490 M. Regnault on the Specific Heat of Simple Bodies y 



To be convinced of this, it is sufficient to compare between 

 themselves the specific heats of some simple bodies taken at diflfer- 

 ent points of the thermometric scale; it is immediately seen that 

 their relations change sensibly with the temperature ; and as the 

 atomic weights remain the same, it is evident that the product 

 of the specific heat by the atomic weight may vary with the 

 temperature unequally in each case. 



In short, the specific heat of the same body is often very dif- 

 ferent, according as it is estimated in the solid, liquid, or gaseous 

 state. Thus, while for all the solid simple bodies which I studied 

 in my previous memoirs the product of the specific heat by the 

 atomic weight varied between 36 and 41, the same product is 

 only 24 for those simple gases, such as oxygen, hydrogen, and 

 nitrogen, which deviate little from Mariotte's law, and it is 29*5 

 for chlorine and gaseous bromine, which show a considerable 

 deviation. Further, the gases which follow Mariotte's law pretty 

 closely, maintain their calorific capacity sensibly constant at dif- 

 ferent points of the thermometric scale ; while this calorific capa- 

 city increases rapidly with the temperature in those gases which 

 deviate considerably from this law, as is the case with carbonic 

 acid. This last observation would of itself be sufficient to prove 

 that the law of Dulong and Petit cannot be admitted as an exact 

 physical law. 



However this may be, no one doubts now that the law of 

 Dulong and Petit, when only applied to simple bodies in the 

 same physical condition, can be of great help in chemistry as a 

 guide in selecting atomic weights for simple bodies, when che- 

 mical considerations permit several equally probable to be adopted. 

 It was from reasoning on this law that I proposed to change the 

 atomic weights of the alkaline metals, and of that of silver, and 

 to take values half those which chemists have usually adopted. 

 It cannot be long before this change, which is in accordance 

 with isomorphism, is adopted. 



Convinced of the utility of obtaining, as speedily as possible, 

 the specific heats of simple bodies which have not yet been deter- 

 mined, I have never neglected to make experiments when I was 

 able to procure any of these bodies in a sufficient quantity and 

 in a sufficiently pure condition. The Universal Exhibition which 

 took place this year at Paris affi)rded me a favourable opportunity, 

 by putting at my disposal simple bodies, often very pure, and on 

 which I had not hitherto made experiments. 



These new experiments were made with the apparatus and by 

 the process described in my first memoir*. The only modifica- 

 tion of the former method consisted in the estimation of the 

 correction which must be made in the elevation of temperature 

 ♦ Annates de Chimie et de Physique, 2nd series, vol. Ixxviii. p. 20. 



