of the combinations of phosphorus. 319 
the tube, large quantities of it may be burnt by the heat of 
a spirit lamp in a retort filled with oxygen, and the absorption 
of oxygen and the quantity of phosphoric acid formed may 
be minutely ascertained. 
The accompanying sketch (PI. XVIII.) will give an idea of 
the apparatus. The neck of the little curved tube, or small dis- 
tilling retort, after the phosphorus is introduced, is drawn out, 
and an aperture left of about of an inch ; it should not be 
smaller, or it becomes choaked by the phosphoric acid formed. 
Regulating the heat by raising or lowering the spirit lamp, 
the combustion may be carried on slowly, or rapidly, at 
pleasure. 
Operating in this way, I have often burnt from 5 to 10 
grains of phosphorus without any accident, and ascertained 
exactly the quantity of oxygen absorbed : there is only one 
source of error — a quantity of phosphorus .remains in the 
upper part of the tube, which cannot be burnt except by a 
greater heat than the retort will bear ; and it is difficult to 
ascertain the precise weight of this, as the tube always 
unites with some phosphoric acid where it is red hot at its 
mouth; but this can be only a trifling source of error. 
In these experiments, and in all the others detailed in this 
paper, I received much useful assistance from Mr. Faraday 
of the Royal Institution ; and much of their value, if they 
shall be found to possess any, will be owing to his accuracy 
and steadiness of manipulation. 
Experiment. 1. 
Six grains of phosphorus. The small tube with the phos- 
phorus weighed before the combustion 36.5 grains ; after the 
