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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

of blood-stains:

(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

bleaches it.

=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.