ced
thereby; but after a while applying my self to consider them
more circumspectly, I became surprised to see them in an oblong
Form; which, according to the receiv'd Law of Refractions, I
expected should have been circular. They were terminated at the
Sides with streight Lines, but at the Ends the Decay of Light
was so gradual, that it was difficult to determine justly what
was the Figure, yet they seemed Semicircular.

Even Newton could not have had the faintest idea of the great
developments which were to be based upon the spectrum.

Now to return to the peculiar property of rare-earth oxides--namely,
their unusual brilliance when heated in a flame--it is easy to
understand the reason for this. For example, when a number of substances
are heated to the same temperature they may radiate the same amount of
energy and still differ considerably in brightness. Many substances are
"selective" in their absorbing and radiating properties. One may radiate
more luminous energy and less infra-red energy, and for another the
reverse may be true. The former would appear brighter than the latter.
The scientific worker in light-production has been searching for such
"selective" radiators whose other properties are satisfactory. The
rare-earths possess the property of selectivity and are fortunately
highly refractory. Welsbach used these in his mantle, whose efficiency
is due partly to this selective property. Recent work indicates that
much higher efficiencies of light-production are still attainable by the
principles involved in the gas-mantle.

Turning again to flames, another interesting physical phenomenon is seen
on placing solutions of different chemical salts in the flame. For
example, if a piece of asbestos is soaked in sodium chloride (common
salt) and is placed in a Bunsen flame, the pale-blue flame suddenly
becomes luminous and of a yellow color. If this is repeated with other
salts, a characteristic color will be noted in each case. The yellow
flame is characteristic of sodium and if it is examined by means of a
spectroscope, a brilliant yellow line (in fact, a double line) will be
seen. This forms the basis of spectrum analysis as applied in chemistry.

Every element has its characteristic spectrum consisting usually of
lines, but the complexity varies with the elements. The spectra of
elements also exhibit lines in the ultra-violet region which may be
studied with a photographic