154 



MATHEMATICAL AND PHYSICAL SCIENCE. 



[Diss. VI. 



(690.) 

 Professor 

 Kelland's 

 Theory of 

 Heat. 



unaccountable yet characteristic rashness, chose to 

 consider as applicable to the communication of heat 

 from particle to particle, as well as to its dissipa- 

 tion from the surfaces of bodies. But the result was 

 worthy of the looseness of the assumptions; the terms 

 arising from the peculiarity of Dulong's law are 

 mostly dispensed with as the investigation proceeds ; 

 and the whole work is to be regarded rather as a 

 mathematical exercitation, than as a serious step in 

 physics. Having elsewhere recorded a criticism on 

 that part of Poisson's work which treats of the Heat 

 of the Globe, I shall not dwell farther on its defects. * 

 Professor Kelland has published a work 2 the only 

 one intended for students on this subject, and he has 

 likewise made a valuable report to the British Asso- 

 ciation, on the best means of comparing the Mathe- 

 matical Theory of Heat with Observation. 3 He has 



suggested, that the results of the Newtonian law of 

 cooling may be used in combination with such an 

 hypothesis as to the relation between quantities of 

 heat and the temperature shown by an air thermo- 

 meter, as will reconcile it with Dulong's law. But 

 there is reason to think, that internal conductivity 

 varies with temperature in the reverse manner of ex- 

 ternal cooling ; in other words, that it diminishes as 

 the absolute temperature increases.* 



MM. Duhamel and Stokes have considered the (691.) 

 propagation of heat in bodies which do not conduct it Case of 



uniformly in all directions ; and their investigations bo , d . ie , s , 

 . , ,. . . i i -i / -i w hich do 



are very interesting in connexion with the beautiful notcondu 



observation of M. de Senarmont, that such is the case beat uni- 

 in crystallized bodies. It is easily shown in a plate f rm ty 

 of gypsum coated with a thin layer of wax, to a small tions! 

 part of which heat is applied. 



7. DULONG. The Law of Cooling. Progress of the Science of Radiant Heat between Leslie's 

 and Melloni's Discoveries ; transmission of Radiant Heat through Glass. Herschel ; De la 

 Roche ; Professor Powell. Theory of Dew ; Wells. 



(692.) The researches of Leslie on Radiant Heat, though 

 Subject of very generally appreciated both at home and abroad, 

 were not, on their publication, immediately repeated 

 sumed. or extended. On some points they were seriously 

 controverted. But the labours of most of his imme- 

 diate contemporaries, of whose names we have given 

 a few at the head of this section, were rather prepa- 

 ratory to the fuller developments of a later time to 

 be made with improved apparatus than demon- 

 strated discoveries. I make an exception, however, 

 with regard to the experiments of Dulong and Petit 

 on the laws of simple radiation and of the cooling 

 of bodies by the contact of gases, not only because 

 they established propositions quite new in an incon- 

 trovertible manner, but also because they introduced 

 into this branch of science a mode of investigation so 

 delicate and precise, and methods of reduction and 

 of physical analysis so beautiful and convincing, as 

 placed the whole science of Heat on a new footing. 

 The main credit is due, we believe, to Dulong, who 

 was one of the most estimable and accomplished phi- 

 losophers of his time. 



PIERKE Louis DULONG was born in 1785, and 

 He was admitted to the 



Polytechnic School at the age of 16, and rose through 

 every grade of that celebrated institution until he 

 became " Directeur des Etudes." It is superfluous 

 to say that he was a good mathematician. He was 



(693.) 



u on S~ showed precocious talents, 

 an eminent 



chemist. 



also a most excellent chemist ; and the honour of 

 being discoverer of the most terrific of fulminating 

 compounds (chloruret of azote) was purchased by the 

 loss of one of his eyes. But his experiments on 

 Heat are those by which he will be longest remem- 

 bered. The most important series of these was de- 

 voted to a rigid examination of the amount of heat 

 radiated under different circumstances, and of that 

 dissipated by the contact of air. 



The law of Cooling. It had been suggested by New- (694.) 

 ton, and tacitly admitted by nearly all writers on heat, Dulong 

 including the most eminent, that simple radiation (or an 

 pulsation, in the language of Leslie) takes place in di- Newtonia 

 rect proportion to the excess of temperature of the hot law of 

 body above the surrounding space. Martine, and par- coolin g- 

 ticularly De la Roche, had indeed thrown doubt on 

 the subject, and had rendered it probable that at high 

 temperatures the velocity of cooling (which is propor- 

 tional to the radiant energy) is considerably greater 

 than Newton's law supposes. Dulong and Petit, 

 however, first demonstrated this. They disengaged 

 the experiment as far as possible from the influence 

 of the contact of air, by using the most perfect ex- 

 haustion which the best air-pumps could produce ; 

 and it appeared very plainly from the result of ob- 

 servations made under successive degrees of rarefac- 

 tion, that instead of there being the slightest appear- 

 ance of radiation ceasing to take place when the vehicle 



1 See Second Report on Meteorology, Brit. Assoc. Reports, 1840. 



2 Theory of Heat. Camb., 1837. 3 Brit. Assoc. Reports, 1841. 



* This at least is the result of an experimental investigation by myself on the conductivity of iron, executed on a principle 

 which I believe to be new, but which I have not yet been able to publish. Mr Airy and Professor Kelland are each in possession 

 of the outline of my method ; and the result noted in the text was briefly announced by me in the Reports of the British Association 

 for 1852. If my health permits, I shall resume and publish these experiments. I may here add, that I pointed out in 1833, 

 from some experiments which I made at that time (Proceedings of the Royal Society of Edinburgh, vol. i. p. 5), that the metals 

 range in the same order as conductors of Heat and of Electricity ; and this law appears to be confirmed by more recent obser- 

 vations. 





