dashed the
needles of a coarse galvanometer violently aside. It is now found
that on substituting for the face of the thermo-electric pile a
combustible body, the invisible rays are competent to set that body on
fire.

********************

6. Visible and Invisible Rays of the Electric Light.

We have next to examine what proportion the non-luminous rays of the
electric light bear to the luminous ones. This the opaque solution of
iodine enables us to do with an extremely close approximation to the
truth.

The pure bisulphide of carbon, which is the solvent of the iodine, is
perfectly transparent to the luminous, and almost perfectly
transparent to the dark, rays of the electric lamp. Supposing the
total radiation of the lamp to pass through the transparent
bisulphide, while through the solution of iodine only the dark rays
are transmitted. If we determine, by means of a thermoelectric pile,
the total radiation, and deduct from it the purely obscure, we obtain
the value of the purely luminous emission. Experiments, performed in
this way, prove that if all the visible rays of the electric light
were converged to a focus of dazzling brilliancy, its heat would only
be one-eighth of that produced at the unseen focus of the invisible
rays.

Exposing his thermometers to the successive colours of the solar
spectrum, Sir William Herschel determined the heating power of each,
and also that of the region beyond the extreme red. Then drawing a
straight line to represent the length of the spectrum, he erected, at
various points, perpendiculars to represent the calorific intensity
existing at those points. Uniting the ends of all his perpendiculars,
he obtained a curve which showed at a glance the manner in which the
heat was distributed in the solar spectrum. Professor Mueller of
Freiburg, with improved instruments, afterwards made similar
experiments, and constructed a more accurate diagram of the same kind.
We have now to examine the distribution of heat in the spectrum of the
electric light; and for this purpose we shall employ a particular form
of the thermo-electric pile, devised by Melloni. Its face is a
rectangle, which by means of movable side-pieces can be rendered as
narrow as desired. We can, for example, have the face of the pile the
tenth, the hundredth, or even the thousandth of an inch in breadth. By
means of an endless screw, this linear thermo-electric pile may be
moved through the entire spectru