RESEARCH FOR ARSENIC AND ANTIMONY. 
541 
this will be brown, black, or strongly metallic, according to 
the quantity of arsenic present, and the time during which 
the heat is continued. Under these circumstances, hydrogen 
only escapes; but as the end of the tube is still immersed in 
the solution of nitrate of silver, no arsenic is lost. The opera¬ 
tor may, at his pleasure, thus procure three separate metallic 
deposits on the three contracted portions of the horizontal 
tube. Having prepared another right-angled tube with a 
cork to fit the flask, this should be rapidly substituted for 
that in w'hich the deposits are collected. More zinc may be 
added to the flask, if necessary, and the gas evolved should 
be allowed to exhaust itself in strong nitric acid . Not a trace 
of gas escapes through this strong acid :—the whole of the 
arsenic is arrested and converted to arsenic acid, which may 
be obtained by evaporating the nitric acid on a sand-bath. 
Fourth stage— tenting the products, —By the methods sug¬ 
gested, the hydride furnishes, 1st, as a result of decompo¬ 
sition by heat, a deposit or mirror of metallic arsenic; 2d, as 
a result of the action of nitrate of silver, arsenious acid; 3d, as 
a result of the action of nitric acid, arsenic acid. 
1st. The conducting-tube should be divided into as many 
parts as there are metallic deposits. The contracted portion 
of the tube being sealed, the metallic deposit may be gently 
heated, and if it be due to arsenic a ring of octahedral crystals 
of arsenious acid, small but well-defined under the micro¬ 
scope, will be procured. If there are several metallic 
deposits, these may be broken up and heated in a wider 
tube. By this means sufficient arsenious acid may be pro¬ 
cured to yield a solution of arsenic, which will give the 
usual yellow and green-coloured precipitates with the ammo- 
nio-nitrate of silver and the ammonio-sulphate of copper. 
There is, however, ample corroboration in the subsequent 
stages, without resorting to the liquid tests. 2d. Arsenious 
acid is obtained in solution by filtering the decomposed 
nitrate of silver into which the gas has been passed, and by 
precipitating the filtered liquid with a slight excess of pure 
hydrochloric acid, so as effectually to remove the undecom¬ 
posed silver as chloride. When the precipitated chloride has 
completely subsided, the liquid may be again filtered until it 
is perfectly clear and free from any silver salt. In this state 
a current of washed sulphuretted hydrogen gas should be 
passed into it, when the well-known yellow sulphide of 
arsenic will be precipitated in greater or less quantity, 
according to the quantity of arsenic present. Independently 
of the colour of the precipitate, which distinguishes sulphide 
of arsenic from all metals excepting cadmium, it will be 
