity contracted by the hydrogen was 

 probably some volatile compound of oxygen nitrogen and sulphur, 

 similar to that with which we are so familiar in the sulphuric acid 

 chambers. Misled, as we have seen, by the indications of sulphur, we 

 continued the reduction — in the experiments whose results have been 

 given on page 47, — far longer and at a far higher temperature than was 

 necessary ; and the apparent increased value of the atomic weight 

 obtained was due to a slight volatilization of the metallic silver. 

 Except in one of the experiments, the temperature had never reached 



* Jour. pr. Ch. (2), xiv. 289, Oct., 1876. 



VOL. XIII. (N. 8. V.) 4 



