ON RADIANT HEAT. 
351 
one eminently deserving of being fully discussed. I can only pretend in 
this imperfect report to have suggested some of the materials which may 
assist in forming some judgement on this point. 
Analogies of Transmission of Light ami Heat by Waves. 
The very important researches of Mr. Joule on heat and the dynamical 
theory to which they lead, though referring directly to heat in its action on 
bodies as temperature or as latent heat, yet are not without a bearing on the 
subject of radiant heat, as has been in some degree pointed out in the excel¬ 
lent Address of our President of last year. (Report, 1853, p. xlvii.) 
Mr. Joule’s theory, though not as such dependent on the wave-theory of 
heat, is yet eminently in accordance with it, and so far lends it much sup¬ 
port If we suppose the temperature of a body to arise from vibrations of 
its molecules, such vibrations may be excited in it by the vibrations of an 
atherial medium surrounding and penetrating that mass of matter. In this 
respect the close analogy with sound is preserved. These vibrations of heat, 
however, produce mechanical changes in the constitution of the medium, they 
cause it to expand, i. e. they drive its molecules to greater distances apart; 
and when carried to a certain extent, cause a fresh and sudden separation to 
alar greater extent; accompanied with anew arrangement of these mole¬ 
cules, or a change of state in the body from solid to fluid or from fluid to 
aeriform. Here the analogy with sound ceases to hold good, except so far 
as that a temporary new arrangement of the molecules is occasioned by the 
sonorous vibrations. The transmission of luminiferous waves has a velocity, 
which, though enormous, is capable of measurement. Whether that of the 
longer non-luminil'erous but oaloriforous waves is the same, has not been, I 
believe, experimentally verified, but must theoretically be supposed the 
same; unless indeed it be only approximately the same for waves within the 
Narrow limits of the luminiferous scale, and diverge from that value beyond 
‘hose limits. 
Again, the passage or process of the vibrations in a body receiving heat 
**« to this conduction of heat there does not seem to be anything strictly 
analogous in sound. In general, the passage of light through transparent 
bodies excites in them no vibrations capable of affecting our eyes with the 
sense of light, i. e. the medium docs not become luminous, unless we except 
he case of the phosphorescence of fluor-spar and some other bodies after 
exposure to light. So far indeed as the transparency is imperfect , and in all 
bodies, the vibrations which constitute light are stopped, or changed iu 
• a manner that they give rise to vibrations in the body constituting heat, 
J u .*f ^ those longer vibrations do which constitute that species of radiatiou 
? I" 1 ‘ s derived from the mere cooling of a hot body; but this does not occur 
n ransparent bodies. It would seem to be the law, that if a ray, or a series of 
wes of the proper length to be luminiferous, impinge on. an opake body, 
tey communicate vibrations to its molecules, which again transmit to the 
Grounding tether other waves of greater length , which in like manner tra- 
f »P >cean d can again excite vibrations in bodies on w hich they impinge; 
./ J ‘rom any source a body have internal r derations of a certain intensity 
ether forming wans, or of what lengths, we have no means of deciding), 
r . lraDii mit to the surrounding mther vibrations which constitute waves of 
^ * greater than u certain given length, via. that which belongs to the 
luminiferous rays. II its internal vibrations are increased in 
a c<;rt;l ' 11 P°’ nt > >t then acquires the power of communicating 
.„„i ( ',. on 'he last) other vibrations to the sether forming waves of other 
Ua 6lualler lengths, so as to give rise to light. 
