BLOOD STAINS 95 bands may not be visible evidently from the effects of putrefaction. In that case the stain should be treated with a drop of a ten per cent solution of potassium cyanide. A cherry red colour will develop due to the conversion of hsemochromogen into cyanhaemochromogen with its characteristic absorp- tion bands similar to those of the former but slightly wider and situated closely at wave lengths 570-550 and 540-527.20 Tl^e^-inllowing procedure21 is recommended for the detection of blood stains' on rusty weapons and red coloured fabrics : — A small portion of the blood stain is taken on a glass slide with a ten per cent solution of caustic potash, a little glucose powder is added and is covered with a cover slip. The glass slide is then warmed over a small flame, when the stain assumes a bright red colour and gives the characteristic haBmochromogen bands. The red dye on fabrics is usually bleached by this procedure. The reduction into haemochromogen may also be brought about by the Takayama reagent as described before or by an alkaline solution consisting of 4 grammes of sodium hydrosulphite, 10 cc. of potassium hydroxide (10 per cent) and 2 cc. of alcohol, which is said to be cleaner and much more efficient. The spectrum of hsemochromogen or of cyanhsemochrornogen is quite enough for purposes of identification and expression of a definite opinion about suspected blood stains. It is not necessary to examine any more spectra of acid or alkaline haematin, hasmatoporphyrin, etc. For practical purposes one chemical test, viz. the benzidine test, and a confirmatory spectro- scopic test for haemochromogen or cyanhasmochromogen, are quite sufficient for a definite opinion, and these are the principal tests usually employed in the laboratories of the Slate Chemical Examiners. For very old and scanty stains where there is not sufficient material for repeating the exami- nations, the Chemical Examiners are required, by the order of the Govern- ment of India, to forward the stains as they are, to the Imperial Serologist, Calcutta, for identification of blood and for determination of its origin by the serological test. Biological Examination.—This is undertaken for the purpose of deter- mining whether the blood of a particular stain is derived from a human being, from a lower mammalian animal or from a bird. Precipitin Test.-^Jhis test is based on the principle that a foreign protein or a protein-containing substance, when injected into an animal, produces antibodies in the blood serum of that animal, which will form a precipitate when mixed with a solution of that foreign protein. The protein thus intro- duced is called the antigen and the antibody capable of forming a precipitate is called precipitin^ Relying on this principle, Uhlenhuth who made several experiments and devised a method for recognizing the different kinds of mammalian blood, found that the test was exceedingly delicate and suggested its applicability for the detection of human blood in medico-legal inquiries. Other workers have elaborated its technique to its perfection and have given it the name of the precipitin test.%The method consists in injecting sub- cutaneously, intraperitoneally or intravenously a rabbit or a fowl with blood serum of an animal, a man for instance, at regular intervals. After a certain number of injections the serum obtained from the injected animal, when sufficiently diluted and added to a clear serum of human blood, produces at first a turbidity and then a fiocculent precipitate but fails to do so with 20. Greval, Roy Chowdhery and Das found that the absorption band on the longer wave length side (red end) was intense and that on the shorter wave length side (violet end) was faint but broader (vide Ind. Jour. Med. Res., Vol. XXXIII, No. 1, May 1945, p. 173). 21. Madras Chera. Exam. Annual Rep., 1940, p. 10.