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. [Illustration: BURNING CLOTH IN THE LABORATORY.] Perhaps he gets a handkerchief with a red stain. If the cloth is white he can apply a test direct to it, but as a rule he prefers to dissolve the stain out. Now, a handkerchief may be stained with a number of different reddish things--Condy's fluid, jam, cochineal log-wood, or red paint. He puts a drop of ordinary ammonia on the cloth. If the stain is caused by currant, gooseberry, or other fruit juice it turns blue or green; if it is Condy's fluid it becomes blue; if it is cochineal it becomes crimson, and so on. But if it is blood, it does not change in the least. Other tests might be described, but we have not the space.