In one case a man was very near being hanged--and in the old days, doubtless, he would have been hanged--mainly because a knife with red stains was found in his possession. The medical witness found that they were rust caused by an acid fruit; and then it was found that the prisoner had actually used a knife for cutting a lemon. But, curiously, this stain is so very like blood that the naked eye of even the most skilful medical jurist would be deceived by it. [Illustration: FOOTPRINTS.--(1) WHEN RUNNING. (2) STANDING. (3) WALKING.] Footprints are usually left to the police to interpret. But, very probably, the result is often a miscarriage of justice. When the police are working up a case they would not be human if they did not view evidence with a certain amount of bias. The scientific witness, on the other hand, has no personal interest one way or the other. And, moreover, the comparison of a naked foot with its supposed print on the ground, or the fitting of a boot to a boot-mark, is a process requiring not only the most exact measurements, but consideration of the kind of mark made on different kinds of soil, and in the various positions taken by the foot in standing, walking, and running. In running we press mainly on the toes, and in walking the greater part of the foot comes down, and the longer the foot rests on the ground the deeper is the impress. In fact, an expert can make a pretty shrewd guess as to the rate at which the owner of the foot was travelling, by considering the size and depth of the footprint. In order to make a comparison a cast has to be taken, if the mark is on soft ground. This is done by heating the footprint with a hot iron, and filling it in with paraffin. From this a plaster cast is taken, and it can be preserved for comparison until someone is arrested. When the footprint is found in snow, gelatine is used to take the form of it, and from this also a plaster cast is made. Of course, these operations have to be carried out with the greatest care, for footprints are frequently the strongest pillars of an indictment. In order to compare the foot of the suspected person, he is made to walk, stand, and run, over a surface similar to that on which the incriminating print has been found. There is one case in which the scientific detective is certain--when the person has stood still on soft, but firm and tenacious, soil. The footprints represented in our sketch are those of course of naked feet, which give the clearest impression. But a corresponding variation occurs in all footprints made by persons wearing boots, so that the attitude or action of the wearer is easily told. Now and again some deformity, such as the possession of six or of only four toes, leaves no room for doubt. When the mark has been made by boots, rather than with the naked foot, it is frequently easy to identify it by the arrangement and number of the nails, by a missing nail, or a patch, or a hole, or a heel worn on one side.