b, Funnel for supplying the bottle with acid.
c, c, Supports.
d, Tube filled with asbestos.
e, Bent tube to convey the liberated gas.
f, Glass vessel containing a solution of nitrate of silver.
Marsh’s Test. Some of the suspected liquid is mixed with dilute sulphuric acid until strongly acid, and is then poured upon some pure granulated zinc, or clippings or other small pieces of zinc, previously placed in the apparatus; hydrogen gas is immediately evolved, and, if arsenic be present, unites with it, forming arseniuretted hydrogen gas, which escapes by the aperture b (see engr.), and may be recognised as follows:—
It possesses a garlic-like odour.
It burns with a bluish-white flame and emits a whitish smoke.
a, a, Bent glass tube, containing dilute sulphuric acid, zinc, and suspected liquid. b, Stop-cock and jet. c, Plate of glass to receive the stain. d, Support. e, e, Bands to keep the tube upright.