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Full text of "Medical Jurisprudence And Toxicology"

510                                              MEDICAL JURISPRUDENCE

ending in a jet. Granulated zinc and dilute sulphuric acid are dropped into
the bottle, when hydrogen will be evolved, and will burn with a pale blue
flame on applying a light to it. It must be remembered that hydrogen is not
ignited at once but after about ten minutes, when all the air in the bottle is
chased out, otherwise the mixture of hydrogen and oxygen will ignite with
a loud explosion and break the apparatus thus injuring those round about.
The other reason for expelling all the air out is that even if traces of air are
left behind, when the flame is applied to the tube, water will be formed to
the detriment of the arsenical deposit which will appear as a greyish-white
cloud.

On adding the suspected mixture of arsenic into the bottle, the flame
begins to burn with a bluish or greenish-violet or purple tint due to the
formation of arseniuretted hydrogen which also emits a garlic-like odour.
If a cold porcelain dish be depressed into the flame, a blackish-brown stain
of metallic lustre is produced. This stain is readily soluble in a solution of
calcium hypochlorite; while the addition of ammonium sulphide does not
dissolve, but detaches it from the porcelain, and on heating turns it yellow.

If the deposit be heated with a few drops of strong nitric acid, and if
silver nitrate be then added, a brick red (reddish-brown) precipitate of
silver arsenate is formed, which is soluble in ammonia.

If the flame be extinguished, and the central part of the tube conveying
arseniuretted hydrogen be heated to redness by means of a spirit lamp for
some time, a brilliant arsenical mirror of a darker and less silvery white
colour appears immediately beyond the heated spot. If the portion be cut
off and heated in a dry test tube a white deposit is formed on its inside,
which shows octahedral crystals under the microscope. Very low mirrors,
such as obtained with 0.006, 0.008 and 0.001 mg. of arsenic trioxide do not
give crystals of arsenious acid when heated in dry test tubes. In order to
determine whether such mirrors are due to arsenic, the following technique
evolved by Mr. K. B,. Ganguly, Assistant Chemical Examiner, United
Provinces, Agra, may be adopted : —

The two ends of the narrower portions of the Marsh's tubes (used for
determination of arsenic by deposition of mirrors) containing the mirror,
should be sealed with air instead of hydrogen inside it. The mirror should
then be passed gently over a flame several times, until the mirror is visible;
the broader portion of the sealed tube should then be- heated to drive the
crystals of arsenious acid in the narrower part. After cooling, the character-
istic crystals of arsenious acid may be detected under the high power of a
microscope.

Commercial zinc and sulphuric acid are often contaminated with
arsenic ; hence a control experiment must be made to prove the purity of
these reagents or the exit tube may be heated for at least thirty minutes
before any of the suspected fluids is introduced into the hydrogen generating
bottle. If the tube remains free from deposit, the purity of the reagents is
established.

To obtain pure hydrogen without any trace of arsenic it is better to use
hydrogen generated by the electrolysis of water.

5. Gutzeit Test.—One cubic centimetre of the suspected solution is
placed in a large test tube with a piece of chemically pure zinc and a few
cc. of dilute sulphuric or hydrochloric acid containing enough solution of
iodine in potassium iodide to colour it yellow so as to remove sulphur dioxide
and hydrogen sulphide, if formed. A plug of absorbent cotton wool is
inserted in the upper part of the tube, and the mouth is covered with a piece
of filter paper moistened with a concentrated solution of silver nitrate
(1 : 1). If arsenic is present the paper is turned yellow owing to the for-