If on iron and mixed with rust the borax solution may be used, with a drop or two of solution of ammonia; use a fine camel-hair pencil dipped in the solution, and brush the stain off into a watch-glass. Becker advises that stains mixed with rust should be digested with a weak solution of ammonia and common salt for a few hours; decant the solution and evaporate it upon a microscope slide to dryness, then test the residue by the “hæmin test.” Ganttner‘s test should be used to a portion of a stain upon metal when thin or mixed with rust. It may be carried out upon the metal itself or upon a scraping of the stain in a watch-glass resting upon a black surface. Moisten the scraping in a watch-glass with a drop or two of distilled water rendered feebly alkaline, then add a minute drop of hydrogen peroxide. Wherever blood is present bubbles of gas develop, which give the material a white beaded appearance. The froth develops from the outside of the drop towards the centre when the stain is mainly composed of blood. In a scraping consisting of mixed particles of rust and blood, the reaction only appears upon the particles of blood, and rust to which blood adheres; it does not take place on those particles of rust free from blood. Before adding the peroxide of hydrogen it may be necessary to dissipate any air-bubbles which may cling to the scraping in the alkaline water by gentle agitation with the point of a fine glass rod. Should the above reaction with peroxide of hydrogen not take place, then one can rest assured that no blood is present. The test, however, is a negative one; it is not a positive test for blood only; other fluids and exudations from the body, such as saliva and pus, give the reaction. The reaction will take place with blood-stains of any age. [Illustration: Fig. 7.--Photo-micrograph of cotton fibres, × 250. (R. J. M. Buchanan.)] In examining a clasp-knife or any hinged weapon for blood-stains, the instrument should be taken to pieces and all the hinges and recesses carefully examined, for in these places blood may be found, although the weapon had previously been wiped clean, and appear free from stains. 5. _On Leather._--The tannic acid in leather forms a compound with blood which is insoluble in the solvents generally used. A thin shaving of the stained portion should be taken and folded, with the stained surface outwards, in the form of a loop. If the outer surface of the loop with the stain be made to touch the surface of the glycerine and water solution, at the same time taking care that the leather itself be not moistened, a recent stain may yield a sufficient quantity of colouring matter for the purposes of examination. Failing this, the shaving should be digested in a small quantity of a two per cent. solution of hydrochloric acid in distilled water (Sorby). Microscopical Examination