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ARSENIC                                                           509

when seen under the microscope, shows octahedral crystals of arsenious acid
with their apices chopped off. If the coating is very thick it may be scraped
off, dissolved in acid and the liquid tests may be applied. Before proceeding
with Reinsch's test a .control test should be tried to prove the purity of
hydrochloric acid and copper foils.

Fig. 171.—Microphotograph  of Arsenious  Acid  Crystals X 300.
(R.B. Dr. K. N. BagcU.)

Sometimes even with a sufficient amount of arsenic present for its
successful detection by Reinsch's test in the normal course, very faint
blackening or only a faint dulling of copper is obtained specially in the case
of solid tissues, such as the liver, or in the case of earthy matter or ashes
containing vomit or stool. In such cases by heating the faintly blackened
01? dulled copper strips, crystals of arsenious acid are not detected. As
arsenic present in such cases is in a higher state of oxidation or the alkali
present in earthy matter, etc., partly or wholly neutralizes the acid in the
Reinsch's ^bath the deposition of arsenic is retarded. In such cases the fol-
lowing modification evolved by Mr. D. N. Chatterji, F.I.C., Chemical
Examiner to Governments, United Provinces and Central Provinces, should
be adopted :-—

Put more hydrochloric acid in the bath and boil with a little sulphurous
acid solution (SO2 water) till sulphur dioxide is removed. Then introduce
the copper foil. A black deposit of arsenic on the copper foil is^ easily
obtained. On now heating the copper foil in the usual way the characteristic
crystals of arsenious acid in the sublimate are detected.

4. Marsh's Test.—This forms such a delicate test for the presence of
arsenic that exceedingly small quantities even upto one thousandth of a
milligramme may be detected.

The test is based on the formation of arseniuretted hydrogen, when the
compounds of arsenic except the metal and its suphides are brought into
contact with nascent hydrogen. It is carried out by means of a WoulnVs
bottle (hydrogen generating bottle) to which is connected a long glass tube