The technique of examination has to be varied in certain details, according to the material upon which the stain is. Stains may have to be examined upon cloth-fabrics, wood, plaster, metal, or leather. These will be taken separately, and the methods of examination described which will prove most reliable in each case.
1. Cloth-fabrics.—Cut out a stain, or part of one, and macerate it in a quantity of one of the solvents mentioned above, sufficient for the purpose. If the stain be very small, squeeze with fine forceps one or more drops upon microscope slides for microscopic, and keep the remainder of the solution for spectroscopic, examination. In dyed fabrics, which have been mordanted, the mordant may fix the blood-stain so as to prevent solution, and especially so when the attempts have been made to wash out the stain with soap and water.
To make a solution of the stain in such cases it is best to use distilled water to which a small quantity of ammonia or citric acid has been added; in one or other of these the colouring matter will dissolve.
2. Wood.—Note the kind of wood, cut off a thin shaving and treat with one of the solvents mentioned above. If on wood containing tannic acid, such as oak or elm, the best solvent is a two per cent. solution of hydrochloric acid.
3. Plaster.—Scrape off some of the stained plaster, and treat as for cloth or wood.
4. Metal.—If the stain be upon a clean and unrusted metal, e.g. the clean blade of a knife, then gently heat the metal on the side opposite to the stain, when the latter, if recent, will peel off or can be easily detached. This requires some care and dexterity. It is easy, however, to scrape the stain off into a watch-glass, and this procedure is necessary when the metal is rusted and the stain mixed with the rust, or when the stain is thin.
Fig. 6.—Photo-micrograph of silk fibres, × 250.
(R. J. M. Buchanan.)
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.