366 HISTORICAL INTRODUCTION TO CHEMISTRY CHAP. 



the atomic heats are much diminished. Between 20 and 

 - 1 80 the average value for the 25 elements is 5-06 

 (Richards and Jackson, Annual Reports, 1910, 16). Be- 

 tween 195 and 253C. it is 2*12 calories (Dewar, Proc. 

 Roy. Soc., 1913, A. 89, 168). The values for different 

 elements are then not even approximately equal, elements 

 which have a large atomic volume (at. wt. -r density) giving 

 large atomic heats, whilst elements with small atomic 

 volumes have small atomic heats. It is only as the tem- 

 perature is raised that Dulong and Petit's Law becomes 

 valid. 



In the range from 20 to 100 all the solid elements 

 except two give values lying between 5*2 and 67. The 

 two exceptions are : 



Boron... At. wt. II Sp. ht. 0*2616 At. ht. 2*9 



Carbon. At. wt. 12 Sp. ht. O'i532 (diamond) At. ht. I '84 (diamond) 

 0-1990 (graphite) 2 -39 (graphite) 



In the case of diamond, the atomic heats at different tem- 

 peratures are as follows (Nernst, Annual Reports, 1912, 9) : 



Temperature... 896 85 -41 -64 - 181 -231 - 243C. 

 Atomic heat ... 5-45 2-12 O'86 O'66 0-03 O'oo o-oo 



the values diminishing from a normal figure at about 

 1000 almost to nothing at temperatures below the boiling 

 point of liquid air. In the case of boron the atomic heats 

 are (Weber, Phil. Mag., 1875, [ivj ^9, 293 ; Dewar, loc. at.) : 



Temperature... 233*2 177-2 125-8 767 26*6 -39*6 -224 

 Atomic heat ... 4*03 3-72 3-38 3-01 2*62 2'ii 0-24 



These exceptions are confined to elements of small atomic 

 weight, to which Avogadro's hypothesis can easily be applied ; 

 they do not, therefore, affect the usefulness of Dulong and 

 Petit's Law as a means for determining the atomic weights 

 of those elements which do not form volatile compounds. 



