1889-90.] Prof. Letts and Mr Blake on Phosphorus. 383 
At the outset we experienced some difficulty in obtaining pure 
oxide of copper ; for any commercial samples we examined, even 
when called “pure” by the manufacturer, were invariably found 
to contain phosphorus or arsenic, often in considerable quantity. 
We therefore prepared it for ourselves, as follows : — 
Commercial sulphate of copper was dissolved in water, and the 
solution saturated with chlorine. On evaporation and crystallisa- 
tion, fairly pure crystals were obtained, but we preferred to re- 
crystallise them once. The cold saturated solution of this salt was 
then electrolysed in a large beaker or shallow glass dish, the elec- 
trodes being platinum plates, of the size generally used in the cell 
of a Grove’s battery. The gas engine used to drive our dynamo was 
1J horse-power, and with it we obtained about 100 grms. of pure 
copper in 7-8 hours. The copper was then dissolved in pure 
nitric acid in a platinum dish, the solution evaporated, and the 
residual nitrate of copper calcined at a dull red heat with constant 
stirring. The resulting oxide formed a black crystalline powder. 
In making the phosphorus determination, a narrow tube about 
50 cm. long and 7-8 mm. bore was employed, which was found to 
contain, when properly charged/about 25 grms. of oxide of copper. 
It was drawn out in the usual way, and sealed at the posterior end ; 
then charged first with about 5 cm. oxide of copper, afterwards 
with about 10-15 cm. of the mixture of substance and oxide, and 
finally with the pure oxide, the whole being kept in its place by a 
plug of copper gauze. The combustion was then proceeded with 
in the ordinary manner, and a current of oxygen passed at its 
conclusion. 
When cold, the tube was removed from the furnace, the plug of 
copper gauze hooked out, and the remainder of the contents shaken 
into a beaker. The tube was then washed two or three times with 
hot nitric acid, to dissolve every trace of adhering oxide of copper. 
To do this, the broken (tail) end was sealed up again, the tube filled 
with nitric acid, and heated in front of an ordinary fire. The nitric 
acid washings were added to the contents of the tube, which had 
been shaken into the beaker, and a further quantity of the same acid 
added — sufficient to dissolve the whole of the oxide of copper, for 
which about 100 c.c. in all were usually required. Solution of the 
oxide was effected by boiling. When all had dissolved, about 200 
