Date. Content of gas in CO. Jan. 12, 1892, 24·7 per cent. „ 20, „ 23·5 „ „ 28, „ 22·2 „ Feb. 8, „ 20·3 „ „ 16, „ 15·5 „ „ 26, „ 10·2 „ March 3, „ 6·3 „ „ 14, „ 4·6 „ „ 22, „ 1·2 „ The same dog was buried on the 12th of January, and exhumed on March 28th, and the gas pumped out from some of the blood; this gas gave 11·7 per cent. of CO; hence it is clear that burial preserves CO blood from change to a certain extent. N. Gréhant[53] treated the poisoned blood of a dog with acetic acid, and found it evolved 14·4 c.c. CO from 100 c.c. of blood. [53] _Compt. Rend._, cvi. 289. Stevenson, in one of the cases detailed at p. 67, found the blood in the right auricle to contain 0·03 per cent. by weight of CO. (2) =Preparation of Hæmatin Crystals=--(Teichmann’s crystals).--A portion of the borax solution is diluted with 5 or 6 parts of water, and one or more drops of a 5 or 6 per cent. solution of zinc acetate added, so long as a brownish-coloured precipitate is thrown down. The precipitate is filtered off by means of a miniature filter, and then removed on to a watch-glass. The precipitate may now be dissolved in 1 or 2 c.c. of acetic acid, and examined by the spectroscope it will show the spectrum of hæmatin. A minute crystal of sodic chloride being then added to the acetic acid solution, it is allowed to evaporate to dryness at the ordinary temperature, and crystals of hæmatin hydrochlorate result. There are other methods of obtaining the crystals. When a drop of fresh blood is simply boiled with glacial acetic acid, on evaporation, prismatic crystals are obtained. Hæmatin is insoluble in water, alcohol, chloroform, and in cold dilute acetic and hydrochloric acids. It may, however, be dissolved in an alcoholic solution of potassic carbonate, in solutions of the caustic alkalies, and in boiling acetic and hydrochloric acids. Hoppe-Seyler ascribes to the crystals the formula C₆₈H₇₀N₈Fe₂O₁₀2HCl. Thudichum considers that the pure crystals contain no chlorine, and are therefore those of hæmatin. It is the resistance of the hæmatin to decomposition and to ordinary solvents that renders it possible to identify a certain stain to be that of blood, after long periods of time. Dr. Tidy seems to have been able to obtain blood reactions from a stain which was supposed to be 100 years old. The crystals are of a dark-red colour, and present themselves in three forms, of which that of the rhombic prism is the most common (see fig.). But crystals like _b_, having six sides, also occur, and also crystals similar to _c_. [Illustration]