With the microscope can be seen those red corpuscles which, in some mysterious manner, seize on the oxygen of the air as it passes into the lungs, shoulder it, so to speak, and rush away with it, like so many ants, to the remotest parts of the body. Unfortunately, they can only be seen in blood that has not been very long shed—that is to say, some weeks or months. To see these, the analyst scrapes the little clot from the piece of cloth, or wood, or iron, and places it on a slip of glass; over this he carefully lays the little film called a cover-glass; and then he gently places, at the edge of the latter, the tiniest possible drop of water. This gradually insinuates itself, and soon dissolves the blood clot; and, when the mixture is placed under a microscope magnifying from 300 to 500 diameters, he sees one of several pictures. The various shapes and arrangements taken by these little bodies are illustrated on the following page. Small as they are—it would take 12¼ millions to cover a square inch—they have the most peculiar way of behaving, and only the practised eye of the microscopist can recognise them in all their disguises.
HUMAN BLOOD CORPUSCLES UNDER THE MICROSCOPE.
Individually, the blood corpuscle is just like a tiny round biscuit, and measures 1/3200 to 1/4000 of an inch across its face. It is these two factors, the shape and measurement, which enable the medical man to say whether the blood is human. The picture above shows how a corpuscle looks under the microscope. Looking at its face, it is like a thick-edged biscuit, with a dark depression in the centre. Some are turned sideways in our illustration. These exist in blood and nothing but blood, so that, when the spectroscope fails, the microscope succeeds.
But it is not always that the analyst can get sufficient blood to place under the microscope. Perhaps he gets a piece of cloth saturated with a trifle of red fluid which he cannot scrape off, or perhaps he gets a stain some months or years old (Dr. Tidy identified a blood stain one hundred and one years old), in which the corpuscles are destroyed. Or perhaps he gets a garment which has been carefully washed, on which there is only the faintest trace of colouring matter. Even then the microscope tells whether the stain is blood.
Our detective mixes the particle of blood-stained wood, or earth, or dust, or cloth fibres, with water and caustic potash, and filters it. Then he takes a drop of the liquid and places it in the useful watch-glass. Into this he puts some glacial acetic acid and a crystal of ordinary table salt. He heats the mixture and lets it cool. And, if it is blood, he gets peculiar crystals visible under the microscope. These, by the way, differ to some extent in different animals.
Another test is so new that it has not yet been given a fair trial. It is as follows:—If a fairly large quantity of blood can be got, it is burned, and the ash is analysed. Now, there are two salts always in blood—sodium and potassium salts. But, while the quantity of the former in human blood is usually twice that of the latter, it is six times as great in the sheep's blood, eight times as great in the cow's blood, and sixteen times as great in the blood of a fowl. Very important results are expected from this principle.
Reliable as are the microscope and spectroscope, the analyst always uses the third means at his disposal—the chemical test. For instance, he gets a knife covered with dark red stains. Are they blood, or are they only the rust formed by vinegar or the juice of a lemon that has deceived so many people? Assuming that he has removed the stain, he places the matter in any kind of tiny vessel, and drops in some tincture of galls. If the thing is only rust, he has some excellent blue ink; if it is blood, he finds that a reddish powder makes its appearance.
BURNING CLOTH IN THE LABORATORY.