Mr. Joule on Specific Heat. 335 



researches of De la Roche ami Berard, Dr. Apjohn*,and Du- 

 long. With compound bodies, however, experimenters have 

 met with better success. Neumann f, in the cases of the car- 

 bonates and sulphates of protoxides, has pointed out the fact 

 that the specijfic heat in each of these classes is inversely pro- 

 portional to the atomic weight. His researches on some 

 oxides and sulphurets also conducted him to a similar law for 

 each of these species of compounds. 



Of late years no philosopher has made more numerous or 

 more accurate experiments on specific heat than Mr. V. Reg- 

 nault. The investigations of this eminent chemist were, in 

 the first instance, directed to the specific heat of simple solid 

 bodies:]:: subsequently they have been directed to a great va- 

 riety of compound bodies§. By these researches Regnault 

 has given the law of Dulong and Petit all the confirmation 

 that could be desired, and has also proved the correctness of 

 Neumann's extension of that law to classes of chemical com- 

 pounds. He has stated a general law in the following terms : 

 — "In all compound bodies of the same atomic composition, 

 and of similar chemical constitution, the specific heats are in- 

 versely proportional to the atomic weights." 



Regnault remarks that the above law holds good only within 

 certain limits, and that the slight differences which are ob- 

 served between the results of theory and observation are not 

 wholly to be attributed to mere errors of experiment. He 

 says that if the specific heat were taken for the temperature at 

 which the bodies present the greatest analogy in their chemi- 

 cal and ph3'sical properties, the most complete isomorphism, 

 the law would probably hold good rigorously. 



Now, without denying altogether the influence of a change 

 of state on the specific heat of a body, I tliink it may be fairly 

 doubted whether it is really so great as is generally imagined. 

 Impressed with this idea, I have been induced to draw up a 

 Table, in which the theoretical specific heats of a variety of 

 bodies impartially selected are calculated on the hypothesis 

 that the capacity for heat of any simple atom remains the same 

 in whatever chemical combination it enters. The law implied 

 by this hypothesis is, that the specific heat of all bodies is di- 

 rectly as the number of atoms in combination, and inversely 

 as the equivalent. * 



• Trans, of the Royal Irish Academy, vol. xviii. part 1. p. \Q. (or Phil. 

 Mag. S. 3. vol. xii. p. 101.) 



t Poggendorff's Jnnalen, vol. xxiii. 



X Annales de Chimie, 1840, vol. Ixxv. p. 1. 



§ Ibid., 1841, vol. i. p. 129. 



