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.
Experiments made by M. Cotton have shown that the blood of different animals varies in the intensity of its action upon hydrogen peroxide. Thus human blood liberates about twice as much oxygen as the blood of the horse or pig, nearly four times as much as that of the ox and guinea-pig, and about ten times as much as the blood of the sheep.
Unfortunately other animal fluids have a similar action upon hydrogen peroxide, and the test can therefore only be regarded as corroborative evidence of the results obtained by other tests.
Attempts have sometimes been made by murderers to remove blood-stains by treatment with chemical agents, so as to prevent their identification.
For instance, in the trial of Misters for murder at Shrewsbury, in 1841, a solution of alum was found in his room, and it was supposed that he had removed the blood from his shirt by treatment with this. He was convicted, however, upon other evidence.
The identification of blood-stains upon rusty weapons is a more difficult matter than in the case of stains upon linen.
The action of the acid salts of fruits upon the iron may produce an appearance very similar to that of a blood-stain, the citrate of iron formed having a reddish colour which on more than one occasion has misled even a surgeon.
A case of this kind happened in 1838 in Paris. A man who had been accused of murdering his uncle, whose heir he was, was found to have a knife on the blade of which were stains, which everyone who saw them said were blood-stains.