146 



MATHEMATICAL AND PHYSICAL SCIENCE. 



[Diss. VI. 



(649.) 

 Prevost 

 moves ble 

 equili- 

 brium of 

 heat. 



(650.) 

 Leslie's 

 Essay on 

 Heat Dif- 

 ferential 

 thermome- 

 ter. 



of surfaces to reflect and absorb it. He noticed the 

 different heights at which a blackened and a bright 

 thermometer stand even when exposed to common 

 daylight ; but so far as I have observed, he did not 

 distinguish the effect of colour in absorbing heat when 

 that heat is accompanied by light or the reverse; and, 

 indeed, this portion of his work stops short on the 

 threshold of most interesting enquii'ies. He showed 

 that radiant heat travels with great velocity, and ob- 

 served the heating and cooling of thermometers in 

 exhausted receivers. His work also contains obser- 

 vations on hygrometry, on some points of meteoro- 

 logy, and on the heat of friction. Indeed, its chief 

 fault is embracing so many topics in so short a com- 

 pass, thus preventing him from thoroughly examin- 

 ing any one of them. To Pictet is due the establish- 

 ment of meteorological observations at the convent 

 of the Great St Bernard, which are amongst the 

 most interesting which have ever been made, and 

 which are still continued. He died in 1825, at the 

 age of seventy-three. 



The interesting experiment of the reflection of cold 

 led PIERRE PREVOST, one of Pictet' s colleagues, to 

 devise the theory of " the Moveable Equilibrium of 

 Heat." His idea is, that heat is a substance associated 

 with bodies, of a highly elastic nature, and continu- 

 ally given off from them in proportion to their tem- 

 perature, which may represent the tension of the 

 imaginary elastic fluid. When the temperature of a 

 body is stationary, it is (according to this view) be- 

 cause it receives, by radiation from surrounding bodies 

 exactly as much heat as it parts with in the same 

 way. The general structure of this theory was sus- 

 tained by the experiments of Leslie, and by some 

 later ones on the law of cooling by Dulong and Petit, 

 which, indeed, realize it in a remarkable manner. 

 Prevost first published his ideas in 1791, in the 

 Journal de Physique, and afterwards in a special 

 work. Prevost was a man of an active and vigorous, 

 rather than profound intellect. He was a foreign 

 member of the Royal Society, and died in 1839, at 

 the advanced age of eighty-eight. 



We now come to speak of Leslie's important Essay 

 on Heat which received from the Royal Society the 

 distinction of the Rumford medals, and which pro- 

 cured for him a European reputation. It is a work 

 difficult to analyze, from the very fact, that its con- 

 struction is fragmentary, and its arrangement desul- 

 tory and obscure. Our limits will only allow us to 

 mention the methods of research and their chief re- 

 sults. As a thermoscopic instrument, he used a 

 modification of the common air thermometer (which 

 last had been employed by Pictet), which, having two 

 balls at a certain distance, connected by a bent tube 

 containing a coloured liquid, showed the difference of 

 temperature of the balls; and being hermetically 

 closed, was free from the disturbing variation of at- 



mospheric pressure. This he called the differential 

 thermometer. A similar instrument had been de- 

 scribed by Sturmius in the seventeenth century. 

 Whether Leslie had any previous knowledge of this 

 does not appear ; but, as Dr Young very correctly 

 observes in one of his anonymous critical articles, 

 " The principle of the differential thermometer is too 

 simple to be called an invention, and it is only by 

 its ingenious application that Professor Leslie has 

 made it an object of attention." He usually em- 

 ployed as a source of heat a canister of block tin, 

 filled with boiling water, and having sides with dif- 

 ferent surfaces. The radiating or emissive effect ofEmissivt 

 these surfaces was measured by the rise of the ther-^ ct of 

 mometer exposed to their successive influence in the Sur f ace8- 

 focus of a metallic reflector. The result showed a 

 great variety of effect, varying from 100 when the 

 surface was blackened, to 12 when it was of polished 

 metal. The absorptive power of surfaces to non- 

 luminous heat is also in exact proportion to their 

 emissive power a property which seems essential 

 for preserving the equilibrium of like temperatures. 



Another and not less important law clearly esta- (651.) 

 Wished by Leslie was this, that the radiation of heat Law oi 

 from a plane surface takes place with unequal force O f ra( ji a , 

 in different directions. 1 When the specific heatingheat. 

 power or density of the calorific rays is estimated in 

 a direction perpendicular to the surface from which 

 it emanates, it is found to be a maximum. At any 

 other angle with the surface, it varies as the sine of 

 the angle. This law (which Fourier showed later to 

 be necessary for the equilibrium of temperature) 

 obtains also in the case of light. Hence the appa- 

 rent specific brightness or warmth of a surface is 

 the same under whatever angle it is viewed with re- 

 ference to the plane of the surface, which, when 

 placed obliquely, contributes rays from a larger ex- 

 tent of surface, owing to the foreshortening, but being 

 weaker in the same proportion from every point, the 

 aggregate effect is the same. Some interesting ex- 

 periments were made on the number of coats of 

 isinglass necessary to effect a complete transformation 

 of the metallic into the gelatinous surface, which was 

 found to be considerable ; and, in like manner, the 

 reflective character of metals was only very gradu- 

 ally destroyed by varnishing. This observation, 

 rightly interpreted, showed that some solids are per- 

 meated by radiant heat, a conclusion which Mr Leslie 

 utterly rejected. 



Another fundamental experiment less decisively (652.) 

 proved was, that the law of radiation varies inversely t ' h e ? ett 

 as the square of the distance. Perhaps the most s q ua re of 

 convincing, as well as the simplest proof of this has the dis- 

 been given more recently by Melloni. If a delicate t&nce - 

 thermometer or other apparatus for measuring radi- 

 ant heat be confined in a case, so as only to admit 

 rays coming within a definite angular space and if 



This result had already been anticipated by Lambert j Pyrometrie, p. 197. 



