32 



with a power to render objects visible, and that there are different 

 degrees of heat in the prismatic spectrum of these invisible rays. 

 This being established, the question now, according to the original 

 enunciation, is in fact, " Whether some of these heat-making rays 

 may not have a power of rendering objects visible, superadded to 

 their own already established power of heating bodies ?" From a ge- 

 neral and comparative view of those among the preceding experiments 

 which apply to this question, we gather that no kind of regularity 

 takes place among the proportions of -the luminous and heating rays 

 which are stopped in their passage, and that hence it might be rea- 

 sonably inferred that heat and light are entirely unconnected. Yet, 

 not to evade the above hypothesis, the Doctor enters into 'a more 

 minute investigation of the subject, and shows that, admitting, ac- 

 cording to the supposition, that the same rays being both luminous 

 and calorific, may in their passage through certain media be so af- 

 fected as to produce the very discordant results observed in the ex- 

 periments, it is yet evident, on a due comparison of those results, 

 that no given proportion that may be ascribed to this operation of 

 the transmitting media, will anyways account for the general phae- 

 nomena ; the degrees of heat being in some instances greatly redun- 

 dant, and in others as much deficient, both ways deviating from any 

 given proportion. Thus it is that he reduces his opponent to the di- 

 lemma of either maintaining that the same agent may under different 

 circumstances produce effects perfectly dissimilar, such as heat with- 

 out light, decreasing heat and increasing light, or the reverse ; or 

 else to admit that there actually is a difference between the rays that 

 give light, and those which produce heat. 



A more direct proof of the difference of the two sorts of rays is 

 deduced from the manifest results of the experiments, in which the 

 stoppage of one sort of rays does by no means occasion the stoppage 

 of the other sort. In investigating this subject the Doctor contro- 

 verts a conjecture that the phenomena observed may be ascribed to 

 a peculiar texture or configuration in the diaphanous substances, 

 which produce differences in the transmission of the rays, though 

 there be no difference in the rays themselves. This hypothesis also 

 is minutely investigated, and its contradiction with the experiments 

 being pointed out, its very foundation seems in . fact to be wholly 

 subverted. 



Lastly, another direct proof of the difference of the two sorts of 

 rays, is deduced from a comparative view of the results of some of 

 the experiments, from which it appears that the stoppage of heat is 

 in general gradually extending as far as five minutes in time, whereas 

 the suppression of light hitherto appears to be instantaneous. This, 

 together with various other arguments derived from the transmission 

 of terrestrial heat, which cannot be properly explained in a manner 

 sufficiently concise for this place, seem to evince that in fact the law 

 by which heat is transmitted is essentially different from that which 

 directs the passage of light, and that hence there is every reason to 

 believe that the rays of heat are different from those of light. 



