on the terrestrial Rays that occasion Heat. 29$ 



and as, in the second, we may obtain it from terrestrial flames, 

 such as torches, candles, lamps, blue-lights, &c. 



Our next division comprehends the heat of coloured radiants. 

 This we obtain, in the first place, from the sun, by separating 

 its rays in a prism ; and, in the second, by having recourse to 

 culinary fires, openly exposed. 



The third division relates to heat obtained from radiants, 

 where neither light nor colour in the rays can be perceived. 

 This, as I have shewn, is to be had, in the first place, directly 

 from the sun, by means of a prism applied to its rays ; and, in 

 the second, we may have it from fires inclosed in stoves, and 

 from red-hot iron cooled till it can no longer be seen in the 

 dark. 



Besides the arrangement in the order of my experiments 

 which would arise from this division, we have another subject to 

 consider. For, since the chief design of this paper is to give a 

 comparative view of the operations that may be performed on 

 the rays that occasion heat, and of those which we already 

 know to have been effected on the rays that occasion light, it 

 will be necessary to take a short review of the latter. I shall 

 merely select such facts as not only are perfectly well known, 

 but especially such as will answer the intention of my compa- 

 rative view, and arrange them in the following order. 



1. Light, both solar and terrestrial, is a sensation occasioned 

 by rays emanating from luminous bodies, which have a power 

 of illuminating objects; and, according to circumstances, of 

 making them appear of various colours. 



2. These rays are subject to the laws of reflection. 



3. They are likewise subject to the laws of refraction. 



4. They are of different refrangibility. 

 mdccc. 2 3 



