364 HISTORICAL INTRODUCTION TO CHEMISTRY CHAP. 



at the present time ; tellurium and cobalt (modern values, 

 127-5 an d 58*97) are exceptions to this statement, on 

 account of errors in the values for their specific heats. 



Significance of the law of atomic heats. The SPECIFIC 

 HEAT of a substance is the amount of heat in calories 

 required to raise the temperature of i gram through i C. 

 The product of the specific heat and the atomic weight of 

 an element is called its ATOMIC HEAT and represents the 

 amount of heat required to raise the temperature of i 

 " gram-atom " (i.e. the atomic weight expressed in grams) 

 through i C. Dulong and Petit's LAW OF ATOMIC HEATS 

 can be expressed algebraically in the form : 



Specific heat x atomic weight atomic heat = constant = 6'o. 



The average value of Dulong and Petit's product was 

 0*3753 (O = i). If reduced to the modern standard, O = 16, 

 it becomes 6;oo, a number which is practically the same as 

 the average value 6 '05 for the atomic heat over the range 

 from 20 to 1 00 of the 25 solid elements tabulated on 

 page 365. 



If the specific heat of an element at ordinary temperatures 

 is known, its atomic weight may be calculated by dividing the 

 specific heat into the figure 6'o. The atomic weights 

 obtained in this way are even less accurate than those 

 deduced from vapour densities ; but they are sufficiently 

 exact to give the valency of the element, i.e. the factor which 

 must be usecl in deducing the atomic weight from the 

 equivalent, as described in the preceding chapter. 



The law of atomic heats can be applied most easily in the 

 case of the metals in which the direct application of 

 Avogadro's hypothesis is most difficult. It was much used 

 by Berzelius, and was regarded by him as a more trust- 

 worthy guide than Avogadro's hypothesis ; but, as Cannizzaro 

 showed, it is not independent of that hypothesis, and merely 

 provides a method for extending it from hydrogen and 



