On definite Proportions. 379 
with a minimum of sulphur, in the supersulphates with a 
maximum. The phosphites answer to combinations in 
which the phosphorus amounts to once and } or twice as 
much as in the phosphurets at a minimum, accordingly as 
the quantity of phosphoric acid, which saturates a basis, 
contains once and 4 or twice as much oxygen as the quan- 
tity of phosphorous acid ‘* that saturates the same basis ;” [or 
rather that is formed from the same quantity of phosphorus.]} 
The phosphates follow, in all probability, the same propor- 
tions as the phosphorets at a minimum. We should expect 
that the same would happen in the two kinds of salts 
containing arsenic: but if we calculate from the two salts 
of lead the respective quantities of arsenic united with 
100 parts of lead, we shall find, in the arsenite 67°578, and 
in the arseniate 29°943 of arsenic. The latter number is 
less than the half of the former; and since twice 29°943 is 
59°886, the deficiency 7*7 is exactly equal to the quantity 
of oxygen taken up by 100 parts of lead. IF it is true that 
the arsenious acid contains 2 as much oxygen as the arsenic, 
and saturates a base containing + as much oxygen as it- 
self, we must indeed never expect to find twice as much 
arsenic in the arsenite formed from a given quantity of lead 
as in the arseniate. This made me doubt the accuracy of 
my experiments, but they have afforded me the same re- 
sults upon a repetition. And since any other law for the 
capacity of saturation of the acids would suppose a very 
great difference in the results, it appears to me demon- 
strated, that a small error in the experiments cannot have 
any essential influence on the conclusions here drawn from 
them. 
We cannot however doubt that arsenic combines with 
the other metals, exactly as sulphur does, in determinate 
proportions ; for in the natural arseniurets, for example 
those of cobalt or of iron, the component parts are mani- 
festly always united with each other in the same propor- 
tions. But if arsenic dues not follow the same progressive 
steps of combination with other metals, as with oxygen, 
such irregularities must occur, in the same manner as I 
have shown that they are observable with respect to sulphur 
and iron, in the subsulphate of the oxide of iron, But these 
irregularities will perhaps no longer be entitled to that ap- 
pellation, if we succeed in discovering, at some future time, 
the lowest stages of combinatign. 
There can be no doubt that the other metals combine 
also with each other in determinate proportions, although 
the possibility of fusing most of them together, in all 
propor- 
