of the combinations of phosphorus . 325 
To ascertain the composition of phosphorous acid, I used a 
new method, that of converting the perchloride of phosphorus, 
or perphosphorane by phosphorus, into the chloride which 
affords phosphorous acid by the action of water. This is easily 
done by heating them together in a close retort ; and it enables 
us to determine with certainty, which opinion is correct, that 
assuming the oxygen in phosphorous acid to be 3, or that 
which supposes it to be 2.5, the oxygen in phosphoric acid 
being 5. 
5 grains of phosphorus were converted into perchloride in 
a small retort of the capacity of 6 cubical inches : it was 
necessary to exhaust this retort twice to remove the residual 
common air mixed with the chlorine, and some perchloride 
must have been lost during this process. A small quantity 
of chlorine, which could have been little more than sufficient 
to compensate for the loss, remained in the retort. 5 grains 
of phosphorus were introduced, and the retort suffered to 
remain filled, principally with common air ; heat was very 
slowly applied ; all the phosphorus, except an atom not so 
big as the head of a small pin, disappeared, and a little of the 
sublimate still remained, when the retort burst from the ex- 
pansion of the vapour of the new chloride formed ; but the 
chloride found on the fragments was pure, and held no phos- 
phorus in solution. 
A second experiment was made in a retort of the capacity 
of 1 1 cubical inches. 5 grains of phosphorus were converted 
into perchloride : the retort was twice completely exhausted, 
by which at least a grain and a half or two grains of perchlo- 
ride must have been lost. 5 grains of phosphorus were 
introduced ; a little of the sublimate was lost by falling into 
