608 poisons : their effects and detection. [§ 777. 
cooling arrangement adjusted. After ten to fifteen minutes the evolution 
of gas has become somewhat slow, and the solution to be tested for 
arsenic is introduced by means of the bulb funnel, little by little, into 
the flask. The bulb is washed out at first with 20 c.c. of dilute 
sulphuric acid (10 per cent.), and then with 10 c.c. of the 1 to 4 acid, 
taking care that the acid only falls into the apparatus drop by drop. 
The most suitable evolution of gas is found to be from 4 to 5 c.c. per 
minute : a guess of the quantity evolved may be made by counting the 
bubbles of gas escaping through the final water in V. 
§ 777. Precipitation as Tersulphide. — The advantages of the 
processes described are great when dealing with minute quantities, 
but the old method of precipitation with hvdric sulphide, SH 2 , is 
best for quantities of arsenic which admit of being directly weighed. 
If this be used, it is well in most cases to pass sulphurous 
anhydride through the liquid until it smells strongly of the gas, for 
by this means any arsenic acid present is reduced ; the sulphurous 
anhydride is quickly got rid of by boiling or by a current of carbonic 
anhydride, and then the liquid is saturated with hydric sulphide. In 
the ordinary way, much time is often wasted in saturating the liquid 
with this gas. Those, however, who have large laboratories, and daily 
employ hydric sulphide, possess (or should possess) a water saturated 
with the gas under pressure ; such a liquid, added in equal volume to 
an arsenical solution, is able to convert the whole of the arsenic into sul¬ 
phide in a very few minutes. Sulphuretted hydrogen water should be 
made always by saturating recently boiled and cooled water. It may 
be kept for some time under a layer of olive oil. Those who do not possess 
this hydric sulphide water can saturate in an hour the liquid to be tested, 
by passing the gas in under pressure. A convenient method is to evolve 
SH 2 from sulphide of antimony and C1H ; the gas passes first into a 
wash-bottle, and then into a strong flask containing the solution under 
trial. This flask is furnished with a safety-valve, proportioned to the 
strength of the apparatus ; the two tubes dipping into the wash-bottle 
and the last flask are provided with Bunsen’s valves, which only allow 
the gas to pass in one direction. The hydric sulphide is then driven 
over by heat, and when sufficient gas has in this way passed into the 
liquid, the flame is withdrawn, and the apparatus allowed to stand 
for some hours, the valves preventing any backward flow of the 
liquid or gas. When the precipitate has settled to the bottom, the super¬ 
natant fluid is carefully passed through a filter, and the precipitate 
washed by decantation in the flask, without transference to the filter, 
if it can be avoided. 1 
1 In precipitating small quantities of arsenic by SH 2 , it is advantageous to shake 
with ether or chloroform, which collects the precipitate rapidly (C. E. Carlson, Zeit. /. 
phys. Chem., 1910). 
