Detection Of Blood-stains Etc
Stains may require detection on clothing, on cutting instruments, on
floors and furniture, etc. The following are the distinctive characters
(a) =Ocular Inspection.=--Blood-stains on dark-coloured materials, which
in daylight might be easily overlooked, may be readily detected by the
use of artificial light, as that of a candle, brought near the cloth.
Blood-spots when recent are of a bri
ht red colour if arterial, of a
purple hue if venous, the latter becoming brighter on exposure to the
air. After a few hours blood-stains assume a reddish-brown or chocolate
tint, which they maintain for years. This change is due to the
conversion of hæmoglobin into methæmoglobin, and finally into hæmatin.
The change of colour in warm weather usually occurs in less than
twenty-four hours. The colour is determined, not entirely by the age of
the stain, but is influenced by the presence or absence of impurities
in the air, such as the vapours of sulphurous, sulphuric, and
hydrochloric acids. If recent, a jelly-like material may be seen by the
aid of a magnifying-glass lying between the fibres. If old, a
cinnabar-red streak is seen on drawing a needle across the stain.
(b) =Microscopic Demonstration.=--With the aid of the microscope, blood
may be detected by the presence of the characteristic blood-corpuscles.
The human blood-corpuscle is a non-nucleated, biconcave disc, having a
diameter of about 1/3500 of an inch. All mammalian red corpuscles have
the same shape, except those of the camel, which are oval. The
corpuscles of birds, fishes, reptiles, and amphibians, are oval and
nucleated. The corpuscles of most mammals are smaller than those of man,
but the size of a corpuscle is affected by various circumstances, such
as drying or moisture, so that the medical witness is rarely justified
in going farther than stating whether the stain is that of the blood of
a mammal or not. Unfortunately, the corpuscles are usually so dried that
little information regarding their size can be given.
(c) =Action of Water.=--Water has a solvent action on blood, fresh
stains rapidly dissolving when the material on which they occur is
placed in cold distilled water, forming a bright red solution. The
hæmatin of old stains dissolves very slowly, so employ a weak solution
of ammonia, and this will give a solution of alkaline hæmatin. Rust is
not soluble in water.
(d) =Action of Heat.=--Blood-stains on knives may be removed by heating
the metal, when the blood will peel off, at once distinguishing it from
rust. Should the blood-stain on the metal be long exposed to the air,
rust may be mixed with the blood, when the test will fail. The solution
obtained in water is coagulated by heat, the colour entirely destroyed,
and a flocculent muddy-brown precipitate formed.
(e) =Action of Caustic Potash.=--The solution of blood obtained in water
is boiled, when a coagulum is formed soluble in hot caustic potash, the
solution formed being greenish by transmitted and red by reflected
(f) =Action of Nitric Acid.=--Nitric acid added to a watery solution
produces a whitish-grey precipitate.
(g) =Action of Guaiacum.=--Tincture of guaiacum produces in the watery
solution a reddish-white precipitate of the resin, 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) =Hæmin 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
hæmin 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
hæmoglobin 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 oxyhæmoglobin.
If we now add a little ammonium sulphide to this solution, we get the
spectrum of reduced hæmoglobin, which is a single broad absorption
band situated in the interval between the preceding oxyhæmoglobin
bands. By shaking the solution, oxyhæmoglobin is again reproduced, and
gives its special absorption bands.
If ammonia be added to the original solution, alkaline hæmatin is
produced, or if acetic acid be chosen, acid hæmatin is produced, and
each gives its appropriate absorption bands.
Methæmoglobin is formed in stains which have been exposed to the air
for a few days, and hæmatin is found in old stains. Hæmochromogen
gives a very characteristic spectrum, and is obtained by reducing
alkaline hæmatin by ammonium sulphide. Carbon monoxide hæmoglobin
gives a spectrum which resembles that of oxyhæmoglobin, 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 serum of this animal's blood will then give a white
precipitate only when brought into contact with dilute solutions of
human blood, but with the blood of no other animal. This is known also
as the 'biologic,' or Uhlenhuth's test.
=Rust Stains.=--These are yellowish-red in colour, and do not stiffen
the cloth. The iron may be dissolved by placing the stain in a dilute
solution of hydrochloric acid, when, on adding ferrocyanide of
potassium, Prussian blue is produced.
=Fruit Stains= are seldom so dark as blood-stains. Solutions of these do
not change colour or coagulate on boiling; ammonia changes the colour to
blue or green; acid brightens the original colour, while chlorine
=Hairs.=--Human hairs must be identified and distinguished from those of
the lower mammals. If the hair has been pulled out from the root, the
microscope will show that the bulbous root has a concave surface which
fitted over the hair papilla, or that the root is encased in a fatty
=Fibres of Clothing.=--Microscopically, wool fibres are coarse, curly,
and striated transversely; cotton fibres appear as flattened bands
twisted into spirals; linen fibres are round, jointed at frequent
intervals, with small root-like filaments; silk fibres are solid,
continuous, and highly glistening.