In the examination of blood-stains the purpose of the medico-jurist is not to demonstrate all the properties of blood, but to identify it. There is not much difficulty in ascertaining whether stains are due to blood or not; but when the question arises as to whether the blood be human or that of some other animal, the identification is more difficult and less certain.
Blood-stains vary in colour, according to the age of the stain, the quantity of blood in it—the thicker the stain the darker—and the nature and colour of the material upon which it is. Recent stains are reddish in colour, old stains brownish. This change of colour depends upon the free access of air and the presence or absence of chemical substances in the air, so that it is almost impossible to infer the age of a blood-stain by its colour. On dark-coloured materials the stains are rendered more visible by the aid of artificial light, such as candle-light; on light-coloured materials, on leather, wood, iron, and stone, they are more visible in good daylight. By reason of the coagulation and the albuminous composition of blood, dry stains stiffen the fabric when thin, and on thicker woollen materials the fine fibres become matted. On metals, such as iron or steel, they appear as dark shiny spots or smears, and when dried are often fissured or cracked. Rust may so alter blood as to produce a difference between the stains on the blade and handle of a knife. In quite recent blood-stains the general appearances are sufficient to give rise to a conclusion as to their nature, especially if the stains are large. The general features as seen by the naked eye are such that one may often recognise blood-stains as arterial by the comet shape they retain when falling slantwise on an object. Venous blood is not spurted in small jets like arterial, but blood from veins may become splashed upon objects and assume shapes similar to those produced from an arterial jet.
EXAMINATION OF BLOOD-STAINS
The examination of blood-stains should be carried out in the following way:
Physical Examination
1. Examine the stains carefully with a good pocket lens or a low power microscope lens. A fabric will show matting of its fibres, red filaments, and minute coagula in its meshes. In old blood-stains coagula may be absent and the fabric appear as if dyed. The characters of any fibres or hairs adhering to the stain and the nature of the substance upon which the stain is should be noted.
Fig. 4.—Photo-micrograph of wool fibres, × 250.
(R. J. M. Buchanan.)
2. Make accurate notes of the position and shape of the stains on the material examined.
3. Take one stain, if there are several, or part if single, and note the solubility of it in water, or in a mixture of water and some other substance. The solubility of the colouring matter is greater if the stain be recent than if it be old. The older the stain the less soluble it becomes, as the hæmoglobin is gradually changed in course of time to insoluble hæmatin.