156 



MATHEMATICAL AND PHYSICAL SCIENCE. 



[Diss. VI. 



(701.) 



(702.) 

 Observa- 

 tionB of Sir 

 W. Her- 

 Bchel on 

 the heat of 

 the solar 

 spectrum. 



(703.) 

 Other ex- 

 periments. 



the necessity of introducing into physics a degree of 

 precision then almost unthought of. Coulomb had 

 done something of the kind in other branches ; but 

 in Heat it was really new. It was speedily succeeded 

 by a rapid advance of precision in almost every kind 

 of delicate experimental research. Nowhere can 

 the student of physics find a better model than in 

 the celebrated memoir on the Law of Cooling, which, 

 we may add, received, as a matter of course, from 

 the Academy of Sciences, the prize in competition 

 for which it had been composed. 1 It is only justice 

 to the countrymen of Dulong to say that they retain 

 the superiority in the deduction of numerical laws 

 from observation, which Coulomb and he conspicu- 

 ously exemplified. The industry of the Germans and 

 of the English have indeed been great ; but in this 

 particular enquiry they have not equalled in address 

 the members of the French Academy. 



In order to complete our sketch of the more im- 

 portant steps connecting the discoveries of Leslie with 

 those of Melloni, we will now, going back a little in 

 point of date, trace the origin of correct experiments 

 on the immediate transmission and refraction of heat 

 by solid substances. 



Sir William Herschel having found it requisite in 

 the course of his arduous observations on the sun to 

 devise means for preventing the intense heat of its rays 

 from reaching the eye, was naturally led to observe the 

 effect of different coloured glasses, and even of coloured 

 liquids in this respect. He also placed thermometers 

 in different parts of the spectrum formed by a prism, 

 in order to discover which of the rays it was most 

 important to exclude. The singular result at which 

 he arrived was this, that the intensity of heat accom- 

 panying the light of the sun not only increases from 

 the violet to the red end of the spectrum (as was 

 already known), but is more intense quite beyond 

 the red, and gradually diminishes in force for a long 

 way farther. This result was keenly contested by Les- 

 lie, butwas confirmed by Englefield and Davy. Berard 

 admitted the existence of invisible heating rays from 

 the sun, but yet found the maximum effect within 

 the red. Seebeck, by numerous experiments, proved 

 that the position of the maximum depended on the 

 nature of the prism, being found even in the yellow 

 ray when a prism of water is employed, whilst with 

 flint glass it always occurs in the space beyond the 

 red. The rationale of this curious result was first 

 discovered by Melloni, whose labours will be detailed 

 in another section ; and it was shown to depend on 

 the different degree of absorption exercised on the heat 

 of the several rays of the spectrum by the differing 

 material of the prisms. 



Herschel made a very great number of experiments 



on the transmission both of solar and fire-heat through 

 different kinds of glass and other bodies. But they 

 were rough trials, giving a sort of practical test of 

 this quality, rather than admitting of accurate esti- 

 mations of the quantities of transmitted heat. It 

 was impossible, for instance, to infer from them the 

 degree in which the warmth induced in the glass or 

 other medium by the heat which it absorbed, tended 

 to raise the indications of the thermometer beyond ; 

 although such an effect was manifest from the re- 

 sults of the experiments themselves. Hence it was 

 open to an objector to deny the direct transmission 

 of radiant heat through such bodies as glass, except 

 in the cases of the sun and of brilliant combustion, 

 when it cannot be doubted. 



Prevost had proved to his own satisfaction the (704.) 

 immediate transmission of heat derived from bodies Important 

 even when below the temperature of visible redness, ex P eri " 

 by using thin screens of glass, and renewing them D G la 

 frequently before they could have absorbed much Roche on 

 heat. Maycock obtained a similar result; but to th . e t . rans " 

 DE LA. ROCHE is due noc only the establishment of ^iant 

 this fact beyond any reasonable doubt, but also the heat 

 discovery of certain laws of its operation which are through 

 inexplicable on any other supposition but thatof gass ' 

 immediate transmission. One of these laws, for in- 

 stance, was this, that when a series of thin glasses 

 are interposed between a source of heat and a ther- 

 mometer, each successive glass transmits a larger pro- 

 portion than the previous ones of the heat which falls 

 upon it. De la Roche rightly accounted for this sig- 

 nificant fact by assuming that heat is not homoge- 

 neous, and that the heat which has once passed 

 through glass has lost the rays which glass most 

 easily intercepts. He farther found that the sus- 

 ceptibility of heat to pass through glass increases 

 rapidly with the temperature of its source. The 

 experiments of De la Roche date from 1812. 



The next step was made by Professor Powell of (705.) 

 Oxford (1825). He showed that the quality of the Professor 

 heat transmitted by glass is not the same as that PoweU< 

 of the incident heat. This he proved by ascertaining 

 the proportion of heat absorbed by a black relatively 

 to a white surface. This proportion was invariably 

 increased by the interposition of glass. Mr Powell 

 concludes that heat consists of two kinds intimately 

 mixed. That of which the absorption depends on the 

 colour of the surface on which it falls, is usually 

 luminous, and is most easily transmitted by glass. 

 That kind of heat which is equally absorbed by black 

 and white surfaces is totally devoid of light, and is 

 sometimes considered as pure radiant heat. 



It will be sufficient here to refer to an interesting (706.) 

 Essay on Dew, published by Dr Wells in 1815, in Wells> 

 which he applies Leslie's experiments on the radiating 



1 As the original paper of Dulong and Petit, in the Memoirs of the Institute, or the Annales de Chimie, is not always acces- 

 sible, I may mention that it is translated almost or quite in extenso in Thomson's Annals of Philosophy, vol. xiii., and iu 

 Mr Lunn's excellent treatise on Heat in the Encyclopaedia Metropolitan. 



