laced 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
12-1/4 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.

[Illustration: 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 blo