If the spot under examination has been scraped off an iron implement the hæmatin is not so easily extracted, but Dragendorff states that borax solution at 50° dissolves it, and separates it from the iron. Felletar has also extracted blood in combination with iron rust, by means of warm solution of caustic potash, and, after neutralisation with acetic acid, has precipitated the hæmin by means of tannin, and obtained from the tannin precipitate, by means of acetic acid, Teichmann’s crystals. A little of the rust may also be placed in a test tube, powdered ammonium chloride added, also a little strong ammonia, and after a time filtered; a small quantity of the filtrate is placed on a slide with a crystal of sodium chloride and evaporated at a gentle heat, then glacial acetic acid added and allowed to cool; in this way hæmin crystals have been obtained from a crowbar fifty days after having been blood-stained.[54] [54] _Brit. Med. Journ._, Feb. 17, 1894. (3) =Guaiacum Test.=--This test depends upon the fact that a solution of hæmoglobin develops a beautiful blue colour, if brought into contact with fresh tincture of guaiacum and peroxide of hydrogen. The simplest way to obtain this reaction is to moisten the suspected stain with distilled water; after allowing sufficient time for the water to dissolve out some of the blood constituents, moisten a bit of filter-paper with the weak solution thus obtained; drop on to the moist space a single drop of tincture of guaiacum which has been prepared by digesting the inner portions of guaiacum resin in alcohol, and which has been already tested on known blood, so as to ascertain that it is really good and efficient for the purpose; and, lastly, a few drops of peroxide of hydrogen. Dragendorff uses his borax solution, and, after a little dilution with water, adds the tincture and then Heunefeld’s turpentine solution, which is composed of equal parts of absolute alcohol, chloroform, and French turpentine, to which one part of acetic acid has been added. The chloroform separates, and, if blood was present, is of a blue colour.