ARSENIC. 
§ 768.] 
593 
stain, the liquid to be tested is run in gradually and the test run for at 
least one hour. 
7. A solution of cadmium sulphate should be added to the contents 
of the flask, to counteract the “ insensitiveness ” of the material. 
The characteristics of the metallic stains which may occur either on 
glass or porcelain in the use of Marsh’s test, may be noted as under :— 
Mirror or Crust of Arsenic. 
Is deposited at a little distance 
from the flame. 
An arsenical stain is in two 
portions, the one brownish, the 
other a glittering black. 
On heating, it is rapidly vola¬ 
tilised as arsenious acid. 
On transmission of a stream of 
SH 2 , whilst immediately behind 
the stain a gentle heat is applied, 
the arsenic is changed to yellow 
sulphide; 1 if dry C1H is now 
transmitted, the arsenical sulphide 
is unchanged. 
Chloride of lime dissolves the 
arsenic completely. 
Protochloride of tin has no 
action on metallic arsenic. 
The arsenic stain, dissolved in 
aqua regia , or C1H and chloride of 
potassium, and then treated with 
tartaric acid, ammonia, and mag¬ 
nesia mixture, gives a precipitate 
of ammonia magnesian arseniate. 2 
Mirror or Crust of Antimony. 
Is deposited close to the flame, 
and on both sides of it, and is 
therefore notched. 
The stain is tolerably homo¬ 
geneous, and usually has a tin¬ 
like lustre. 
Volatilisation very slow; no 
crystalline sublimate obtainable. 
The same process applied in 
the case of antimony produces the 
orange or black sulphide ; and on 
passing dry C1H, chloride of anti¬ 
mony volatilises without the appli¬ 
cation of heat. 
Antimony not affected. 
Dissolves slowly but completely 
the antimony stain. 
No precipitate with antimony. 
1 It is desirable to dissolve away the free sulphur often deposited with the arsenical 
sulphide by bisulphide of carbon. 
2 Schonbein has proposed ozone as an oxidiser of arsenical stains. The 
substance containing the stain, together with a piece of moist phosphorus, is placed 
under a shade, and left there for some time ; the oxidisation product is, of course, 
coloured yellow by SH 2 if it is arsenious acid, orange if antimony. The vapour of 
iodine colours metallic arsenic pale yellow, and later a brownish hue ; on exposure to 
the air it loses its colour. Iodine, on the other hand, gives with antimony a carmelite 
brown, changing to orange. 
An arsenical ring may be also treated as follows :—Precipitated zinc sulphide is 
made into a paste with a little water, and introduced into the end of the tube ; the 
same end is then plunged into dilute sulphuric acid, and the ring heated, when the 
arsenical sulphide will be produced. 
38 
