368 MESSRS. V. H. VELEY AND J. J. MANLET ON 



PUEIFICATION OF WATER. 



Ordinary distilled water was purified by redistillation with a few crystals of 

 potassium permanganate and a few cubic centimetres of concentrated sulphuric acid 

 instead of potassium hydrogen sulphate, as recommended by STAS. A slow current of 

 purified hydrogen was passed through the water during the operation, which was 

 conducted in a laboratory used for no other purpose. The condensation was purposely 

 kept somewhat imperfect to prevent any ammonia gas from being redissolved in the 

 cool water ; the first and last fifths of the distillate were rejected, and the inter- 

 mediate portion again distilled over barium hydrate to remove any sulphuric acid. 

 On the day before any conductivity experiments were made the water was distilled 

 a third time in a platinum still fitted with a condensing tube and receiver of the 

 same material, and again also with imperfect condensation. It was observed that 

 after several litres of the water had been distilled a deposit of silica, distinctly 

 crystalline, was formed on the still. Finally, a few hours previous to the commence- 

 ment of a series of experiments, the water contained in small bottles was partially 

 frozen (by immersion in a freezing mixture) so as to obtain a hollow tube of ice ; the 

 central core of water was poured off, the ice quickly melted and mixed with a 

 quantity of nitric acid necessary for the concentration required. It is not, of course, 

 presumed that, by the method adopted, water was obtained of the degree of purity 

 arrived at by KoHLRAUSGH and HEYDWEILER ; * indeed, it was not considered 

 necessary to use such precautions, when the solutions to be measured would possess 

 ,i conductivity many million times greater than that of the purified water. 



PURIFICATION OF NITRIC ACID. 



Two different methods were adopted for this purpose according as the concentration 

 required was less or greater than 68 per cent. In the former case, acid of sp. gr. = 1*4, 

 sold as pure, and nearly colourless, was distilled continuously under a pressure of a 

 few millimetres in an apparatus constructed entirely of glass, as to the essential parts, 

 namely, the still, condenser and receiver. The acid could be drawn up into the still, 

 and syphoned off from the receiver by glass tubes . provided with stop-cocks; it was 

 stored in tubes of hard glass (a precaution adopted by STAS in his investigations upon 

 atomic weights) the ends of which were drawn off but not sealed, as it was found 

 that the vapour of the acid given off at ordinary temperatures decomposed to produce 

 nitrous fumes in the operation of sealing. 



When acids of concentration greater than 68 per cent, were required, a proportion 

 of the water was removed by distillation of such an acid with an equal bulk of 

 sulphuric acid ; the pale yellow acid thus obtained was then redistilled with barium 



* ' Zeits. Physikal Chem.,' vol. 14, p. 317. 



