§ 190. For the quantitative estimation of chloroform the method recommended by Schmiedeberg[177] is, however, the best. A combustion-tube of 24 to 26 cm. long, and 10 to 12 mm. in diameter, open at both ends, is furnished at the one end with a plug of asbestos, while the middle part, to within 5-6 cm. of the other end, is filled with pieces of caustic lime, from the size of a lentil to that of half a pea. The lime must be pure, and is made by heating a carbonate which has been precipitated from calcic nitrate. The other end of the tube is closed by a cork, carrying a silver tube, 16-18 cm. long, and 4 mm. thick. The end containing the asbestos plug is fitted by a cork to a glass tube. The combustion-tube thus prepared is placed in the ordinary combustion-furnace; the flask containing the chloroform is adapted, and the distillation slowly proceeded with. It is best to add a tube, bent at right angles and going to the bottom of the flask, to draw air continuously through the apparatus. During the whole process, the tube containing the lime is kept at a red heat. The chloroform is decomposed, and the chlorine combines with the lime. The resulting calcic chloride, mixed with much unchanged lime, is, at the end of the operation, cooled, dissolved in dilute nitric acid, and precipitated with silver nitrate. Any silver chloride is collected and weighed and calculated into chloroform.[178] [177] _Ueber die quantitative Bestimmung des Chloroforms im Blute._ Inaug. Dissert., Dorpat, 1866. [178] S. Vidali has made the ingenious suggestion of developing hydrogen in the usual way, by means of zinc and sulphuric acid, in the liquid supposed to contain chloroform, to ignite the hydrogen, as in Marsh’s test, when it issues from the tube, and then to hold in the flame a clean copper wire. Since any chloroform is burnt up in the hydrogen flame to hydrochloric acid, the chloride of copper immediately volatilises and colours the flame green. VI.--Other Anæsthetics. § 191. When chlorine acts upon marsh-gas, the hydrogen can be displaced atom by atom; and from the original methane (CH₄) can be successively obtained chloromethane or methyl chloride (CH₃Cl), dichloromethane, or methene dichloride, methylene dichloride (CH₂Cl₂), trichloromethane, or chloroform (CHCl₃), already described, and carbon tetrachloride (CCl₄). All these are, more or less, capable of producing anæsthesia; but none of them, save chloroform, are of any toxicological importance. Methene dichloride, recommended by Sir B. W. Richardson as an anæsthetic, has come somewhat into use. It is a colourless, very volatile liquid, of specific gravity 1·360, and boiling at 41°. It burns with a smoky flame, and dissolves iodine with a brown colour.