120 POISONS : THEIR EFFECTS AND DETECTION. [§ 95 . 
1. Nitrates, treated with mercury or copper and strong sulphuric 
acid, develop nitric oxide, recognised by red fumes, if mixed with air 
or oxygen. 
2. A nitrate dissolved in a small quantity of water, with the addition 
of a crystal of ferrous sulphate (allowed to partially dissolve), and then 
of strong sulphuric acid—poured through a funnel with a long tube 
dipping to the bottom of the test tube, so as to form a layer at the 
bottom—strikes a brown colour at the junction of the liquid. When 
the test is properly performed, there will be three layers—the upper¬ 
most being the nitrate solution, the middle ferrous sulphate, and the 
lowest sulphuric acid ; the middle layer becomes of a smoky or black 
hue if a nitrate is present. Organic matter interferes much with the 
reaction. 
3. Nitrates in solution, treated in the cold with a zinc-copper couple, 
are decomposed first into nitrites, and then into ammonia. The nitrites 
may be detected by a solution of metaphenyldiamine, which strikes a # 
red colour with an infinitesimal quantity. Hence, a solution which gives 
no red colour with metaphenyldiamine, when submitted to the action 
of a zinc-copper couple, and tested from time to time, cannot contain 
nitrites ; therefore no nitrates were originally present. 
4. Nitrates, on being treated with strong sulphuric acid, and then 
a solution of indigo carmine dropped in, decolorise the indigo ; this is 
a useful test—not conclusive in itself, but readily applied, and if the 
quinine method of separation has been resorted to, with few sources 
of error. 
There is a process of separating nitric acid direct from any organic 
tissue, which may sometimes be useful :—Place the substance in a strong, 
wide-mouthed flask, closed by a caoutchouc cork, and in the flask put 
a small, short test tube, charged with a strong solution of ferrous 
chloride in hydrochloric acid. The flask is connected to the mercury 
pump (see fig., p. 50), and made perfectly vacuous by raising and 
lowering the reservoir. When this is effected, the tube is adjusted so 
as to deliver any gas evolved into a eudiometer, or other gas-measuring 
apparatus. By a suitable movement of the flask, the acid ferrous 
chloride is allowed to come in contact with the tissue, a gentle heat 
applied to the flask, and gases are evolved. These may be carbon 
dioxide, nitrogen, and nitric oxide. On the evolution of gas ceasing, 
the carbon dioxide is absorbed by passing up under the mercury a little 
caustic potash. When absorption is complete, the gas, consisting 
of nitrogen and nitric oxide, may be measured. The nitric oxide may 
now be absorbed by a strong solution of sodic metasulphite, and from 
the contraction the nitric oxide determined. 
It is also obvious that, by treating nitric oxide with oxygen, and 
absorbing the nitric peroxide present by an alkaline liquid of known 
