14 Prof. R, Threlfall on the Electrical 



slightly, and was boiled under diminished pressure for two or 

 three hours before use. The solution still gave a distillate 

 with sulphuric acid which reacted with starch and iodine, 

 but after a time so long that it was possible that the exposure 

 to air during the experiment had been sufficient to supply the 

 nitrite discovered. In fact a control experiment made with 

 some ammonia, known to be free from nitrites or nitrates, 

 yielded a distillate which coloured to nearly the same extent 

 on standing. The solution was made up to the strength re- 

 commended for use in organic analysis. 



Silver Nitrate. — Crystals of silver nitrate bought as pure 

 were dissolved in platinum distilled water to make a ten-per- 

 cent, solution ; to this, when in the tube, a little of the potash 

 solution was added — so as to form a little silver hydroxide. 

 This hydroxide darkened gradually in the bright light to 

 which it was exposed. It was not considered necessary to 

 investigate the solution for traces of free nitrous compounds, 

 as the gas on leaving it passed through another potash-tube. 



Solid Potash and Potash- Lime. — The above-mentioned 

 stick potash was used, and the potash-lime was made by 

 adding a little of the solution to some marble-lime prepared 

 in the laboratory ; the drying process was carried out in a 

 clean iron dish. 



Phosphorus Pentoxide. — This, being the last reagent tra- 

 versed by the nitrogen, requires to be exceptionally carefully 

 treated. I began by acting on a hint obtained in 1889 from 

 Prof. Josiah P. Cook, of Harvard, and endeavoured to pre- 

 pare a pure sample of the pentoxide by burning phosphorus 

 in a very strong draught of air, rather than by attempting to 

 purify the ordinary commercial reagent. For this purpose 

 an elaborate sheet-iron cylindrical chamber, measuring 3 feet 

 by 2 feet, was prepared and fitted with suitable contrivances 

 for carrying out the combustion of several pounds of phos- 

 phorus. The air-blast was obtained from a Root's blower 

 worked by a gas-engine, and the filtered air travelled through 

 a tin tube about 4 feet long and 7 inches in diameter, filled 

 with calcium chloride in the state usually described as "rough 

 dried." After many attempts a large quantity of excellent 

 pentoxide was obtained, which, however, though indefinitely 

 better than the commercial product, still retained traces of 

 phosphorus. The tests I employed for the purpose of dis- 

 covering whether the pentoxide contained free phosphorus or 

 not were the following : — (1) Dissolving 10 or 15 grammes 

 weight of the pentoxide in clear distilled water and observing 

 (a) whether anything remains undissolved, (/3) whether, at 

 the moment of throwing the phosphorus pentoxide on the 



