FORMATION OF ARSENICAL ACID. 
423 
Mr. Pelletier, however, fays that we succeed equally well by using 
the nitric acid alone, in the proportion of six parts of the acid to one 
of the white arsenic. 
The acid comes off in red fumes of nitrous gas, 
and the white arsenic afsumes the true characters of the arsenical acid 
(to it is now called). It muſt be kept a good while in a ſtrong heat, 
to expel all the redundant nitrous acid. (Fourcroy II. 507. Ed. 1786.) 
This is an exact enough account of the phenomena, and perfectly 
instructive in the nature of the operation; but having practised both 
methods, I agree with Dr. Scheele, that his previous solution in mu 
riatic acid enables the nitric acid to act on a much greater quantity 
of the arsenical oxyd. 
The manner in which this process produces its effect is sufficiently 
evident. White arsenic muſt be considered as a metallic oxyd, contain 
ing a very moderate quantity of oxygen, and capable of a higher de 
gree of oxydation. I always viewed it in this light; and on this 
principle, I explained Mr. Macquer’s experiments, and the effects 
which it produces with nitre; a part of which is the change of the nitric 
acid into nitrous acid, in consequence of the abstraction from it of a 
part of its oxygen, attracted by the arsenic. In Mr. Scheele’s process, 
the nitric acid alone supplies oxygen to the arsenic, and thus oxydates 
it to the greatest degree of which it is capable; in which high state of 
oxydation, the abundance of oxygen which it contains gives it the 
qualities of an acid, and deprives it of attraction for other acids, but 
dispoſes it to unite ſtrongly with alkalis. This is the proper explica 
tion of the process according to the principles of the new theory; 
and there is an experiment described by Mr. Scheele, which gives 
great support to the French explication of the phenomena, and esta 
bliſhes it without a doubt. This experiment is made with the acid 
of arsenic. If some of this acid be put into a retort by itself, and ex 
posed to the action of heat alone, it endures a low red heat without 
change, or is only melted. But if the heat be increased, and con 
tinued, the greater part of the acid arises gradually into the neck of