594 ON DETECTING THE 
If diluted sulphuric acid or hydro-chloric acid 
be allowed to act upon zinc combined with arsenic, 
or upon zinc in contact with a solution of arsenious 
acid, or other compound of arsenic, the gas gene- . 
rated is arsenuretted hydrogen—hydrogen hold- 
ing arsenic in combination. When, therefore, Mr. 
Marsh has a liquor for examination which is sus- 
pected to contain arsenic, and which could not 
easily be made sufficiently clear, and free from 
objectionable matter, to admit of being tested by 
the other usual means, or when he has solid mat- 
ter, such as pastry, pudding, or the like, which, 
on being treated with water, gives such a liquor, 
he mixes the liquor with diluted sulphuric acid, 
and allows the mixture to act upon pure zinc, 
whereby hydrogen gas is produced, to which the 
arsenic is transferred, if any was present in the 
matter the subject of examination. The gas col- 
lected he causes to burn from a jet; and in con- 
tact with the flame holds a piece of cold window 
glass, or the like, on the surface of which a thin 
film of metallic arsenic immediately deposits when 
that metal was present in the matter suspected to 
contain it; or, having set fire to the gas as it 
issues from the jet, he receives the flame within 
a glass tube open at the ends, which becomes 
dimmed by a white powder, if arsenic be present. 
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