922 
ARSENIC—ARSENICAL ACID. 
alkali ; and why arsenic does not decompose common salt, though it 
decomposes nitre. 
But the whole of this fubject has been cleared up by the experi 
ments of Scheele, who made much more progress in discovering the 
nature of arfenic, and has given us principles by which all the phe 
nomena are explained. He learned, by a feries of instructive experi 
ments, that one reaſon why the crystallizable arsenical salt, and the 
common arfenicated alkali, have not the fame property, is, that the 
arsenic in the crystallizable arfenical falt has undergone a change from 
the ſtate of common white arsenic ; the acid of the nitre having acted 
upon it as it does upon fugar and some other substances, so as to 
change it into an acid. Of this he gave the most fatisfactory demon 
ſtration, by applying the nitric acid to white arsenic by other different 
ways, by which he changed it into an acid, which he obtained sepa 
rate from any other matter; and afterwards, combining this acid with 
the vegetable alkali in fufficient quantity, he formed a perfectly crys 
tallizable arsenical ſalt. 
He contrived two processes by which he changed white arsenic into 
an active acid. The first of these is entirely an imitation of the pro 
cess by which sugar is changed into an acid, with this difference only, 
that some muriatic acid is first employed to dissolve the arsenic, that 
the nitric acid may act on it with more advantage. 
Scheele’s process is as follows : Into a tubulated retort, fitted with a 
receiver, put two parts of powdered white arsenic, and seven of mu 
riatic acid, and diffolve by a gentle boiling heat. When all is dissolved, 
pour back what is in the receiver, and add three and one half parts 
aquafortis, and distil. The nitric acid rises in red fumes, and after 
ſome time they cease. Now add one part arsenic, and one and a half 
aquafortis. Red vapours arise again. Distil to dryneſs, and make the 
retort red hot. 
In the retort you have the arsenical acid, fixed in the fire, and deli 
quescent in the air, and soluble in twice its weight of water,