88 On the Alomic Theory. 



off, and because the light of the beam in the room unites to the 

 free caloric which accompanies it. 



It is probable, as Sir Isaac Newton and other philosophers 

 supposed, that the particles of light, like those of ponderable 

 matter, possess their repulsive and attractive poles, and that it is 

 upon this account that they dart from luminous bodies in straight 

 lines ; and it is also likely, while in the act of moving or radi- 

 ating with velocity almost incalculable, that they are incapable of 

 uniting to the free particles of caloric which accompany them iu 

 their flight ; and that so soon as this rapid motion ceases, by 

 the interposition of opaque bodies, their particles may assume a 

 new arrangement in relation to each other, which arrangement 

 may enable them to unite the more readily to caloric, and pro- 

 bably to certain ponderable bodies ; and that by this union they 

 become incapable of illuminating upon principles already ex- 

 plained. Light diminishes in regular progression through a 

 medium of imiform density as we recede from the luminous body, 

 as Count Rumford lias siiown in a very ingenious way by means 

 of his photometer, a verv simple instrument. This is occasioned 

 by the absorption of the particles of light as they move along. 

 Indeed the feeble light we receive from the fixed stars, which are 

 considered suns as bright as our own, affords a strong illustra- 

 tion of the subject. 



When the bright light of the sun, as it radiates immediately 

 to us from that luminary, is collected into a focus by means of a 

 large burning lens, prodigious heat and light are produced. Per- 

 haps the intensity of the illumination is increased by the libera- 

 tion of a portion of the combined light of the caloric from the 

 concentration of its particles ; and it is also more than probable 

 that the intensity of the heat is in some degree increased by a 

 diminution of the combined or atmospheric light of the particles 

 of caloric, so as to set them more at liberty. 



When moon-light is treated in the same way by a lens, very 

 brilliant light is produced in the focus, but no sensible degree of 

 heat whatever ; yet bv means of the prism, moon-light is sepa- 

 rated into the seven coloured rays. Hence we may infer that 

 the rays of light do not heat ; but I am inclined to believe that 

 moon-light contains some small portions of caloric, yet sufficient 

 to absorb all the light when its source is interrupted by opaque 

 bodies. Dr.Herschel the celebrated a-slrononier has shown that 

 heat and light may be in some degree separated from each other, 

 by placing a thermometer in the different coloured rays produced 

 bvtbe prism. The red ray being the least refrangible has the most 

 lieating power, and this power diminishes progressively according 

 to the degree of the refrangibility of the different rays : conse- 

 quently the violet ray, which is the most refrangible, has the 



smallest 



