MOLECULAR HEAT OF SOLIDS 69 



determined by Bunsen to be 0-057, and the choice is then 

 no longer doubtful, since 



0-057x76 =4.3, 



0-057x114 = 6-5, 



so that only the atomic weight 114 agrees with Dulong and 

 Petit's law. 



(y) Additive Character of the Specific Heat of Solid Bodies 

 (Neumann's Law). 



Whilst Dulong and Petit determined the relation between 

 the elements with regard to their specific heat, Neumann 

 did the same for the relation between a compound and its 

 components, and arrived at a simple additive rule, that the 

 heat necessary for the molecular quantity of a compound 

 (M w ) is the sum of the atomic heats (A w ) of the components ; 

 a compound therefore behaves in this respect, as in respect 

 to its weight, as a mixture, although only approximately. 

 Symbolically this may be expressed as 



M w = 2A K , 

 or since the atomic heats are approximately 6*5, 



^4 = 6-5 x n, 



where n is the number of atoms of which the molecule is 

 composed. It must be remembered that the last expression 

 only holds for compounds of such elements as follow Dulong 

 and Petit's law. 



As example we may take iodide of lead PbI 2 , whose 

 elements, being of high atomic weight, fall under Dulong 

 and Petit's rule. Then we should have 



-W = 3 * 6 '5 = 19-5. 



and the specific heat (8 W ) of lead iodide, with molecular 

 weight 207 + 2 x 127 = 461, 



An application of Neumann's rule is to determine approxi- 



