July 29, 1898.] 



SCIENCE. 



115 



space, as compared with the rate of cooling 

 of another body taken as the standard. 



The process of intermixture with water 

 was used by the earlier experimenters in the 

 last century, and some of the best results 

 extant have been obtained by this method, 

 which, however, is not so easy as it appears 

 when the highest degree of accuracy is de- 

 sired. 



Lavoisier and Laplace, in 1780, devised 

 the ice calorimeter which bears their name ; 

 and in a most interesting memoir, which is 

 reprinted among Lavoisier's works, they 

 show that they were familiar with the idea 

 which in modern times is expressed as the 

 principle of the conservation of energy. In 

 this memoir they give the results of experi- 

 ments, in which the specific heats of iron, 

 mercury and a few other substances are es- 

 timated with a very tolerable approach to 



Dulong and Petit* seem to have used at 

 first the method of mixtures, and to have 

 found, by direct experiment, that the 

 specific heat of solids (metals and glass) in- 

 creases 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 re- 

 markable general expression which is asso- 

 ciated with their names. f 



After pointing out that all the results of 

 previous experiments except those of La- 

 voisier and Laplace are extremely incor- 

 rect they describe their own conclusions 

 obtained by the method of cooling, con- 

 ducted with many precautions to avoid 

 error. The numerical expression of their 

 experimental results is given in the follow- 

 ing table : 



Copy of Table by Petit and Dulong. 

 (Ann. Chim. Phys. 1819, X ., 403.) 



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. 



The statement of the relation indicated 

 in the last column of figures is expressed 

 in the following words of the authors, page 

 405 : " Les atomes de tons les corps sim- 

 ples out exactement la m^me capacity pour 

 la chaleur." 



Here the question rested, till resumed 



*Ann. Chim., 1817, VII., 144. 

 t Ibid., 1819, X., 395. 



