140 
MEDICO-LEGAL CONTRIBUTIONS ON ARSENIC. 
tilized and readily oxidizable, and which furnishes, hy oxi¬ 
dation, octahedral crystals soluble in water, is sufficient evi¬ 
dence of arsenic. 
Various plans may he adopted to bring the sulphide of 
arsenic, obtained in the manner just mentioned, into a state 
fit for making the various tests. 
The following method is convenient and simple :—A part, 
or the whole, of the sulphide of arsenic, according to the 
quantity obtained, is to he placed in a small porcelain dish, 
and concentrated nitric acid added and evaporated. This 
operation is to he repeated a number of times. The residue 
is to be repeatedly moistened with water, and as often eva¬ 
porated ; by this means every trace of nitric acid is finally 
removed (a condition essential to the success of the succeed¬ 
ing operations); the sulphide of arsenic is by this treatment 
converted into arsenic acid. The mass remaining in the 
dish is dissolved in a small quantity of water, and, if par¬ 
ticles of sulphur are present, it is to be filtered, the filter 
thoroughly washed, and the washings mixed with the original 
solution. The mixed fluid may be divided into two parts, 
yl and £. 
A is to be divided into two parts, which may be used 
respectively in (1) Marshes apparatus, and (2) according to 
Ileinsch’s method. 
B is to be evaporated nearly to dryness, then thoroughly 
mixed with twice its bulk of carbonate of soda, and gently 
heated until perfectly diy. It may then be divided into 
two parts, to be used respectively in (1) Fresenius’ and 
Babo’s apparatus, and (2) in the ordinary reduction tubes. 
3. Marshes apparaUis» —Mr. Marsh, of Woolwich, in the 
year 1836, proposed an exceedingly simple and delicate 
method for the detection of arsenic. The principle upon 
which it is founded may be briefly stated as follows:— 
in the presence of nascent hydrogen, arsenious and 
arsenic acids are deoxidized, water is formed, and the me¬ 
tallic arsenic combines with hydrogen, forming arseniuretted 
hycl rogen gas (AsOy + 6H, = AsHg + 3HO). The simplest 
form of Marsh’s apparatus consists of a bottle closed with a 
cork containing two perforations, in which are placed a 
funnel tube and a tube bent at right angles, drawn out to a 
point at the free end,—zinc and water being placed in the 
bottle, and a little sulphuric acid added, hydrogen will be 
evolved; if now a solution of arsenious or arsenic acid 
be added, arseniuretted hydrogen will be formed. This gas 
is inflammable, burning with a bluish-white flame. If a cold 
porcelain plate be held in the flame, a brownish stain of 
