The solution of cuprous chloride is prepared by dissolving 10·3 grms. of copper oxide in 150 c.c. of strong hydrochloric acid and filling the flask with copper turnings; the copper reduces the cupric chloride to cuprous chloride; the end of the reduction is known by the solution becoming colourless. The colourless acid solution is poured into some 1500 c.c. of water, and the cuprous chloride settles to the bottom as a precipitate. The supernatant fluid is poured off as completely as possible and the precipitate washed into a quarter litre flask, with 100 to 150 c.c. of distilled water and ammonia led into the solution until it becomes of a pale blue colour. The solution is made up to 200 c.c. so as to contain about 7·3 grms. per cent. of cuprous chloride. Such a solution is an absorbent of carbon monoxide; it also absorbs ethylene and acetylene. A solution of cuprous chloride which has absorbed CO gives it up on being treated with potassic bichromate and acid. It has been proposed by Wanklyn to deprive large quantities of air of oxygen, then to absorb any carbon monoxide present with cuprous chloride, and, lastly, to free the cuprous chloride from the last gas by treatment with acid bichromate, so as to be able to study the properties of a small quantity of pure gas. By far the most reliable method to detect small quantities of carbon monoxide is, however, as proposed by Hempel, to absorb it in the lungs of a living animal. A mouse is placed between two funnels joined together at their mouths by a band of thin rubber; one of the ends of the double funnel is connected with an aspirator, and the air thus sucked through, say for half an hour or more; the mouse is then killed by drowning, and a control mouse, which has not been exposed to a CO atmosphere, is also drowned; the bodies of both mice are cut in two in the region of the heart, and the blood collected. Each sample of blood is diluted in the same proportion and spectroscopically examined in the manner detailed at p. 58. Winkler found that, when large volumes of gas were used (at least 10 litres), 0·05 per cent. of carbon monoxide could be readily detected. II.--Chlorine. § 43. Chlorine is a yellow-green gas, which may, by cold and pressure, be condensed into a liquid. Its specific gravity is, as compared with hydrogen, 35·37; as compared with air, 2·45; a litre under standard conditions weighs 3·167 grms. It is soluble in water. The usual method of preparation is the addition of hydrochloric acid to bleaching powder, which latter substance is hypochlorite of lime mixed with calcic chloride and, it may be, a little caustic lime. Another method is to treat manganese dioxide with hydrochloric acid or to act on manganese dioxide and common salt with sulphuric acid.