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224                                                 MEDICAL JURISPHUDENCE

with carbon is found in the barrel of the firearm, if black gunpowder was

used.   For the first five or six hours this deposit forms a strong alkaline

solution with distilled water and emits an offensive odour of sulphuretted

hydrogen.   If the solution is filtered,  and the  nitrate is  treated with a

solution of lead acetate,  a black precipitate of lead sulphide is formed.

After exposure to air  and moisture for a few days potassium  sulphide

t becomes converted into thiosulphate, thiocyanate and finally into potassium

? sulphate, which forms a neutral solution with distilled water and gives a

* white precipitate with lead acetate.    At later periods oxides of iron  (iron

rust) with traces of iron sulphate are formed in the barrel.

Smokeless nitro-powders leave a dark grey deposit in the barrel of a
recently discharged firearm. It does not change with the lapse of time. It
forms a neutral solution with distilled water, and contains nitrites and
nitrates, but does not contain sulphides. If the chromate or bichromate
powder is used, the residue in the barrel is usually of a greenish tint.10

It should be borne in mind that the composition of the deposit would
vary considerably, if the firearm was dirty at the time of its discharge, and
the medical practitioner has no means to know its condition prior to dis-
charge.   Again, the deposit would not be found, if the weapon had been"
thoroughly cleansed after discharge.

Direction from which the Weapon was fired.—The question regarding
the direction from which the weapon was fired may arise in a case where
it is alleged that it was fired from a certain point in a quarrel. To ascertain
this it is necessary to know the position of the victim at the ffine- of the
discharge of the bullet, when a straight line drawn between the entrance
and exit wounds and prolonged in front should indicate the line of direction.
In some cases it is difficult to determine the direction as the bullet is so
often deflected by the tissues that its course is very irregular.

10. For further details of the examination of the weapon and the residue the reader
is referred to Sydney Smith and Glaister's Recent Advances in Forensic Medicine, Ed.
n, Chapts, HI and IV.