31 



in the situations of their maxima. And we also collect that the pro- 

 jecting part of the heating rays, being on the side of the red or least 

 refrangible coloured ray, the aggregate of the former may be said to 

 be less refrangible than that of the latter. 



The Doctor now goes on to prove by experiments, that the sines 

 of refraction of the heat-making rays are in a constant ratio to their 

 sines of incidence, and points out the results of a correction of the dif- 

 ferent refrangibility of heat, by contrary refractions in different media. 



Experiments are also described which show that the focus pro- 

 duced by a lens is in fact twofold, that which is produced by the rays 

 of heat being in the same axis, but at some distance further from the 

 lens than the luminous focus, a property that might have been in- 

 ferred, a priori, from the less refrangibility of the heat-making rays. 



In the fifth article, which treats of the transmission of heat- making 

 rays through diaphanous bodies, besides the accurate description of 

 the various apparatus which it was necessary to contrive for the 

 purpose, and which can only be clearly understood by inspecting the 

 figures added to the paper, we find the results of 170 experiments 

 distinguished under the six following sections. 1 . On the transmis- 

 sion of solar heat through colourless substances; through glasses of 

 the different prismatic colours ; through liquids, such as well and sea- 

 water, and different spirits ; and through scattering substances, such 

 as ground glasses, paper, linen, silk, &c. 2. On the transmission 

 of the heat of terrestrial flame through various substances. 3. On 

 the transmission of the solar rays, which are of an equal refrangibility 

 with the red prismatic rays. 4. On the transmission of fire -heat 

 through various substances. 5. On the transmission of invisible 

 rays of solar heat. And lastly, (the subject which appears most preg- 

 nant with useful inferences for the common purposes of life,) on the 

 transmission of invisible terrestrial heat. Not only the general po- 

 sition, that the rays of heat, both solar and terrestrial, are detained 

 in their passage through various bodies, appears to be here completely 

 evinced, but the great variety in the power of the transmitting bodies 

 seems also to be determined with abundance of accuracy, and affords 

 matter of much consideration and curiosity. 



From the sixth article, in which it is intended to prove that the 

 rays of heat, both solar and terrestrial, are liable to be scattered on 

 rough surfaces, it appears that all bodies, even the most polished, are 

 sufficiently rough to scatter heat in all directions. And the chief ob- 

 ject of the twenty-four experiments here described, is to compare the 

 effects of rough surfaces on heat with their simultaneous effects on 

 light. The general and rather unexpected result is here brought for- 

 ward, that colours have no concern whatever in the laws that relate 

 to the scattering of heat. 



The chief object of the whole of this inquiry follows next in the 

 seventh article, where the question is discussed, " Whether light and 

 heat be occasioned by the same or by different rays ?" One of the 

 leading facts deduced from the experiments in the fourth section, is 

 that there are rays of heat, both solar and terrestrial, not endowed 



