HEAT. 



heated body, 9.2 marked the heating 

 power. A still greater effect, equal to 

 1 8, was produced, when the surface of 

 the 'glass, next to the source of heat, 

 was blackened, the metallic coating 

 being entirely removed. 



Endeavouring to discover the velo- 

 city with which radiant heat moves 

 through space, Pictet placed two con- 

 cave metallic reflectors opposite to each 

 other, at the distance of sixty-nine feet 

 apart: he interposed a thick screen, a 

 few inches distant from the focus of the 

 miiTor in which the heated body was to 

 be placed, and an air thermometer in 

 the focus of the other mirror ; the ball, 

 heated a little below ignition, being intro- 

 duced, the screen was removed, and in- 

 stantly the thermometer began to show 

 an increase of temperature. When 

 the screen was again suddenly inter- 

 posed, the effect produced upon the ther- 

 mometer ceased at the same moment ; 

 from which experiments he inferred that 

 radiant heat moves with such velocity, 

 as to require no perceptible interval of 

 time to enable it to traverse sixty-nine 

 feet of space. Sir William Herschel, in 

 his investigation concerning the consti- 

 tution of the sun's rays, found that the 

 different coloured rays of the prismatic 

 spectrum produced different degrees of 

 effect upon a thermometer, the red occa- 

 sioning the greatest rise of temperature, 

 the violet the least. The heat of the red 

 rays, compared with that of the green, 

 was considered to be as 55 to 26, and 

 with that of the violet rays as 55 to 16. 

 In ten minutes a thermometer, placed 

 in the full red rays, rose 7 of Fahren- 

 heit's scale, and beyond the red ray the 

 increase, in the same time, appeared to 

 be nine degrees. These experiments 

 were repeated and verified by Sir H. 

 Englefield. He discovered also that in- 

 visible heating rays exist beyond the 

 coloured rays, and imagined that the 

 greatest degree of heat was produced 

 at the distance of half an inch beyond 

 the red rays. Other philosophers have 

 confirmed the existence of these invisible 

 heating rays, but M. Berard affirms that 

 the greatest heating effect is produced 

 within the red ray. The experiments of 

 M. Berard on the sun's rays, (Amiales 

 de Chimie, March, 1813, and Annals of 

 Philosophy, September, 1813) were con- 

 ducted with the aid of a heliostate* by 



* From two Greek words signifying the standing 

 still of the sun; because, by machinery, the image 

 of the sun i kept stationary uiion a wall or sheet 

 01 paper. 



which he was enabled to obtain an im- 

 movable coloured spectrum. In the dif- 

 ferent coloured rays, of which this spec- 

 trum is composed, he suspended deli- 

 cate thermometers, and ascertained their 

 different heating powers with accuracy. 

 The greatest heating effect was pro- 

 duced upon a thermometer, when its 

 bulb was entirely covered by the red 

 ray ; the instruments being placed beyond 

 the red ray, in the space where Sir W. 

 Herschell imagined the greatest degree 

 of heat to exist, the rise of temperature 

 was only one -fifth as much as that which 

 had been produced within the red rays. 



The same philosopher formed a prism 

 of Iceland spar, which divided a beam of 

 light, made to pass through it, into two 

 similar coloured spectra, the properties 

 of which were the same as those of the 

 spectrum obtained by decomposing light 

 with a glass prism. 



He also polarized a portion of the 

 light by receiving it upon glass ; this 

 polarized portion being intercepted by a 

 second glass which was made to revolve, 

 the rays were then collected by a mirror, 

 and directed upon a thermometer, and 

 it was found that as long as light was 

 reflected from the second glass, the tem- 

 perature of the thermometer was raised ; 

 but when the position of the second 

 glass was such, that all the light was 

 transmitted, the whole of the heat was 

 transmitted along with it, as the ther- 

 mometer ceased then to be affected. 

 The experiments and discoveries of 

 Herschell, which have been alluded to, 

 are published in the Philosophical Trans- 

 actions for 1800, as were also the ac- 

 counts of other experiments, by the 

 same philosopher, yet to be mentioned. 



Having ascertained, from the pheno- 

 mena of the prismatic spectrum, the 

 refraction of the heating rays accom- 

 panying light in the beam, he deter- 

 mined to try if the calorific rays pro- 

 jected from a heated body were also 

 subject to refraction. He placed a lens 

 near to a burning candle, having first 

 interposed a screen, with an aperture 

 nearly of the same size as the lens ; the 

 rays from the candle passing through 

 this aperture, were refracted by the lens 

 to a focus, and in three minutes raised 

 the temperature of a thermometer two 

 and a half degrees. 



Experiments of the same kind were 

 made upon the rays projected from a 

 common fire, and from a mass of iron 

 not quite heated to redness^ and they 

 were found to be equally subject to the 



