n ounce of
liquid, the wife's draught contained fifteen grains, showing that the
surgeon's ingenious explanation was a lie!

Blood is so characteristic a fluid that it might be supposed a skilful
analyst could never have any difficulty in recognising it. Of course, if
he were given, say, a cupful in its ordinary state, he could not make a
mistake. But he never gets a chance of earning his fee so easily.

When the police seek his assistance they give him, perhaps, a suit of
dirty clothes, which may be stained by two or three small dark spots
that might be anything.

Or perhaps he is given a rusty knife, or a perfectly clean hatchet, and
is asked to say if there is blood on it. And when he comes into court he
is expected to tell the jury whether the blood is human or animal, how
old it is, was it spilled from a living blood vessel, and in what part
of the body was this blood vessel.

Take an actual case. Years ago a celebrated murder was committed in
Eltham, and in the report of Dr. Letheby, the analyst, is the following
note:--

"On the evening of May 3rd I received from Mr. Mulvaney" (of the police)
"a brown paper parcel containing a pair of dark trousers, a man's shirt,
and a man's wide-awake hat. On the following evening I received from Mr.
Mulvaney a brown paper parcel containing a lock of hair, a pair of men's
boots, and a plasterer's hammer."

These were all very dirty, but that did not prevent the analyst from
finding a number of blood stains and hairs, and giving valuable and
decisive evidence at the trial.

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.

[Illustration: HUMAN BLOOD MAGNIFIED 400 TIMES.]

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.

[Illustration: PIG'S BLOOD MAGNIFIED MANY TIMES.]

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, poss