The red corpuscles differ in size and shape according to the species of animal. Thus in human blood and in the blood of most mammalia they appear as double concave circular discs, while in the blood of the camel and in that of birds, reptiles and fish the red corpuscles are elliptical in form. The number of corpuscles present is also subject to great variations, the blood of amphibia and reptiles, for instance, containing remarkably few. The following numbers in 100 parts of the blood of different animals have been recorded: Horse, 53; pig, 43·5; ox, 35; dog, 35·7; and man, 48 corpuscles. The colour of blood is due to a compound known as hæmoglobin, which constitutes about 40 per cent. of the substance of the corpuscles. In the bright red arterial blood the hæmoglobin is present in the form of oxyhæmoglobin, and the latter may be separated in crystalline form by suitable treatment of the separated red blood corpuscles. These crystals differ in the case of different animals both in their chemical and physical characteristics, and have very different forms. There are also similarly pronounced differences between the microscopical appearance of oxyhæmoglobin crystals from human blood and from that of various animals. The crystals from human blood are in the form of long rhombic needles; those from the blood of the horse are quadrilateral prisms; the blood of the guinea-pig, rat, and many birds yield rhombic tetrahedea; while that of the squirrel gives hexagonal plates. Crystals of other compounds of hæmoglobin, such as hæmin, differing in the case of different species of animals may also be prepared, and the identity of oxyhæmoglobin may also be proved by its characteristic appearance in the spectroscope. It is, therefore, under favourable conditions, not a very difficult matter to distinguish between the fresh blood of, say, a man and a squirrel by means of these characteristic differences. It is rarely, however, that the problem is presented in such a simple form in criminal work, in which usually all that is available for the investigation is the dried stain upon some garment or the clot upon a rusty knife. One of the most widely employed tests is to dissolve a little of the material in acetic acid containing a little common salt, to apply a gentle heat to the microscope slide, and then to notice under the microscope whether hæmin crystals are formed. Where the stain is upon iron it is often impossible to prepare hæmin crystals, and in such cases hydrogen peroxide is used as a reagent. This compound, when brought into contact with a fragment of the material moistened with alkaline water, gives off in the presence of blood, bubbles of oxygen, which gradually form a white scum.