EADIATION OF HEAT BY GASES AND VAPOUES. 
35 
shown that every atom absorbs in a special degree those waves which are synchronous 
with its own periods of vibration. Now, besides presenting broader sides to the ether, 
the association of simple atoms to form groups must, as a general rule, render their 
motions through the ether more sluggish, and tend to bring the periods of oscillation 
into isochronism with the slow undulations of obscure heat, thus enabling the molecules 
to absorb more effectually such rays as have been made use of in our experiments. 
Let me here state briefly the grounds which induce me to conclude that an agreement 
in period alone is not sufficient to cause powerful absorption and radiation; that in 
addition to this the molecules must be so constituted as to furnish 'points d'appui to 
the ether. The heat of contact is accepted with extreme freedom by rock-salt, but a 
plate of the substance once heated requires a great length of time to cool. This 
sm’prised me when I first noticed it. But the effect is explained by the experiments of 
Mr. Balfoue Stewaet, by which it is proved that the radiative power of heated rock- 
salt is extremely feeble. Periodicity can have no infiuence here, for the ether is capable 
of accepting and transmitting impulses of all periods ; and the fact that rock-salt 
requires more time to cool than alum, simply proves that the molecules of the former 
glide through the ether with comparatively small resistance, and thus continue moving 
for a longer time ; while those of the latter presenting broad sides to the ether, speedily 
communicate to it the motion which we call heat. This power of gliding through still 
ether, possessed by the rock-salt molecules, must of course enable the moving ether to 
glide round them, and no coincidence of period could, I think, make such a body a 
powerful absorber. 
Many chemists, I believe, are disposed to reject the idea of an atom, and to adhere to 
that of equivalent proportions merely. They figure the act of combination as a kind of 
interpenetration of one substance by another. But this is a mere masking of the funda- 
mental phenomenon. The value of the atomic theory consists in its furnishing the 
physical explanation of the law of equivalents ; — assuming the one the other follows ; and 
assuming the act of chemical union as Dalton figured it, we see that it blends harmo- 
niously with the perfectly independent conception of an ether, and enables us to reduce 
the phenomena of radiation and absorption to the simplest mechanical principles. 
Considerations similar to the above may, I think, be applied to the phenomena of 
conduction. In the Philosophical Transactions for 1853, I have described an instru- 
ment used in examining the transmission of heat through cubes of wood and other sub- 
stances. When engaged with this instrument, I had also cubes of various crystals pre- 
pared, and determined with it their powers of conduction. With one exception, I found 
that the conductivity augmented with the diathermancy. The exception was furnished 
by a cube of very perfect rock-crystal, which conducted slightly better than my cube of 
rock-salt. The latter, however, had a very high conductive power ; in fact rock-salt, 
calcareous spar, glass, selenite, and alum, stood in my experiments, as regards conducti- 
vity, exactly in their order of diathermancy in the experiments of Melloni. I have 
already adduced considerations which show that the molecules of rock-salt glide with 
