ON THE CONDUCTION OF HEAT. $ 



Newton s Imo of cooling, — The author is constructing a scale of tempera- 

 tures ; he is comparing, for instance, the heat of boiling water with that of 

 the human body. The comparison is made immediately, to the extent to 

 which the thermometer affords an indication of the temperature ; beyond this 

 it is requisite to have recourse to some process which involves computation ; 

 and to this end Newton admits the hypothesis, to which we apply the de- 

 signation given above. His words are as follows (translated) : " This table 

 was constructed by the use of a thermometer and red-hot iron. By means 

 of a thermometer I found the measure of the heat up to the point at which 

 tin {stannum) is melted, and by heated iron I found the measure of the rest. 

 For the temperature which heated iron communicates to cold bodies con- 

 tiguous to it, in a given time, is as the total temperature of the iron. There- 

 fore, if the times of cooling are taken in arithmetical progression, the tem- 

 peratures will be in geometrical progression, and may be found by a table of 

 logarithms." 



It is affirmed by most modern writers that Newton was led to this law by 

 experiment. This was very probably the case, for to the extent of tempera- 

 ture indicated by his thermometer it would be very nearly verified. 



The inaccuracy of this law was first pointed out by Martine*. He found, 

 that although it appears very exact when the temperature of the heated body 

 does not differ much from that of the surrounding air, yet when the tempera- 

 tures differ considerably it is very far from being the case. Erxlebenf also 

 proved that the law is at fault \n proportion to the excess of the temperature 

 of the body. Mr. DaltonJ, in his ' New System of Chemical Philosophy,' ia 

 a truly philosophical manner attempted to re-establish the law of Newton by 

 altering the thermometric scale. The hypothesis on which he bases his views 

 is, that the dilatation of all liquids is subject to the same law. MM. Dulong 

 and Petit conceive that Dalton's views are untenable, arguing that, " even 

 supposing the accuracy of the principles of this new scale to have been proved, 

 we should be constrained to acknowledge that it does not satisfy the con- 

 dition of rendering the losses of heat of a body proportional to the excess of 

 its temperature above that of the air which surrounds it, or, in other words, 

 that it does not re-establish the law of Richmann § ; for it would be necessary 

 in that case that the law of cooling should be the same for all bodies, and our 

 experiments rigorously prove the contrary || ." 



We presume MM. Dulong and Petit's argument to be based, not, as would 

 appear from the phrase quoted, on the variability of the law of cooling, so 

 much as on the fact that for different substances the two portions whose sura, 

 according to these authors, constitutes the law, are affected with very different 

 multipliers; so that their relative values depend altogether on the nature of 

 the body. To this matter we shall return in the sequel^. 



M. de la Roche of Geneva** likewise pointed out the deviation from New- 

 ton's law, at the same time admitting that it is sufficiently accurate to 212° 

 Fahr., which is perhajis rather more than subsequent discoveries warrant us 

 in assuming. 



We come now to the time when the law was established in its correct form, 

 so far as we can see at present. The whole merit of the discovery is due to 



* Martine, Essays on Heat, 1740, p. 236, art. 4. 

 t Novi Commentarii, Soc. Gott., vol. viii. p. 74. 

 X New System of Chemical Philosophy, 1808, p. 12. 

 § Kraft and Richmann, Novi Commentarii, Petrop. i. p. 195. 

 II Dulong and Petit, Journal de TEcole Polytechnique, 1820, torn. xi. p. 237. 

 % Consult their Memoir, p. 190. 



** Journal de Physique, 1812, torn. Ixxv, p. 201, Prop. 6. Aunals of Philosophy, vol. ii, 

 p. 100. 



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