THE VALENCY AND SPECIFIC HEAT OF THE METALS 685 



of the specific heat Q into the atomic weight A is an almost constant 

 quantity. This means that to bring different elements into a known 

 thennal state an equal amount of work is required if atomic quantities 

 of the elements are taken ; that is, the amounts of heat expended in 

 heating equal quantities by weight of the elements are far from equal, 

 but are in. inverse proportion to the atomic weights. For thermal 

 changes the atom is a unit ; all atoms, notwithstanding the difference 

 of weight and nature, are equal. This is the simplest expression of the 

 fact discovered by Dulong and Petit. The specific heat measures that 

 quantity of heat which is required to raise the temperature of one unit 

 of weight of a substance by one degree. If the magnitude of the 

 specific heat of elements be multiplied by the atomic weight, then we 

 obtain the atomic heat that is, the amount of heat required to raise 

 the temperature of the atomic weight of an element by one degree. It 

 is these products which for the majority of the elements prove to be 

 approximately, if not quite, identical. A complete identity cannot be 

 expected, because the specific heat of one and the same substance 

 varies with the temperature, with its passage from one state into 

 another, and frequently with even a simple mechanical change of 

 density (for instance by hammering), not to speak of allotropic changes, 

 &c. We will cite several figures 4 proving the truth of the conclu- 



4 The specific heats here given refer to different limits of temperature, but in the 

 majority of casea between and 100 ; only in the case of bromine the specific heat ia 

 taken (for the solid state) at a temperature below 7, according to Regnault's deter- 

 mination. The variation, of the specific heat with a change of temperature is a 

 very complex phenomenon, the 5 consideration of which I think would here be out of place, 

 I will only cite a few figures as an example. According to Bystrom, the specific heat of 

 iron at = 0'1116, 'at 100=0-1114, at 200 = 0'1188, at 800 = 0'1267, and at 1,400 

 = 0-4031. Between these last limits of temperature a change takes place in iron (a spon- 

 taneous heating, recalescence), as we shall see in Chapter XXII. For quartz SiO 9 

 Pionchon gives Q = OT787 + 894*10- -27< 2 10- up to 400, for metallic aluminium 

 (Richards, 1892) at 0^ 0'222, at 20 0'224, at 100 0'232 ; consequently, as a rule, 

 the specific heat varies slightly with the temperature-. Still more remarkable are 

 H. E. Weber's observations on the great variation of the specific heat of charcoal, the 

 diamond and boron : 



100 200 600 900 



Wood charcoal 0-16 0'28 0-29 0'44 0-46 



Diamond O'lO 019. 0'22 0'44 0'45 



Boron 0'22 0'29 0'85 



These determinations, which have been verified by Dewar, Le Chatelier (Chapter VIII., 

 Note 18), Moissan, and Gauthier, the latter finding for boron AQ = 6 at 400, are of especial 

 importance as confirming the universality of Dulong and Petit's law, because the 

 elements mentioned above form exceptions to the general rule when the mean specific 

 heat is taken for temperatures between and 100. Thus in the case of the diamond 

 the product of Ax Q at = 1'2, and for boron =2'4. But if we take the specific heat 

 towards which -there is evidently a tendency with a rise of temperature, we obtain 

 a product approaching to 6 as with other elements. Thus with the diamond and 

 charooa), it is evident that the specific heat tends towards 0'47, which multipled by 12 



