Methane .................................. .04

The oxygen, nitrogen, hydrogen, methane and carbon monoxide are either
harmless or are present in such small quantities as to be neglected. The
phosphoretted hydrogen and silicon hydride are self-inflammable gases when
exposed to the air, but their quantity is so very small that this
possibility may be dismissed. The ammonia and sulphuretted hydrogen are
almost entirely dissolved by the water used in the gas generator. The
surest way to avoid impure gas is to use high-grade calcium carbide in the
generator and the carbide of American manufacture is now so pure that it
never causes trouble.

The first and most important purification to which the gas is subjected is
its passage through the body of water in the generator as it bubbles to the
top. It is then filtered through felt to remove the solid particles of lime
dust and other impurities which float in the gas.

Further purification to remove the remaining ammonia, sulphuretted hydrogen
and phosphorus containing compounds is accomplished by chemical means. If
this is considered necessary it can be easily accomplished by readily
available purifying apparatus which can be attached to any generator or
inserted between the generator and torch outlets. The following mixtures
have been used.

"_Heratol,_" a solution of chromic acid or sulphuric acid absorbed in
porous earth.

"_Acagine,_" a mixture of bleaching powder with fifteen per cent of
lead chromate.