736 



Professor W. A. Tilden 



[May 13, 



few other substances are estimated with a very tolerable approach to 

 accuracy. Although many of the metals were known to them, and 

 supposing they had persisted in this work, it would not have been 

 possible for them to make the discovery which was reserved for 

 Dulong and Petit thirty-five years later, for the atomic theory had 

 not then been conceived, and no elemental combining proportions 

 had been determined. 



Dulong and Petit * seem to have used at first the method of mix- 

 tures, and to have found, by direct exiDcriment, that the specific heat of 

 solids (metals and glass) increases with the temperature. They also 

 studied (after Leslie) the laws of cooling of bodies ; and two years 

 after the publication of their first paper on the subject, they (Petit 

 and Dulong, sic) arrived at the remarkable general expression which 

 is associated with their names.f 



After pointing out that all the results of previous experiments 

 except those of Lavoisier and Laplace are extremely incorrect, they 

 describe their own conclusions obtained by the method of cooling, 

 conducted with many precautions to avoid error. The numerical 

 expression of their experimental results is given in the following 

 table : — 



Copt of Table by Petit and Dulong. 

 (Ann. Cliim. Phys. 1819, x. 403.) 



The statement of the relation indicated in the last column of 

 figures is expressed in the following words of the authors, p. 405 : 

 " Les atomes de tous les corps simples ont exactement la memo 

 capacite pour la chaleur." 



Here the question rested, till resumed many years later (1840) by 

 Eegnaultj who in his first memoir | pointed out the difficulties which 



Ann. Chira. 1817, vii. 144. f Ibid. 1819, x. 395. 



Ibid. 73, 5. 



