The phenomenon has been examined by
Mauricheau-Beaupre, who finds that frothing is not characteristic of pure
carbide and that it cannot be attributed to any of the impurities
normally present in commercial carbide. If, however, the carbide contains
calcium chloride, frothing is liable to occur. A 0.1 per cent. solution
of calcium chloride appears to yield some foam when carbide is decomposed
in it, and a 1 per cent. solution to foam in a pronounced manner. In the
absence of calcium chloride, the main cause of frothing seems to be the
presence in the generator of new paint or tar. If a generator is taken
into use before the paint in any part of it which becomes moistened by
warm lime-water has had opportunity of drying thoroughly hard, frothing
is certain to occur; and even if the carbide has been stored for only a
short time in a tin or drum which has been freshly painted, a production
of froth will follow when it is decomposed in water. The products of the
polymerisation of acetylene also tend to produce frothing, but not to
such an extent as the turpentine in paint and the lighter constituents of
coal-tar. Carbide stored even temporarily in a newly painted tin froths
on decomposition because it has absorbed among its pores some of the
volatile matter given off by the paint during the process of desiccation.
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