inch in thickness,
which scarcely perceptibly affected the luminous intensity, absorbed
the entire chemical energy of the concentrated beam of the electric
light.

The close relation subsisting between a liquid and its vapour, as
regards their action upon radiant heat, has been already amply
demonstrated. [Footnote: 'Phil. Trans.' 1864; 'Heat, a Mode of
Motion,' chap, xii; and P. 61 of this volume.] As regards the nitrite
of amyl, this relation is more specific than in the cases hitherto
adduced; for here the special constituent of the beam, which provokes
the decomposition of the vapour, is shown to be arrested by the
liquid.

A question of extreme importance in molecular physics here arises:
What is the real mechanism of this absorption, and where is its seat?
[Footnote: My attention was very forcibly directed to this subject
some years ago by a conversation with my excellent friend Professor
Clausius.]

I figure, as others do, a molecule as a group of atoms, held together
by their mutual forces, but still capable of motion among themselves.
The vapour of the nitrite of amyl is to be regarded as an assemblage
of such molecules. The question now before us is this: In the act of
absorption, is it the molecules that are effective, or is it their
constituent atoms? Is the _vis viva_ of the intercepted light-waves
transferred to the molecule as a whole, or to its constituent parts?

The molecule, as a whole, can only vibrate in virtue of the forces
exerted between it and its neighbour molecules. The intensity of
these forces, and consequently the rate of vibration, would, in this
case, be a Junction of the distance between the molecules. Now the
identical absorption of the liquid and of the vaporous nitrite of amyl
indicates an identical vibrating period on the part of liquid and
vapour, and this, to my mind, amounts to an experimental proof that
the absorption occurs in the main _within_ the molecule. For it can
hardly be supposed, if the absorption were the act of the molecule as
a whole, that it could continue to affect waves of the same period
after the substance had passed from the vaporous to the liquid state.

In point of fact, the decomposition of the nitrite of amyl is itself
to some extent an illustration of this internal molecular absorption;
for were the absorption the act of the molecule as a whole, the
relative motions of its constituent atoms would remain unchanged, and
there would be no mechani