sin, but on addition of an
aqueous solution of peroxide of hydrogen, or of an ethereal solution of
the same substance (known as _ozonic ether_), a blue or bluish-green
colour is developed. This test is delicate, and succeeds best in dilute
solutions. It is not absolutely indicative of the presence of blood, for
tincture of guaiacum is coloured blue by milk, saliva, and pus.

(h) =Haemin Crystals (Teichman's Crystals).=--These are produced by
heating a drop of blood, or a watery solution of it, with a minute
crystal of sodium chloride on a glass slide and evaporating to dryness.
A cover-glass is placed over this, and a drop of glacial acetic acid
allowed to run in. It is again heated until bubbles appear. Crystals of
haemin may now be detected by the microscope. They are dark brown or
yellow rhombic prisms.

An improvement on this test is the use of formic acid alone; on slowly
evaporating it, numerous very small dark crystals are visible if
haemoglobin has been present (Whitney's test).

(i) =Spectroscopic Appearances.=--If a solution of a recent stain be
examined by the spectroscope, we get two absorption bands situated
between the lines D and E, the one nearer E being doubly as broad as the
other. These bands indicate _oxyhaemoglobin_.

If we now add a little ammonium sulphide to this solution, we get the
spectrum of _reduced haemoglobin_, which is a single broad absorption
band situated in the interval between the preceding oxyhaemoglobin
bands. By shaking the solution, oxyhaemoglobin is again reproduced, and
gives its special absorption bands.

If ammonia be added to the original solution, _alkaline haematin_ is
produced, or if acetic acid be chosen, _acid haematin_ is produced, and
each gives its appropriate absorption bands.

_Methaemoglobin_ is formed in stains which have been exposed to the air
for a few days, and _haematin_ is found in old stains. _Haemochromogen_
gives a very characteristic spectrum, and is obtained by reducing
alkaline haematin by ammonium sulphide. _Carbon monoxide haemoglobin_
gives a spectrum which resembles that of oxyhaemoglobin, but it is not
reduced by ammonium sulphide.

(j) =Precipitin Test.=--This allows us to tell whether the blood is from
a human being or not. A specific serum must be obtained from a rabbit
which is sensitized as follows: 10 c.c. of human blood is injected into
its peritoneal cavity at intervals, until from three to five injections
have been given. The ser