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To secure the purity of the chemicals, it would be of advantage to
issue them from a depot* or depots in each province.
In any case chemicals should be tested before being taken into use,
and for this purpose specimens should be sent to the Chemical Examiner
for report as to whether they are sufficiently pure to be efficient.
Where it is not possible to obtain the assistance of the Chemical
Examiner, the following rough tests may be employed, as they require
for their performance no special apparatus except a few test-tubes with
corks to fit them, and a 9-per cent. solution of caustic soda.
Perchloride of mercury should be of a white or glassy colour; heated
on an iron spoon over a flame it should disappear without leaving more
than a trace of residue. It is soluble in 16 parts of cold water—60 °F.
The impurity most likely to be present is calomel, which is insoluble
in water.
Take a little of the ' perchloride ' to be examined, powder it, place
it in a test-tube and add pure water. Shake and pour off the clear
fluid into a second test-tube. The clear fluid should contain the
perchloride of mercury. Add some of the caustic soda solution to it, a
yellow precipitate indicates perchloride of mercury.
If any salt is left in the bottom of the original test-tube, a little of
the caustic soda solution should be poured on it. Blackening indicates
the presence of calomel.
The examination of the carbolic acid is less satisfactory. A long
test-tube should be taken and sufficient of the carbolic acid poured into
it to fill it for 1½ inches. Twice the quantity of the 9 per cent. so-
lution of caustic soda should then be added to fill the test-tube up to 4½
inches. The tube should then be corked, well shaken, and the con-
tents allowed ample time to settle. If tar oils are present, they will
separate in a dark layer, and if this layer measures more than 1/20 of an
inch in the test-tube, the carbolic acid contains too much tar oil to be
efficient.
8. The strength † of the disinfecting perchloride solution should be
1 in 1,000 with the addition of 2 per thousand of hydrochloric acid, and a
little aniline blue or other colouring matter. The solution of perchlo-
ride of mercury should be prepared in concentrated form ‡ and specific
directions should be given on the label of the vessel containing it to
show the amount of the concentrated preparation necessary to make a
solution of 1 part of perchloride in 1,000 in strength, on the addition of
one gallon, or other convenient quantity, of water. But care must be
taken that the proper dilution is always observed, and that the solution
is not stored in a porous receptacle or brought into contact with metal.
* Note.—Collectors and Chairmen of Municipal Councils should obtain their supplies
of disinfectants by submitting indents to the Medical Store Depot, Madras, through the
Surgeon-General. (P.C.'s Proceedings, No. 774, dated 18th March 1899.)
† Vide Rule 15, page 77 and the Sanitary Commissioner's letter printed in Plague
Commissioner's Proceedings, No. 308, dated 1st February 1899.
‡ Note.—A useful alternative is to carry 4 oz. packets of the perchloride of mercury
in a bamboo and the acid packed with sawdust in a tin.