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. 47), and made perfectly vacuous by raising and lowering the reservoir. When this is effected, the tube SS′P 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. A bubble or two of oxygen is now passed into the eudiometer; if nitric oxide is present, red fumes at once develop. On absorbing the excess of oxygen and the nitric peroxide by alkaline pyrogallate, and measuring the residual gas, it is easy to calculate how much nitric oxide was originally present, according to the principles laid down in “Foods,” p. 587. It is also obvious that, by treating nitric oxide with oxygen, and absorbing the nitric peroxide present by an alkaline liquid of known strength and free from nitrates or ammonia, the resulting solution may be dealt with by a zinc copper couple, and the ammonia developed by the action of the couple directly estimated by titration by a decinormal hydrochloric acid, if large in quantity, or by “_nesslerising_,” if small in quantity. Crum’s method of estimating nitrates (“Foods,” p. 568) in the cases of minute stains on fabrics, &c., with a little modification, may be occasionally applicable. IV.--Acetic Acid. § 90. In the ten years ending 1893 nine deaths (four males and five females) occurred in England and Wales from drinking, by mistake or design, strong acetic acid. A few cases only have been recorded in medical literature although there have been many experiments on animals. The symptoms in the human subject consist of pain, vomiting, and convulsions. In animals it causes colic, paralysis of the extremities, bloody urine, and œdema of the lungs. The lethal dose for plant-eating animals is about 0·49 gramme per kilo.