326 Mr. Powell on Solar Light and Heat. [May, 



ing effect. Prof. Leslie considers the proportion to be precise 

 and undeviating. 



(12.) Whatever we suppose to be the state in which the heat 

 exists when it thus inseparably accompanies the luminous rays, 

 it is evident that there must be some peculiar circumstance in 

 the mode of its union which makes its effects sensible only under 

 some particular circumstances ; and under others endows it with 

 properties which heat in its simple radiant state does not possess. 

 It evidently exists in a state essentially different from that of 

 simple radiant heat :, and we may in general say, that it is never 

 developed or rendered sensible except under such circumstances 

 as produce at the same time some modification or change in the 

 light itself. 



Upon considering all these well established facts, I think, 

 instead of using such terms as " calorific rays," and " luminous 

 rays," it is much more conformable to facts, and involves no 

 hypothetical ideas, to describe the phenomena by the terms 

 " rays of light," and the " heating poioer or property " of those 

 rays". 



III. (13.) Thus far my remarks have been confined to the 

 nature of the heating power of the sun when its rays are in that 

 state in which they naturally are, as coming directly from that 

 luminary. In the next place we have to inquire whether by any 

 modification which these rays may be made to undergo by artifi- 

 cial means, we can attain to any further elucidation of the nature 

 of this heating power. 



(14.) It has been shown by the experiments of M. Berard 

 (Biot, Traite de Phys. vol. iv. p. 603, &c), that when light under- 

 goes polarization, the heating power participates in that effect. 



It has also been shown by the same distinguished philosopher 

 (see Biot as above), that simple radiant heat when unaccompa- 

 nied by light is susceptible of being polarized also. In consi- 

 dering these results, we must be careful not to confound them 

 together ; because simple rays of heat are capable of displaying 

 the effects of polarization, and the heating effect in the solar 

 beam also obeys the same impulse, we must not hastily conclude 

 that the same agent existing in the same form is, therefore, the 

 common heating principle in both cases. 



(15.) The heating power of the sun is well known to be 

 capable of being collected in a focus along with the luminous 

 rays, both by reflexion and refraction. By the former means 

 simple radiant heat may also be concentrated : this circumstance 

 again shows a similarity, but does not prove an identity in the 

 agents or powers. 



In one of Sir W. Herschel's experiments (Phil. Trans. 1800, 

 IS T o. 15, Exper. 23), a focus of heat different from that of light 

 seems to be proved. This opinion it is not my design at pre- 

 sent either to maintain or controvert. I have only to observe, 

 that granting its truth, it must not be applied as an argument to 



