650 POISONS : THEIR EFFECTS AND DETECTION. [§ 827. 
hexagonal plates, if dissolved in hot water, crystallise from the last in 
needles. The stannous chloride reaction is one of the finest among the 
many beautiful micro-chemical reactions, and appears, so far as the 
writers know, not to ha\e been appreciated. 
Lead in solution can be converted into sulphide ; in this case it is, 
however, absolutely necessary that there should be no great excess of 
acid, since as little as 2*5 per cent, of free hydrochloric acid will pre¬ 
vent all the lead going down. On obtaining the sulphide, the latter, as 
already described, can be converted into chloride by hydrochloric acid, 
and the crystalline chloride is extremely characteristic. 
The British Pharmacopoeia (1914) Quantitative Limit Test 
for Lead. 
Apparatus. —Nessler glasses, thin and of lead-free glass, about 
150 mm. long, and of such diameter that the mark indicating a content 
of 50 mm. is at the height of 100 mm. from the base. 
Solutions and Reagents.— The special solutions and reagents for 
the quantitative limit test for lead are distinguished by the letters 
“ PbT.” 
Strong Solution of Lead. —Dissolve 0*16 grm. of lead nitrate 
in distilled water, adding 50 millilitres of nitric acid, and dilute with 
distilled water to 100 millilitres. This solution contains 0-001 grm. 
of lead in 1 millilitre. 
Dilute Solution of Lead PbT. —Dilute 1 millilitre of the strong 
solution of lead PbT, measured from a burette, with distilled water so 
that the resulting solution measures 100 millilitres. This solution con¬ 
tains 0-00001 grm. (-01 mgrm.) of lead in 1 millilitre. 
Solution of Potassium Cyanide PbT. —Dissolve 10 grms. of 
potassic cyanide in distilled water, add 2 millilitres of solution of hydrogen 
peroxide, and make up to 100 millitres with distilled water. This solution, 
after being allowed to stand, when tested by the quantitative limit test 
for lead gives no colour with the dilute solution of lead PbT. 
Solution of Sodium Sulphide PbT. — Dissolve 10 grms. of 
sodium sulphide in distilled water, and make up to 100 millilitres with 
distilled water. 
Mode of Testing (General). —Two solutions of the substances under 
examination are made in hot distilled water :— 
1. The primary solution, containing 12 grms. of the substance. 
2. The auxiliary solution, containing 2 grms. of the substance. 
Each solution is filtered (if necessary), made alkaline with solution 
of ammonia, and treated with 1 millilitre of the solution of potassium 
cyanide PbT. If the colours of the solution differ much, the difference 
may be rectified by the cautious addition of a highly diluted solution of 
