56 POISONS : THEIR EFFECTS AND DETECTION. [§ 38 . 
need only here consider tin, antimony, and arsenic, and these are best 
separated by Carnot’s method. 1 The ammonium sulphide solution is 
made acid by means of HC1, and the dissolved sulphides (with sulphur) 
are thrown down ; these are collected on a filter, and dissolved by 
means of either HC1 and potassic chlorate or HC1 with a little nitric 
acid ; to the solution is added ammonium oxalate and ammonia, but 
not in sufficient quantity to form a precipitate. The clear solution is 
heated to ebullition and a solution of sodic hyposulphite added ; this 
throws down (with sulphur) the red sulphide of antimony, should 
antimony be present. On separating the precipitate by filtration the 
filtrate is made acid by hydrochloric acid, and a current of hydric 
sulphide passed through, and well boiled to get rid of sulphur dioxide ; 
any arsenic is precipitated as yellow sulphide, and any tin remains in 
solution ; the latter may be separated as bisulphide by saturating the 
filtrate with ammonia, then adding ammonium sulphide and, after a few 
minutes, acetic acid. 
( b) Precipitate from acetic H 2 S solution. See Zinc. 
(c) Precipitate from ammonium sulphide. See Chromium , Nickel , 
Cobalt , etc. 
(d) Examine the filtrate for alkalies and alkaline earths. Any 
residue remaining after destroying organic matter and dissolving in 
acids may be specially treated for the detection of silver and barium 
salts, should these be present in an insoluble form. 
The residue is dried and intimately mixed with three times its 
weight of a mixture containing two parts of sodic nitrate and one part 
of sodium hydrate. This is placed, little by little, in a red-hot porcelain 
crucible and melted. The melted mass is cooled, dissolved in a little 
water, a current of C0 2 passed through the solution to convert any 
caustic soda into carbonate, and the solution boiled. The insoluble 
portion consists of carbonates of lead and baryta, and of metallic silver. 
The mixture is filtered ; the insoluble residue on the filter is warmed 
for some time with dilute nitric acid ; the solution of nitrates of silver, 
lead, and barium is concentrated on the water-bath nearly to dryness 
so as to get rid of any excess of acid, and the nitrates dissolved in water ; 
then the silver is precipitated by hydrochloric acid, the lead by SH 2 , 
and the barium by sulphuric acid. 
VII.—Microscopic Investigation of Crystals. 
§ 38. Of late years much attention has been paid to the microscopic 
investigation of crystals. For this purpose a petrological microscope 
is necessary. A good microscope of the kind has a circular rotary 
1 Traiti d* analyse des substances miner ales, Paris, 1898. 
