24 Prof. S. P. Thompson on the Conservation of Electricity, 



the most careful conditions. These researches show that the 

 rates of loss of charge for equal "vitreous" and "resinous" 

 electrifications are for feeble charges imperceptibly alike, but 

 that with stronger charges the rate of loss of charge was always 

 greater for "resinous" than for "vitreous " electrification. 



Now this is precisely what analogy with the law of dis- 

 sipation of heat would lead us to expect if " resinous " be the 

 true plus electrification. The careful researches of Dulong 

 and Petit showed that Newton's law of cooling (that the 

 rate of loss of heat is proportional to the excess of tempera- 

 ture of the body above that of the circumambient air and of 

 an enclosure in which it is placed) is inexact; for they found 

 the rate of cooling of a hot body to be higher (for same excess) 

 at higher temperatures than at lower. The somewhat compli- 

 cated law which they finally enunciated as representing their 

 results gives the absolute amount of heat emitted as being pro- 

 portional to (1*0077) 6 



where 6 represents the temperature in Centigrade degrees — a 

 function which for higher temperatures has values a little 

 higher than if the radiation were taken as proportional to the 

 absolute temperature, which might, a priori, have been ex- 

 pected to be the law. 



A formula given by Peclet to express the number of calo- 

 ries per hour lost by a square metre of surface of a sub- 

 stance whose temperature was T° (Centigrade), cooling in air 

 of temperature t° C, is as follows : — 



Q=£T(0-9556 + 0-0037*) (1 + 0-0056T), 

 where h is a coefficient depending only on the nature of the 

 body. Here the first of the two factors in brackets is a term 

 very nearly proportional to the absolute temperature, since the 

 formula was calculated from observations in which the tem- 

 perature = 12° C. 



A still more decisive result was obtained by Provostaye and 

 Desains*, who investigated the law of rate of heating — the 

 body to be experimented upon being placed within a vacuous 

 enclosure surrounded by a steam-jacket, the rise of tempera- 

 ture from about 60° C. to 100° C. being watched. They found 

 that the complete law of exchange of heat between two bodies 

 at different temperatures might be represented, either for heat- 

 ing or cooling, by an expression of the general form 



7na T '. 



where T and T[ are the two temperatures. But they found 



that the coefficient m had less values for the case of heating than 



for the case of cooling. They gave the following cases in illus- 



* Ann, de Chim, et de Physique, ser. 8, t. xyi. p. 409. 



