ed that of the chloroform when the source of heat was the flame
of bisulphide of carbon. Comparing the radiation from a Leslie's cube
coated with isinglass with that from a similar cube coated with
lampblack, at the common temperature of 100 deg.C, it was found that, out
of eleven vapours, all but one absorbed the radiation from the
isinglass most powerfully; the single exception was chloroform.

It is worthy of remark that whenever, through a change of source, the
position of a vapour as an absorber of radiant heat was altered, the
position of the liquid from which the vapour was derived underwent a
similar change.

It is still a point of difference between eminent investigators
whether radiant heat, up to a temperature of 100 deg.C, is monochromatic
or not. Some affirm this; some deny it. A long series of experiments
enables me to state that probably no two substances at a temperature
of 100 deg.C. emit heat of the same quality. The heat emitted by
isinglass, for example, is different from that emitted by lampblack,
and the heat emitted by cloth, or paper, differs from both. It is
also a subject of discussion whether rock-salt is equally diathermic
to all kinds of calorific rays; the differences affirmed to exist by
some investigators being ascribed by others to differences of
incidence from the various sources employed. MM. de la Provostaye
and Desains maintain the former view, Melloni and M. Knoblauch
maintain the latter. I tested this point without changing anything
but the temperature of the source; its size, distance, and
surroundings remaining the same. The experiments proved rock-salt to
be coloured thermally. It is more opaque, for example, to the
radiation from a barely visible spiral than to that from a white-hot
one.

In regard to the relation of radiation to conduction, if we define
radiation, internal as well as external, as the communication of
motion from the vibrating atoms to the aether, we may, I think, by
fair theoretic reasoning, reach the conclusion that the best radiators
ought to prove the worst conductors. A broad consideration of the
subject shows at once the general harmony of this conclusion with
observed facts. Organic substances are all excellent radiators; they
are also extremely bad conductors. The moment we pass from the metals
to their compounds we pass from good conductors to bad ones, and from
bad radiators to good ones. Water, among liquids, is probably the
worst c