§ 37-1 PROCEDURE IN SEARCHING FOR MINERAL POISONS. 55 
duced ; the vapours are caused to pass first into a flask cooled with 
water, then through bulbs or other apparatus, the final portion of which 
is charged with a weak solution of potash. The action is aided by 
heat ; the addition of sulphuric acid is continued until no more yellow 
vapours are produced and the carbon disappears. The gas is decom¬ 
posed by the first washing of water into hydrochloric acid and chromic 
acid, Cr0 2 Cl2+H 2 0=Cr0 3 +2HCl. The heat being continued, sulphur 
dioxide is produced, which, in its turn, reduces the chromic acid. The 
final green solution, with the various washing waters, are freed from 
sulphur dioxide and submitted to hydric sulphide, while the non¬ 
volatile metals are tested for in the residue remaining in the flask, 
the residue for this purpose being exhausted with hot water, and the 
solution filtered. 
In all cases the amount of acidity of the solution of the inorganic 
salts should be ascertained bv titrating with normal soda an aliquot 
part of the same. 
The liquid is now saturated with a current of gaseous sulphuretted 
hydrogen until it smells strongly of the gas. The flask should now be 
corked and set aside for at least twelve hours, any precipitate is filtered 
off, the liquid is shaken and warmed to expel the excess of sulphuretted 
hydrogen. Sodic acetate is now added in slight excess of the acidity, 
as determined by the titration above mentioned, so as to replace the 
mineral acid by acetic acid. For every 10 c.c. of normal soda 1*36 
grm. of sodic acetate in theory would exactly replace the mineral acid. 
For example, supposing that the original liquid measured 510 c.c., 
10 c.c. of which was neutralised by 5 c.c. of normal soda, then the 
proper quantity to add of sodic acetate to the 500 c.c. would be 34 
grms. to exactly replace the acid, and an extra couple of grms. so as to 
ensure an excess—in all, 36 grms. The liquid is now again saturated 
with sulphuretted hydrogen in order to throw down any zinc as sulphide. 
Should a precipitate occur, this is filtered off, and the filtrate 
saturated with ammonium sulphide. 
(a) Precipitate from acid H 2 S solution .—From the acid solution, 
the sulphide of a large number of substances may theoretically be 
present in the precipitate, e.g. arsenic, antimony, tin, germanium, 
molybdenum, selenium, tellurium, gold, platinum, iridium, silver, 
mercury, lead, bismuth, copper, cadmium, palladium, osmium, rhodium, 
and ruthenium ; but it is obvious that many of the above substances 
are not likely to occur in a routine toxicological investigation. These 
sulphides are treated with ammonium sulphide, which dissolves the 
members of the above group up to and including iridium, and leaves 
insoluble the remainder, which may be searched for in the ordinary 
manner (see Lead, Mercury, etc.). 
With regard to the sulphides soluble in ammonium sulphide, we 
