248 Prof. Forbes on the Undulatory Theory of Heat. 
Or the distinction may be founded on the extent of displacement 
of the ethereal particles, or on a want of coincidence with the 
law of force produced by displacement as commonly assumed, 
or on a thousand other causes, on which I do not wish to 
dwell because I see little advantage in presenting premature 
hypotheses which a year or two may demolish. I cannot help 
observing, however, in an experimental point of view, that if 
Sir D. Brewster’s analysis of the solar spectrum he adopted, 
we have a difficulty in the case of light identical with that in 
the case of heat. It surely would have been unreasonable to 
urge against that analysis that it could not be true, because it 
is contrary to the assumption that colour depends on frequency 
of vibration ;—and refrangibility solely upon the velocity of a 
wave :—these are tne very points to be proved, and if we have 
no breach of analogy between light and heat but on ground 
still debateable as regards the former, the supporters of calo- 
rific waves have little to tremble for. 
Since the experiments of M. Matteucci respecting the in- 
terference of calorific rays have been treated (and [ am in- 
clined to think justly) as inconclusive, the proof of the polari- 
zation and double refraction of heat is the only one to which 
we can refer with any confidence as a basis of analogical rea- 
soning. The phenomena of polarization and depolarization 
of perfectly dark heat I have now succeeded in making as 
obvious as most of the more ordinary experiments on trans- 
mission, and I have lately succeeded in completing the ana- 
logy in one case which seems to put the nature of the calorific 
emanations beyond a doubt. Fresnel’s marvellous prediction 
of the circular polarization of light by two internal total re- 
flexions at certain angles, is justly appealed to as one of the 
most conclusive evidences in favour of a theory which could 
foresee so singular a result. By employing a rhomb of rock 
salt I have obtained precisely analogous results in the case of 
heat wholly unaccompanied by light*. ‘The loss is so trifling 
in passing through this amazing substance (the discovery of 
whose properties I hold to be the most valuable part of M. 
Melloni’s valuable labours), and the total reflexion so far com- 
plete, that this curious and complex experiment is almost as 
easily tried as any of those in common polarization. 
With such evidence before me I cannot for a moment doubt 
that the waves (if such there be either in light or heat) pro- 
duced by non-luminous hot bodies are identical in character 
with those producing light, that is, that the vibrations are 
transversal. 
Before concluding, I beg to mention briefly a decisive ex- 
* Communicated to the Royal Society of Edinburgh, Ist Feb. 1836. 
