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.
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