PRINCIPLES OF CHEMISTRY 111 



The theory by which the process of electrolysis is now ex- 

 plained has been very considerably modified within recent 

 times, but Faraday's two quantitative laws remain unaltered, 

 and constitute a firm basis on which researches continued since 

 Faraday's time securely rest. 



In the meantime almost before Dalton's Atomic Theory had 

 become familiar to the majority of chemists, and long before 

 Faraday's discoveries in electricity were made known, another 

 discovery was made, the importance of which has in later times 

 been fully recognised. In 1819 the two French physicists, 

 Dulong and Petit, discovered the relation between specific heat 

 and atomic weight, which is expressed by their law, as follows : 

 the specific heat of an element in the solid state is inversely 

 proportional to its atomic weight. 



Consequently the number expressing the specific heat, multi- 

 plied by the atomic weight, gives a constant which is approxi- 

 mately 6-4 for all temperatures between the freezing and boiling 

 points of water. Four exceptions among the elements, namely, 

 carbon, boron, silicon, and the metal glucinum are known, but 

 are accounted for, and the principle is universally accepted and 

 acted upon for the purpose of regulating the value to be assigned 

 to those atomic weights which cannot be fixed by appeal to other 

 rules, such as that of Avogadro. 



Dulong and Petit expressed their law in the following words : 

 " Les atonies de tous les corps simples ont exactement la meme 

 capacite pour la chaleur." That is the atoms of all the elements 

 have exactly the same capacity for heat. 



An equally remarkable fact was observed some years later by 

 Neumann, who found that there was a similar relation between 

 the specific heats of chemically similar compounds, and the sum 

 of the atomic weights of the elements composing them. Since 

 that day these results have been corrected and extended, so that 

 it may now be said that the specific heat of the molecule of a 

 compound is the sum of the specific heats of the atoms com- 

 posing it, very approximately, on condition that elements are 

 excluded such as oxygen, hydrogen, nitrogen, of which the 

 specific heat in the solid state cannot be found by experiment. 

 In other words the capacity of the elementary atoms for heat is 

 the same whether they are in the elemental, uncombined state 

 or form part of a chemical compound. The independence of the 

 atom in any condition is the interesting point. 



