n commercial use. His first mantles
were unsatisfactory, but they were improved in 1886 by the use of
thoria, an oxide of thorium, in conjunction with other rare-earth
oxides. His mantle was now not only stronger but it gave more light.
Later he greatly improved the mantles by purifying the oxides and
finally achieved his great triumph by adding a slight amount of ceria,
an oxide of cerium. Welsbach is deserving of a great deal of credit for
his extensive work, which overcame many difficulties and finally gave to
the world a durable mantle that greatly increased the amount of light
previously obtainable from gas.

The physical characteristics of a mantle depend upon the fabric and upon
the rare-earths used. It must not shrink unduly when burned, and the ash
should remain porous. It has been found that a mantle in which thoria is
used alone is a poor light-source, but that when a small amount of ceria
is added the mantle glows brilliantly. By experiment it was determined
that the best proportions for the rare-earth content are one part of
ceria and ninety-nine parts of thoria. Greater or less proportions of
ceria decreased the light-output. The actual percentage of these oxides
in the ash of the mantle is about 10 per cent., making the content of
ceria about one part in one thousand.

Mantles are made by knitting cylinders of cotton or of other fiber and
soaking these in a solution of the nitrates of cerium and thorium. One
end of the cylinder is then sewed together with asbestos thread, which
also provides the loop for supporting the mantle over the burner. After
the mantle has dried in proper form, it is burned; the organic matter
disappears and the nitrates are converted into oxides. After this
"burning off" has been accomplished and any residual blackening is
removed, the mantle is dipped into collodion, which strengthens it for
shipping and handling. The collodion is a solution of gun-cotton in
alcohol and ether to which an oil such as castor-oil has been added to
prevent excessive shrinkage on drying.

The materials and structure of the fabric of mantles have been subjected
to much study. Cotton was first used; then ramie fibers were introduced.
The ramie mantle was found to possess a greater life than the cotton
mantle. Later the mantles were mercerized by immersion in ammonia-water
and this process yielded a stronger material. The latest development is
the use of an artificial silk as the base fabric, which res