. Any small body which was a good absorber of dark
rays was rapidly heated to redness when placed at the focus. Platinized
platinum (platinum foil upon which a thin film of platinum had been
deposited electrolytically) and charcoal were rendered incandescent,
black paper and matches immediately inflamed, ordinary brown paper
pierced and burned, while thin white blotting-paper, owing to its
transparency to the invisible rays, was scarcely tinged. A simpler
arrangement, also employed by Tyndall, is to cause the rays to be
reflected outwards parallel to one another, and to concentrate them by
means of a small flask, containing the iodine solution and used as a
lens, placed some distance from the camera. The rock-salt and cold water
circulation can then be dispensed with.

Since the rays used by Tyndall in these experiments are similar to those
emitted by a heated body which is not hot enough to be luminous, it
might be thought that the radiation, say from a hot kettle, could be
concentrated to a focus and employed to render a small body luminous. It
would, however, be impossible by such means to raise the receiving body
to a higher temperature than the source of radiation. For it is easy to
see that if, by means of lenses of rock-salt or mirrors, we focused all
or nearly all the rays from a small surface on to another surface of
equal area, this would not raise the temperature of the second surface
above that of the first; and we could not obtain a greater concentration
of rays from a large heated surface, since we could not have all parts
of the surface simultaneously in focus. The desired result could be
obtained if it were possible, by reflection or otherwise, to cause two
different rays to unite without loss and pursue a common path. Such a
result must be regarded as impossible of attainment, as it would imply
the possibility of heat passing from one body to another at a higher
temperature, contrary to the second law of thermodynamics (q.v.).
Tyndall used the dark rays from a luminous source, which are emitted in
a highly concentrated form, so that it was possible to obtain a high
temperature, which was, however, much lower than that of the source.

A full account of Tyndall's experiments will be found in his _Heat, a
Mode of Motion_. (J. R. C.)

CALORIMETRY, the scientific name for the measurement of quantities of
heat (Lat. _calor_), to be distinguished from thermometry, which
signifies the measuremen