thoria
in the burner mantles were absolutely pure or contained traces of some
of the other elements that were so difficult to separate.

This sudden change of venue from pure to applied science came about
through a Viennese chemist, Dr. Carl Auer, later and in consequence
known as Baron Auer von Welsbach. He was trying to sort out the rare
earths by means of the spectroscopic method, which consists ordinarily
in dipping a platinum wire into a solution of the unknown substance and
holding it in a colorless gas flame. As it burns off, each element gives
a characteristic color to the flame, which is seen as a series of lines
when looked at through the spectroscope. But the flash of the flame from
the platinum wire was too brief to be studied, so Dr. Auer hit upon the
plan of soaking a thread in the liquid and putting this in the gas jet.
The cotton of course burned off at once, but the earths held together
and when heated gave off a brilliant white light, very much like the
calcium or limelight which is produced by heating a stick of quicklime
in the oxy-hydrogen flame. But these rare earths do not require any such
intense heat as that, for they will glow in an ordinary gas jet.

So the Welsbach mantle burner came into use everywhere and rescued the
coal gas business from the destruction threatened by the electric light.
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. It is the ceria that gives the light, yet a little more of i