RADIANT HEAT, AND ITS CONVERSION THEREBY INTO SOUND. 
319 
b. The Plates of Rocksalt. 
Having thus, I trust effectually, disposed of the hypothesis which ascribed my 
results to liquid layers covering the interior surface of the experimental tube, I 
proceed to the examination of the plates of rocksalt which have also been credited 
with a liquid deposit. This hypothesis is to a great extent disposed of in a paper of 
mine published in the Philosophical Transactions for 1864 ; which, like other papers, 
has been overlooked by writers on this question. An apparatus is there described 
which enabled me to operate upon gaseous strata of different thicknesses, the thick¬ 
ness, in each case, being measured with great accuracy by aid of a vernier. Let 
two polished plates of rocksalt be supposed in close contact with each other, and let 
them be gradually separated by a suitable rack-and-pinion movement. From the first 
moment of their separation let the space between them be copiously supplied with 
vapour. Liquid films, if they form at all, will be deposited when the distance between 
the plates of salt is but small. As far as the films are concerned, they will be as 
influential when the plates are ^-th of an inch apart as when they are 2 inches apart. 
Hence, if the hypothesis of my opponents be correct, the absorption ought to declare 
itself with the former amount of separation as clearly as with the latter. But if, as 
I allege, the absorption be the act of the vapour molecules, then the deepening of the 
vapour stratum will be accompanied by an increase of the absorption. The following 
experiment bearing directly on this subject was executed in 1864. My plates of salt 
were first fixed at a distance of Aoth of an inch asunder, and the space between them 
was copiously supplied with air saturated with sulphuric ether vapour. The distance 
between the plates of salt was then augmented by steps, as shown in the following 
table, and at each step the absorption was determined. Here are the results :— 
Thickness of vapour stratum. 
Absorption. 
0'05 inch. 
2T per cent. 
OT 
D ..... 
. . 4-6 
0-2 
.. 
. . 87 
0’4 
..... 
. . 14-3 
0-8 
» ..... 
. . 2L0 
1*5 
.. 
. . 34-6 
2'0 
.. 
. . 35T 
We thus see that when the depth of the vapour stratum augments from - 2 - 0 -th of 
an inch to 2 inches, the absorption augments from 2 per cent, to 35 per cent, of the 
total radiation. 
It is only with sulphuric ether that an absorption of 2 per cent, by the thinnest 
stratum could be obtained. With most other vapours the action of a layer - 2 L 0 -th of an 
inch in thickness proved insensible. We may thus begin with a layer which yields an 
