90 MEDICAL JURISPRUDENCE (ii) Hydrogen Peroxide (3 per cent) Solution.—This is Merck's ' 10 volume' hydrogen peroxide. Instead of * 10 volume' solution ordinary Merckozone or ' 12 volume ' solution (equivalent to 3.6 per cent) may be used safely. The solutions are usually kept in our laboratory for about a month after which they are discarded although they may be used for a longer period. The best way of performing the test is to clip off a small fragment of the stained material or to tease out a fibre from the stained fabric and to place it on a porcelain tile. At first a drop of benzidine solution and then a drop of hydrogen peroxide solution are added, when an intense blue colour radiating out on the tile is produced immediately if blood is present. The test may also be obtained by gently pressing a piece of white blotting or filter paper moistened with a few drops of distilled water on the stain and by adding the reagents to the moistened paper. The advantage of this test is that the same specimen with the blue colour streaming out may be transferred to a slide for microspectroscopy. For this purpose the material is treated at first with a drop of a 10 per cent solution of potassium cyanide and then with a drop of ammonium sulphide; it is then covered with a cover slip and looked for cyanhsemochromogen bands. Coloured fabrics are, however, not suitable for direct spectroscopy.8 It must be borne in mind that a positive reaction may be obtained from certain other substances, such as sputum, pus, nasal secretion, plant juices, formalin 9 and a clay, called bentonite,10 but the reaction is decidedly weaker and differs in its sensitiveness and in its shade and depth of colour. They show a slow and faint colouration which should be ignored. A control test performed side by side with a known blood stain decides the issue. Gum1-1 also gives a positive reaction with the benzidine test; hence it is not desirable " to use gum for sticking labels on medico-legal exhibits of fabrics which are suspected to have blood stains. The benzidine test never fails to detect blood even in very old, de- composed stains with every sort of dirt. The negative result is undoubtedly valuable, but the positive results obtained so far in thousands of our cases never failed to satisfy the confirmatory tests for blood. 3. Kastle-Meyer Test (Phenolphthalein Test).—The principle of this test is based on the fact that if' ordinary phenolphthalein of the laboratory is reduced by zinc dust in an alkaline solution, phenolphthalein is produced which, if oxidized in the presence of an alkali, gives the characteristic red colour. The reagents required for carrying out this test are— 1. Hydrogen peroxide solution (20 volumes or 6 per cent strength). 2. A mixture containing 2 grammes of phenolphthalein, 20 grammes of potassium hydrate and distilled water in sufficient quantity to make up 100 cc. of the solution. These three ingredients are boiled, and during the process 10 to 30 grammes of powdered zinc are added. Boiling is further continued until the solution becomes colourless. The solution, thus prepared, will remain effective for a long period, if a small quantity of powdered zinc is left deposited at the bottom of the reagent bottle to ensure reduction. Jtfjen to twenty drops of the phenolphthalein reagent are added to a solution extracted from the stain with distilled water, a deep permanganate colour will be obtained instantaneously on the addition of a drop or two of hydrogen peroxide solution if blood be present. 8. Imperial Serologists' Annual Report, Calcutta, 1937-38. 9. Sydney Smith, Foren. Med., Ed. IX, p. 211. 10. Greval, S. D. S. and Roy Chowdhery, A. B., Ind. Med. Gaz., Sep. 1941, p. 535 11. S. D. S. Greval, Ind. Med. Gaz., Nov. 1945, p. 556.