XV Detection of Blood-Stains, etc.
Stains may require detection on clothing, on cutting instruments, on floors and furniture, etc. The following are the distinctive characters of blood-stains:
(a) Ocular Inspection.—Blood-stains on dark-coloured materials, which in daylight might be easily overlooked, may be readily detected by the use of artificial light, as that of a candle, brought near the cloth. Blood-spots when recent are of a bright red colour if arterial, of a purple hue if venous, the latter becoming brighter on exposure to the air. After a few hours blood-stains assume a reddish-brown or chocolate tint, which they maintain for years. This change is due to the conversion of hæmoglobin into methæmoglobin, and finally into hæmatin. The change of colour in warm weather usually occurs in less than twenty-four hours. The colour is determined, not entirely by the age of the stain, but is influenced by the presence or absence of impurities in the air, such as the vapours of sulphurous, sulphuric, and hydrochloric acids. If recent, a jelly-like material may be seen by the aid of a magnifying-glass lying between the fibres. If old, a cinnabar-red streak is seen on drawing a needle across the stain.
(b) Microscopic Demonstration.—With the aid of the microscope, blood may be detected by the presence of the characteristic blood-corpuscles. The human blood-corpuscle is a non-nucleated, biconcave disc, having a diameter of about 1/3500 of an inch. All mammalian red corpuscles have the same shape, except those of the camel, which are oval. The corpuscles of birds, fishes, reptiles, and amphibians, are oval and nucleated. The corpuscles of most mammals are smaller than those of man, but the size of a corpuscle is affected by various circumstances, such as drying or moisture, so that the medical witness is rarely justified in going farther than stating whether the stain is that of the blood of a mammal or not. Unfortunately, the corpuscles are usually so dried that little information regarding their size can be given.
(c) Action of Water.—Water has a solvent action on blood, fresh stains rapidly dissolving when the material on which they occur is placed in cold distilled water, forming a bright red solution. The hæmatin of old stains dissolves very slowly, so employ a weak solution of ammonia, and this will give a solution of alkaline hæmatin. Rust is not soluble in water.
(d) Action of Heat.—Blood-stains on knives may be removed by heating the metal, when the blood will peel off, at once distinguishing it from rust. Should the blood-stain on the metal be long exposed to the air, rust may be mixed with the blood, when the test will fail. The solution obtained in water is coagulated by heat, the colour entirely destroyed, and a flocculent muddy-brown precipitate formed.
(e) Action of Caustic Potash.—The solution of blood obtained in water is boiled, when a coagulum is formed soluble in hot caustic potash, the solution formed being greenish by transmitted and red by reflected light.
(f) Action of Nitric Acid.—Nitric acid added to a watery solution produces a whitish-grey precipitate.
(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.
Hairs.—Human hairs must be identified and distinguished from those of the lower mammals. If the hair has been pulled out from the root, the microscope will show that the bulbous root has a concave surface which fitted over the hair papilla, or that the root is encased in a fatty sheath.
Fibres of Clothing.—Microscopically, wool fibres are coarse, curly, and striated transversely; cotton fibres appear as flattened bands twisted into spirals; linen fibres are round, jointed at frequent intervals, with small root-like filaments; silk fibres are solid, continuous, and highly glistening.