438 
MEDICO-LEGAL CONTRIBUTIONS ON ARSENIC. 
materials used in the following operations must be free from 
chlorine, otherwise a portion or the whole of the arsenic, 
according to the quantity present, will be converted into the 
chloride and may be volatilized. If the precipitate by sulph- 
ydric acid be of a yellow colour, it may contain arsenic, antimo¬ 
ny, or tin; if of a dark colour, approaching to blackness, it may 
contain copper, lead, bismuth, or mercury, together with the 
metals first named. It is now to be treated with ammonia 
and sulphide of ammonium, which will dissolve the sulphides 
of arsenic, antimony, and tin. The ammoniacal solution 
may be poured upon the filter (stirring the precipitate mean¬ 
while with a feather or with a glass rod covered with 
caoutchouc) until all soluble matter is removed. The preci¬ 
pitate is then to be washed, and the first filtrate, mixed with 
the washings, is to be evaporated to dryness at a moderate 
temperature. The dry residue is to be treated with fuming 
nitric acid, and gently evaporated. This operation is to be 
repeated until the residue, while moist, is of a clear yellow' 
colour. A few' drops of water are to be added, and gradually 
a solution of carbonate of soda to neutralise the acid. A 
mixture of two parts of nitrate of soda and one part of 
carbonate of soda is then added, the whole is carefully mixed 
together and dried, then placed in a porcelain crucible, and 
gradually heated to fusion. By this operation all organic 
matter has been destroyed, and the metals present oxidized ; 
the antimony is converted ir.to antimoniate of soda, the tin 
into binoxide of tin, and the arsenic into arseniate of soda; 
the latter only is soluble in cold water. The fused mass, 
when cold, is therefore treated with cold water, and the 
solution placed in a porcelain dish ; sulphuric acid is then 
added until the liquid is strongly acid. This expels any 
nitric and nitrous acids that may be present; it is then 
evaporated, and, when quite concentrated, more sulphuric 
acid is added. If no red lumes appear uj)on tlie last addition, 
it has been added in sufficient quantity ; if they are produced, 
the operation must be repeated. The solution, finally, must 
be evaporated until the white fumes of sulphuric acid are 
given off. Any arsenic present is now in solution as arsenic 
acid. It is desirable, in order to determine the weight of 
arsenic present, to convert it into arseuious acid ; tliis is 
readily done by adding concentrated suljjhurous aciil solution 
until its odour is distinctly percejjtible, then evaporating 
the liquid until its odour disappears. 
