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. A chemical examination, however, which was made in the presence of the magistrate and the prisoner, proved that they consisted of citrate of iron, and had been produced by cutting a lemon and neglecting to wipe the blade after use. It has frequently happened in the past that the opinion of policemen or witnesses without any special knowledge of the subject has been taken in criminal cases on the point whether stains upon clothes or on a weapon consisted or did not consist of blood. This practice was obviously a dangerous one, since even by the modern methods of examination it is not always a simple matter to be sure of the fact. Until a comparatively recent date the tests for blood-stains were based upon bringing the colouring matter of the blood into solution and applying chemical tests to establish its identity. The necessity for scientific proof of the presence of blood-stains is shown by numerous cases in which stains of similar colour have at first been attributed to blood. Thus in a case related in Taylor's _Forensic Medicine_ a man was arrested in 1840 on suspicion of being connected with a murder in Islington. He had in his possession a sack on which were numerous stains supposed to be dried and coagulated blood. When these were examined, however, they were found to be due to red paint.