HUMAN BLOOD MAGNIFIED 400 TIMES.
PIG'S BLOOD MAGNIFIED MANY TIMES.
What the analyst first does, when he receives such an article as a pair of trousers, is to scrutinise every inch of its surface with a magnifying glass. If he finds a little lump of dark-coloured stuff he scrapes it off and puts it into a watch glass. If he discovers merely a dark stain, he cuts out the piece of cloth and puts it into a small quantity of distilled water.
Now he has to find out whether the suspicious-looking thing is really blood, or whether it is merely red paint, or logwood, or cochineal, or madder, or iron-mould. There are three ways of doing this, and he nearly always utilises them all.
First, there is the marvellous spectroscope test. This test will reveal the presence of the minutest trace of blood, and it is practically infallible. It depends on the curious property, possessed by nearly all bodies, of absorbing certain parts of the light that passes through them. Sunlight passing through a prism is split up into the familiar seven colours of the rainbow. But if a little blood dissolved in water is placed in a glass tube, and if the light is made to pass through it on its way to the prism, the blood takes something out of it; for now among the seven bright colours are seen two dark bands near the middle of the yellow ray. Nothing but blood gives these two bands in that particular place, with the exception of two or three substances that are not likely to be found on criminals' clothes. These are cochineal, mixed with certain chemicals, hot purpurin sulphuric acid, and the red dye of the banana-eater.
Blood, however, changes after it is shed. In stains a few weeks old the colouring matter changes from what is technically called hæmoglobin to methæmoglobin, and, later still, to hæmatin. All of these give different spectra. The analyst has standard spectra already mounted, and he invariably looks at the mounted or standard specimen and the suspected liquid at the same time, placing them side by side, so that a mistake is impossible. All the red colours in the world, in fact, have been tried, and, with the exceptions named above, none of them gives a spectrum like the colouring matter of blood in any of its forms.
But though the spectroscope is a certain discoverer of blood, it can draw no distinction between human and animal blood. That duty remains to the microscope.
| Man. | Mouse. | Horse. | Camel. |
| Toad. | Pike. | Pheasant. | Pigeon. |
| LITTLE LINKS IN THE CHAIN OF EVIDENCE. THE CORPUSCLES INTHE BLOOD OF DIFFERENT CREATURES. | |||