CHEMISTRY. 



Element! red end. This led him to suspect, that the heating 



. 



. power does not stop at the end of the spectrum. On 

 ^' trying the experiment, he found that a thermometer 

 placed a little beyond the spectrum rose still higher 

 than when in the red ray. This important experi- 

 ment was successfully repeated by Sir Henry Engle- 

 field. Hence it follows, that there are rays emitted 

 from the sun which produce heat, but hare not the 

 power of illuminating : consequently caloric is emit- 

 ted from the sun in rays, and the rays of caloric are 

 not the same with the rays of light. 



All the illuminating rays have the power of ex- 

 citing heat. It is probable that they derive this 

 power from rays of caloric mixed with them ; for the 

 rays from the moon, though they consist of the 

 seven prismatic rays, do not, even when concentrated, 

 affect the most delicate thermometer. 



Thus it appears that solar light is composed of 

 three sets of rays, the colorific, the calorific, and the 

 de-oxidizing. 



The rays of caloric are refracted and reflected pre- 

 cisely as the rays of light. They obviously move 

 with a very considerable velocity, though what that 

 velocity is we do not at present know. It has been 

 ascertained that caloric produces no sensible effect 

 upon the weight of bodies ; the weight remaining 

 sensibly the same, whether a substance be hot or 

 cold. In this respect it agrees with light. It agrees 

 with light also in another property, its particles are 

 never found cohering together iii masses. 



SECT. II. Of the Motion of Caloric. 



Of the mo When heat radiates from the surfaces of bodies, it 

 tiou of moves with great velocity ; but, when it makes its 

 caloric. wa y through bodies, it moves comparatively slowly. 

 Let ua consider these two kinds of motions. 



1. Escape of Heat from Surfaces. 



EsApc of When bodies, artificially heated, are exposed to 

 heat from the open air, they emit heat, and continue to do so 

 surfaces. tj|j they sink to the temperature of the surrounding 

 atmosphere. The rapidity of their cooling depends 

 upon the nature of their surface. For the investiga- 

 tion of this branch of the subject, we are indebted 

 chiefly to the sagacity of Professor Leslie. A globe 

 of bright tin, filled with hot water, lost a certain 

 number of degrees of heat in 156 minutes. But, 

 when covered with a thin coat of lamp-black, it lost 

 the same number of degrees in 81 minutes. The 

 rate of cooling was likewise increased by covering it 

 over with a coat of linen, and by painting it with 

 black or white paint. This difference is only con. 

 ipicuous in still air. In a strong wind it diminishes, 

 or nearly disappears. 



Llie'ti- When a canister of tin, filled with hot water, is 



pcrlmcnts. placed before a concave mirror of bright polished tin, 



having a delicate thermometer in the focus, the ther- 



mometer experiences a certain elevation. The dif- 



ferential thermometer, invented by Mr Leslie, an- 



swers best for these experiments. It consists of a 



small glass tube, bent into the shape of the letter U, 



and terminating at each extremity in a small hollow 



ball. The tube is filled with sulphuric acid, tinged 



red with carmine. An ivory scale is affixed to one 



VOL. VI. FART I. 



Chemiitnr. 



of the legs, and the top of the liquid stands at the Elements 

 division of the scale marked 0. This thermometer is 

 not affected by any change in the temperature of the 

 room. But if one of its balls be heated, while the 

 other is not affected, the air within it expands and 

 pushes away the sulphuric acid, which rises in the 

 other leg. Hence it indicates changes of heat in a 

 particular point, as the focus of a mirror. The ball 

 of it which is applied to the point, and undergoes the 

 change, is called the focal ball. 



When the experiment is made in the way above 

 specified, the rise of the thermometer depends upon 

 the distance of the canister from the mirror, being al- 

 ways the greater the nearer the canister is to the mir- 

 ror. From Mr Leslie's experiments it follows, that 

 the effect on the thermometer is very nearly inversely 

 proportional to the distance of the canister from the 

 reflector. 



When the nature and position of the canister is the 

 same, the rise of the thermometer is always propor- 

 tional to the difference between the temperature of 

 the hot canister and that of the surrounding air. 



Heat radiates from the surfaces of hot bodies in 

 all directions, but the radiation is most copious in 

 the direction perpendicular to the surface of the hot 

 body. 



When different bodies are applied in succession to 

 the surface of the canister, their power of radiation 

 becomes evident, by the effect they produce upon 

 the thermometer. The following Table exhibits this 

 effect, as ascertained by the experiments of Mr Leslie. 



Lamp-black, 100 Radiating 



Water, by estimate, . . 100-f- power of 



Writing paper, .... 98 bodi "r 



Rosin, 96 



Sealing wax, 95 



Crown glass, 90 



China ink, 88 



Ice 85 



Minium, 80 



Isinglass, 80 



Plumbago, 75 



Tarnished lead, .... 45 



Mercury, 20+ 



Clean lead, 19 



Polished iron, 15 



Tin-plate, 12 



Gold, silver, copper, . . 12 



Thus it appears that metals radiate heat worst, and 

 that lamp-black, paper, and glass, are among the best 

 radiators of it tried. The radiating power of the 

 metals is increased by tarnishing and by scratching 

 their surface. 



The radiating powers of these bodies were ascer- 

 tained by applying thin coats of them to the surface 

 of the canisters. Now, it appears that the radiating 

 power increases somewhat with the thickness of the 

 coat, till that coat amounts to the i 6 ' a g of an inch, 

 when it remains stationary. But this does not hold 

 with respect to metallic bodies, the thinnest coat of 

 which produces as great an effect as the thickest. 



When the focal ball of the thermometer is glass, 

 let us suppose that it rises 100. If we coat it with 

 E 



