Blood Stains. In criminal trials it is sometimes a matter of importance to determine the exact nature of stains on clothing, knives, etc., and in particular to ascertain whether the discoloration in question is a blood stain. In investigations of this kind the ordinary tests for blood are employed. A microscopical examination is made, the guaiacum test applied, and an attempt made to obtain haemin crystals. Perhaps the most valuable means of diagnosis at disposal, however, is afforded by the spectroscope. The spectrum of oxyhaemoglobin when examined in appropriately dilute solution, presents two absorption bands - a narrower band in the yellow part of the spectrum and a broader one in the green. On shaking up the solution with a reducing agent, such as sulphide of ammonium, the two bands become replaced by a single band in the yellowish-green. This test for blood is an extremely delicate one.
Various stains may be confused with blood stains, e.g. certain red dyes and iron rust; none of these, however, give the characteristic reactions of blood when examined spectroscopically. It must of course be remembered that the blood of any vertebrate animal will give the haemoglobin spectra, and it is, as a rule, impossible to say to what species of animal the blood originally belonged.