(g) Action of Guaiacum.—Tincture of guaiacum produces in the watery solution a reddish-white precipitate of the resin, but on addition of an aqueous solution of peroxide of hydrogen, or of an ethereal solution of the same substance (known as ozonic ether), a blue or bluish-green colour is developed. This test is delicate, and succeeds best in dilute solutions. It is not absolutely indicative of the presence of blood, for tincture of guaiacum is coloured blue by milk, saliva, and pus.
(h) Hæmin Crystals (Teichman's Crystals).—These are produced by heating a drop of blood, or a watery solution of it, with a minute crystal of sodium chloride on a glass slide and evaporating to dryness. A cover-glass is placed over this, and a drop of glacial acetic acid allowed to run in. It is again heated until bubbles appear. Crystals of hæmin may now be detected by the microscope. They are dark brown or yellow rhombic prisms.
An improvement on this test is the use of formic acid alone; on slowly evaporating it, numerous very small dark crystals are visible if hæmoglobin has been present (Whitney's test).
(i) Spectroscopic Appearances.—If a solution of a recent stain be examined by the spectroscope, we get two absorption bands situated between the lines D and E, the one nearer E being doubly as broad as the other. These bands indicate oxyhæmoglobin.
If we now add a little ammonium sulphide to this solution, we get the spectrum of reduced hæmoglobin, which is a single broad absorption band situated in the interval between the preceding oxyhæmoglobin bands. By shaking the solution, oxyhæmoglobin is again reproduced, and gives its special absorption bands.
If ammonia be added to the original solution, alkaline hæmatin is produced, or if acetic acid be chosen, acid hæmatin is produced, and each gives its appropriate absorption bands.
Methæmoglobin is formed in stains which have been exposed to the air for a few days, and hæmatin is found in old stains. Hæmochromogen gives a very characteristic spectrum, and is obtained by reducing alkaline hæmatin by ammonium sulphide. Carbon monoxide hæmoglobin gives a spectrum which resembles that of oxyhæmoglobin, but it is not reduced by ammonium sulphide.
(j) Precipitin Test.—This allows us to tell whether the blood is from a human being or not. A specific serum must be obtained from a rabbit which is sensitized as follows: 10 c.c. of human blood is injected into its peritoneal cavity at intervals, until from three to five injections have been given. The serum of this animal's blood will then give a white precipitate only when brought into contact with dilute solutions of human blood, but with the blood of no other animal. This is known also as the 'biologic,' or Uhlenhuth's test.
Rust Stains.—These are yellowish-red in colour, and do not stiffen the cloth. The iron may be dissolved by placing the stain in a dilute solution of hydrochloric acid, when, on adding ferrocyanide of potassium, Prussian blue is produced.
Fruit Stains are seldom so dark as blood-stains. Solutions of these do not change colour or coagulate on boiling; ammonia changes the colour to blue or green; acid brightens the original colour, while chlorine bleaches it.