4 REPORT — 1841. 



MM. Daloiig and Petit, to whom the Academy of Sciences awarded the 

 prize in 1818, and whose admirable memoir ' On the Measure of Tempera- 

 ture and the Laws of Communication of Heat' the reader will do well to con- 

 sult*. All that we can do is to give a very brief outline of their researches. 

 The first step requisite for them to take was the determination of a correct 

 measure of temperature. To present to the eye an indication of the state of 

 heat of a body the principle of dilatation has been most commonly applied, 

 but it becomes a question to ascertain what substance will by its dilatation 

 express the state of heat the most simply. MM. Dulong and Petit, having 

 determined "that all the gases dilate absolutely in the same manner and by 

 the same quantity for the same change of temperature," conclude that the air- 

 thermometer is the best indicator of the state of heat. They argue, " that the 

 well-known uniformity in the principal physical properties of all the gases, 

 and particularly the perfect identity of the laws of their dilatation, renders it 

 very probable that in this class of bodies the disturbing causes have not the 

 same influence as in solids and liquids, and that, consequently, the changes 

 in volume produced by the action of heat upon them are more immediately 

 dependent on the force which produces them. It is therefore probable (they 

 think) that the greater number of the phsenomena relating to heat will pre- 

 sent themselves under a more simple form if we measure the temperatures on 

 the air-thermometer. It is at least by these considerations (they inform us) 

 that we have been determined constantly to employ this scale t-" Having 

 thus settled that the air-thermometer is to be taken as the measure of tem- 

 perature, they proceeded in the next place to obtain the laws of cooling in 

 vacuo. And here we cannot but express our regret that the original unre- 

 duced observations of the authors are not presented to the world in some 

 work generally known. We have never seen them, nor are we sure that they 

 have been published at all. We take the present opportunity of further ex- 

 pressing our astonishment that experiments on which so much depends have 

 never been repeated in this country. We do not know any more desirable 

 exercise of the funds and energies of public scientific bodies than the repe- 

 tition of all experiments, and the institution of others in a trying form, on 

 which laws of nature have been partially or totally founded. In the case 

 before us we do not doubt the accuracy or fidelity of the ingenious experi- 

 ments, but we wish to be assured by cumulative evidence that the constant 

 introduced into their law is determined with sufficient accuracy. To return 

 from this digression. 



The velocity of cooling was experimented on by our authors by means of 

 heated thermometers placed in a balloon nearly free from air ; but the ob- 

 servations were subjected to two corrections. In the first place the stem of 

 the thermometer without the balloon soon becomes cooled down to the tem- 

 perature of the surrounding air. Every temperature observed therefore was 

 too low by a number of degrees equal to that to which the mercury in the 

 stem would dilate, when heated from the temperature of the surrounding 

 atmosphere to that of the bulb. A correction on this account was applied to 

 all the temperatures observed. The second correction was destined to re- 

 duce the observations actually made on the mercurial thermometer to the 

 corresponding indications of the air-thermometer. Besides these corrections, 

 rendered requisite by the nature of the experiments, there was a third which 

 arose out of the necessary imperfection of the vacuum. This was applied to 

 the resulting velocities, and its value was ascertained by making correspond- 



* Annales de Chimie, torn. vii. p. 225, &c. Thomson's Annals of Philosophy, vol. xiii. 

 p. 113, &c. Journal tie I'Ecole Polytechnique, 1820, torn. xi. p. 189. 

 t Journal de I'Ecole Polytechnique, torn. xi. p. 232. 



