128 Mr. Smithson on Arsenic and Mercury. [Ave. 
potash, arseniate of potash is produced, of which the solution 
affords a brick-red precipitate with nitrate of silver. 
In cases where any sensible portion of the potash of the nitre 
has become set free, it must be saturated with acetous acid, 
and the saline mixture dried and redissolved in water. 
So small is the quantity of arsenic required for this mode of 
trial, that a drop of a solution of oxide of arsenic in water, which, 
at a heat of 54-5° Fahr. contains not above 1-80th of oxide of 
arsenic,* put to nitrate of potash in the platina spoon and fused, 
affords a considerable quantity of arseniate of silver. Hence 
when no solid particle of oxide of arsenic can be obtained, the 
presence of it may be established by infusing in water the mat- 
ters which contain it. : 
The degree in which this test is sensible is readily determined. 
With 5:2 grains of silver, I obtained 6:4 grains of arseniate of 
silver ; but 0°65 grain of silver was recovered from the liquors, 
so that the arseniate had been furnished by 4°55 grs. of silver. 
In a second trial 7-7 grains of silver, but of which only 6-8 
grains precipitated, yielded 9-5 grs. of arseniate. 
The mean is 140-17 from 100 of silver. 
If we suppose 100 of silver to form 107-5 of oxide, we shall 
have 
CRE Of EILVGE. smitniieis =a les ss oes 107-50 
Acid, of arsenic, . 0 Bases oe are 8ie 90: 
Consequently 1 of acid of arsenic will produce 4:29 of arse- 
niate of silver; 1 of white oxide of arsenic, 4:97; and 1 of 
arsenic, 6°56. 
Of Mercury. 
All the oxides and saline compounds of mercury laid in a drop 
of marine acid on gold with a bit of tin, quickly amalgamate the 
old. 
3 A particle of corrosive sublimate, or a drop of a solution of it, 
may be thus tried. The addition of marine acid is not required 
in this case. 
Quantities of mercury may be rendered evident in this way 
which could not be so by any other means. 
This method will exhibit the mercury in cinnabar. It must be 
previously boiled with sulphuric acid in the platina spoon to 
convert it into sulphate. 
Cinnabar heated in sclution of potash on gold amalgamates it. , 
A most minute quantity of metallic mercury may be disco- 
vered in a powder by placing it in nitric acid on gold, drying, 
and adding muriatic acid and tin. 
A trial [ made to discover mercury in common salt by the 
present method was not successful, owing, perhaps, to the small- 
ness of the quantity which I employed. 
Iam, Sir, yours, &c. James SMITHSON. 
% Chimie de Thenard, ii p. 167. 
