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90                                                MEDICAL  JURISPRUDENCE

(ii) Hydrogen Peroxide (3 per cent) Solution.—This is Merck's ' 10
volume' hydrogen peroxide. Instead of * 10 volume' solution ordinary
Merckozone or ' 12 volume ' solution (equivalent to 3.6 per cent) may be
used safely.

The solutions are usually kept in our laboratory for about a month after
which they are discarded although they may be used for a longer period.

The best way of performing the test is to clip off a small fragment of
the stained material or to tease out a fibre from the stained fabric and to
place it on a porcelain tile. At first a drop of benzidine solution and then
a drop of hydrogen peroxide solution are added, when an intense blue
colour radiating out on the tile is produced immediately if blood is present.
The test may also be obtained by gently pressing a piece of white blotting
or filter paper moistened with a few drops of distilled water on the stain and
by adding the reagents to the moistened paper.

The advantage of this test is that the same specimen with the blue colour
streaming out may be transferred to a slide for microspectroscopy. For this
purpose the material is treated at first with a drop of a 10 per cent solution
of potassium cyanide and then with a drop of ammonium sulphide; it is
then covered with a cover slip and looked for cyanhsemochromogen bands.
Coloured fabrics are, however, not suitable for direct spectroscopy.8

It must be borne in mind that a positive reaction may be obtained from
certain other substances, such as sputum, pus, nasal secretion, plant juices,
formalin 9 and a clay, called bentonite,10 but the reaction is decidedly weaker
and differs in its sensitiveness and in its shade and depth of colour. They
show a slow and faint colouration which should be ignored. A control test
performed side by side with a known blood stain decides the issue. Gum1-1
also gives a positive reaction with the benzidine test; hence it is not desirable "
to use gum for sticking labels on medico-legal exhibits of fabrics which are
suspected to have blood stains.

The benzidine test never fails to detect blood even in very old, de-
composed stains with every sort of dirt. The negative result is undoubtedly
valuable, but the positive results obtained so far in thousands of our cases
never failed to satisfy the confirmatory tests for blood.

3. Kastle-Meyer Test (Phenolphthalein Test).—The principle of this
test is based on the fact that if' ordinary phenolphthalein of the laboratory is
reduced by zinc dust in an alkaline solution, phenolphthalein is produced
which, if oxidized in the presence of an alkali, gives the characteristic red
colour. The reagents required for carrying out this test are—

1.    Hydrogen peroxide solution (20 volumes or 6 per cent strength).

2.    A mixture containing 2 grammes of phenolphthalein, 20 grammes of
potassium hydrate and distilled water in sufficient quantity to make up 100 cc.
of the solution.   These three ingredients are boiled, and during the process
10 to 30 grammes of powdered zinc are added.   Boiling is further continued
until the  solution becomes  colourless.   The  solution,   thus   prepared,  will
remain effective for a long period, if a small quantity of powdered zinc is
left deposited at the bottom of the reagent bottle to ensure reduction.

Jtfjen to twenty drops of the phenolphthalein reagent are added to a
solution extracted from the stain with distilled water, a deep permanganate
colour will be obtained instantaneously on the addition of a drop or two of
hydrogen peroxide solution if blood be present.

8.   Imperial Serologists' Annual Report, Calcutta, 1937-38.

9.    Sydney Smith, Foren. Med., Ed. IX, p. 211.

10.    Greval, S. D. S. and Roy Chowdhery, A. B., Ind. Med. Gaz., Sep. 1941, p. 535

11.   S. D. S. Greval, Ind. Med. Gaz., Nov. 1945, p. 556.