36 REPORT—1840. 
sources is not explained by the slight difference of refrangibility ; 
and Prof. Forbes is of opinion that we must in consequence 
look for its solution to a mechanical theory of heat in some re- 
spects at least different from that of light. It is even a ques- 
tion of some difficulty, why any portion of the heat should not 
be subject to the law of polarization which the rest obeys, unless 
we suppose the heating effect to be of so complex a nature, that 
some part of it only is properly due to rays analogous to those 
of light, while the other part of the effect is produced by a mode 
of action altogether different. 
To any such questions, however, we are hardly yet in a condi- 
tion to give a satisfactory answer; but among the numerous 
points open to inquiry, I have dwelt more particularly on those 
which appear to me pre-eminently to require more extended in- 
vestigation before we can hope to obtain materials for construct- 
ing any substantial and unexceptionable theory. 
