It was no longer necessary to enrich the gas with oil to make its flame
luminous, for a cheaper fuel gas such as is used for a gas stove will
give, with a mantle, a fine white light of much higher candle power than
the ordinary gas jet. The mantles are knit in narrow cylinders on
machines, cut off at suitable lengths, soaked in a solution of the salts
of the rare earths and dried. Artificial silk (viscose) has been found
better than cotton thread for the mantles, for it is solid, not hollow,
more uniform in quality and continuous instead of being broken up into
one-inch fibers. There is a great deal of difference in the quality of
these mantles, as every one who has used them knows. Some that give a
bright glow at first with the gas-cock only half open will soon break up
or grow dull and require more gas to get any kind of a light out of
them. Others will last long and grow better to the last. Slight
impurities in the earths or the gas will speedily spoil the light. The
best results are obtained from a mixture of 99 parts thoria and 1 part
ceria.
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