On definite Proportions. 375 
nious acids, according to which the former contains from 
50 to 56, and the latter 33°33 of oxygen to 190 of metal; 
or the arsenic acid consists of * metal and + oxygen, and 
the arsenious of 3 metal and ! oxygen. If however the 
composition of the latter is rightly determined, the former 
must contain, according to the laws which J have disco- 
vered, either 50 or 66 parts of oxygen, that is, either once 
and + or twice as much as the latter. 
In order to ascertain this point, I dissolved 10 grammes 
of metallic arsenic in nitric acid, evaporated the solution, 
dissolved the acid in a very little water, and mixed it in a 
platina crucible with a solution of 30 gr. of oxide of lead 
in nitric acid. I evaporated the mixture to dryness, and 
ignited it: the residuum after ignition’ weighed 44°95 gr. 
Consequently 100 paris of metal had taken up 49°5 of oxy- 
gen. The same experiment, repeated with 3 gr. of arsenic, 
afforded 4-5 [4:45?] of arsenic acid; so that 100 parts of 
metal had taken up 48-3 of oxygen. In another repetition, 
a single gramme of the metai afforded 1°53 gr. of the acid, 
The experiments, which I performed in this manner, in 
order to avoid the presence of water, afforded therefore very 
different results: partly because the arsenic acid retains in 
them a little muriatic acid, which is expelled with the prot- 
oxide of lead; partly because the two acids, which are at 
liberty, begin to contend with each other in a high tem- 
perature for the protoxide, whence a little of the uncom- 
bined arsenic acid is decomposed and dissipated. In order 
to satisfy myself that metallic arsenic contains no hydrogen, 
that could have influenced the result, I heated some of it 
with some oxide of tin in a small glass retort ; some traces 
of moisture appeared in the operation, but they were too 
slight to be estimated by weight: and in the neck of the 
retort some arsenious acid had been sublimed. Although 
all these experiments afford no very accurate determina- 
tion of the quantity of oxygen in the arsenic acid, they still 
sufficiently show that 100 parts of metal cannot be com- 
bined with 66 of oxygen in it, and consequently that the 
arsenic acid can only contain half as much more oxygen 
than the arsenious. In order to assure myself more fully 
of the composition of these acids, I examined their com- 
binations with the protoxide of lead. 
Arsenite of the protoxide of lead. 1 dissolved 20 grammes 
of protoxide of lead in nitric acid, and evaporated the so- 
jution, in order to expel the superfluous acid. The nitrate 
of the protoxide, dissolyed in water, was mixed with the 
arsenite of potass, as long as a precipitate appeared ; this 
Aaé4 arsenite 
