It
should be noted, however, that particles of lime dust in the gas may
cause the atmospheric flame to be reddish or yellowish (by presence of
calcium or sodium) quite apart from ordinary impurities; and for various
other reasons this appearance of the non-luminous flame is scarcely to be
relied upon. The simplest means of ascertaining definitely whether a
purifier is sufficiently active consists in the use of the test-papers
prepared by E. Merck of Darmstadt according to G. Keppeler's
prescription. These papers, cut to a convenient size, are put up in small
books from which they may be torn one at a time. In order to test whether
gas is sufficiently purified, one of the papers is moistened with
hydrochloric acid of 10 per cent. strength, and the gas issuing from a
pet-cock or burner orifice is allowed to impinge on the moistened part.
The original black or dark grey colour of the paper is changed to white
if the gas contains a notable amount of impurity, but remains unchanged
if the gas is adequately purified. The paper consists of a specially
prepared black porous paper which has been dipped in a solution of
mercuric chloride (corrosive sublimate) and dried.
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