CALORIC. 



neap. There are three species ; viz. the 

 serraticornis, hispicornis, and pygmxus. 



CALORIC, in chemistry, a word used 

 to signify that substance or property, by 

 which the phenomena of heat are pro- 

 duced. Concerning the nature of caloric 

 there are two opinions, which have divid- 

 ed philosophers ever since they turned 

 their attention to the subject. Some sup- 

 pose, that caloric, like gravity, is merely 

 a property of matter, and that it consists 

 in a peculiar vibration of its particles ; 

 others, on the contrary, think that it is a 

 distinct substance. Each of these opinions 

 has been supported by the greatest phi- 

 losophers ; and till lately the obscurity 

 of the subject has been such, that both 

 sides have been able to produce exceed- 

 ingly plausible and forcible arguments. 

 The recent improvements, however, in 

 this branch of chemistry, have gradually 

 rendered the latter opinion much more 

 probable than the former : and a recent 

 discovery made by Dr. Herschell, has at 

 last nearly put an end to the dispute, by 

 demonstrating that caloric is not a pro- 

 perty, but a peculiar substance ; or at 

 least that we have the same reason for 

 considering it to be a substance, as we 

 have for believing that light is material. 

 Dr. Herschell had been employed in mak- 

 ing observations on the sun, by means of 

 telescopes. To prevent the inconve- 

 nience arising from the heat, be used co- 

 loured glasses : but these glasses, when 

 they were deep enough coloured to in- 

 tercept the light, very soon cracked, and 

 broke in pieces. This circumstance in- 

 duced him to examine the heating power 

 of the different coloured rays. He made 

 each of them in its turn fall upon the 

 bulb of a thermometer, near which two 

 other thermometers were placed, to serve 

 as a standard. The number of degrees 

 which the thermometer exposed to the 

 coloured ray rose above the other two 

 thermometers indicated the heating pow- 

 er of that ray. He found that the most 

 refrangible rays have the least heating 

 power, and that the heating powr gra- 

 dually increases as the refrangibility di- 

 minishes. The violet ray therefore has 

 the smallest heating power, and the red 

 ray the greatest. Dr. Herschel found, 

 that the heating power of the violet, 

 green, and red rays, are to each other as 

 the following numbers : 



Violet 

 Green 

 Red . 



= 16 

 = 22.4 

 = 55. 



It struck Dr. Herschel as remarkable, 

 VOL. lit. 



that the illuminating power andtheheat : 

 ing power of the rays follow such differ- 

 ent laws. The first exists in greatest 

 perfection in the middle of the spectrum, 

 and diminishes as we approach either ex- 

 tremity ; but the second increases con- 

 stantly from the violet end, and is greatest 

 at the red end. This led him to suspect, 

 that perhaps the heating power does not 

 stop at the end of the visible spectrum, 

 but is continued beyond it. He placed 

 the thermometer completely beyond the 

 boundary of the red ray, but still in the 

 line of the spectrum, and it rose still 

 higher than it had done when exposed to 

 the red ray. On shifting the thermome- 

 ter still farther,! t continued to rise, and the 

 rise did not reach its maximum till the 

 thermometer was half an inch beyond 

 the utmost extremity of the red ray. 

 When shifted still farther, it sunk a little, 

 but the power of heating was sensible at 

 the distance of 1^ inch from the red ray. 

 These important experiments have been 

 lately repeated and fully confirmed by Sir 

 Henry Englefield, in the presence of 

 some very good judges. 



From these it follows, that there are 

 rays emitted from the sun which produce 

 heat, but have not the power of illumi- 

 nating; and that these are the rays 

 which produce the greatest quantity of 

 heat. Consequently caloric is emitted 

 from the sun in rays, and the rays of ca- 

 loric are not the same with the rays of 

 light. On examining the other extremity 

 of the spectrum, Dr. Herschel ascertain- 

 ed that no rays of caloric can be traced 

 beyond the violet ray. He had found, 

 however, that all the coloured rays of the 

 spectrum have the power of heating 1 ; it 

 may be questioned, therefore, whether 

 there be any rays which do not warm. 

 The coloured rays must either have the 

 property of exciting heat as rays of light, 

 or they must derive that property from a 

 mixture of rays of caloric. If the first of 

 these suppositions were 'true, light ought 

 to excite heat in all cases; but it has 

 been long known to philosophers, that 

 the light of the moon does not produce 

 the least sensible heat, even when con- 

 centrated so strongly as to surpass in 

 point of illuminatio.n the brightest can- 

 dles or lamps, and yet these produce a 

 very sensible heat. Here then are rays of 

 light which do not produce heat : rays, 

 too, composed of all the seven prismatic 

 coloured rays. We must conclude from 

 this well known fact, that rays of light do 

 not excite heat ; and consequently that 

 the coloured rays from the sun and com- 

 bustible bodies, since they excite heat,. 



G 



