the Periodic Law of the Chemical Elements. Ill 



shim, whereas, according to Dulong and Petit's law, it should 

 be less. The experimental values for gallium and germanium, 

 on the other hand, are, according to Dulong and Petit's law, 

 apparently much too low, being less even than that of bromine, 

 although this element has an atomic weight about 10 units 

 higher. The atomic heat of gallium is only 5 '4, and that of 

 germanium only 5*5; whereas the specific heats calculated 



from the expression j= would give rather less than 6*7 



and 6*9 respectively. 



It may be said that of the 55 elements with which the 

 comparison can be made, the calculated specific heats agree 

 very closely with the experimental values in the case of 45 of 

 them, while the other ten are elements the specific heats of 

 which, according to Dulong and Petit's law, are more or less 

 abnormal. 



The agreement becomes even more marked when the com- 

 parison is made with specific heats determined at temperatures 

 above those at which specific heats have been more commonly 

 determined. 



Diagram II. shows graphically the close agreement between 

 the calculated and experimental specific heats. It is con- 

 structed by taking the atomic weights as abscissas and the 

 specific heats as ordinates. The dotted curve is constructed 

 from the calculated and the continuous one from the experi- 

 mental specific heats. The specific heats used in the latter 

 case are chiefly those given on page 89 of Lothar Meyer's 

 i Modern Theories,' translated by Williams and Bedson. If, 

 however, those determinations which agree more nearly with 

 the calculated numbers were used, the two curves would coin- 

 cide even more closely. 



In the equation A=c(m + \/v) the constant c is equal to 

 6* 6, and has been assumed to represent the atomic heat of the 

 elements, whereas the atomic heat, on the basis of Dulong and 

 Petit's law, is usually taken to be 6*4. This latter value, 

 however, is obtained because the specific heats employed are 

 mostly those which have been determined between 0° and 

 100° C, whereas if the specific heats obtained at higher tem- 

 peratures (say 0° to 300°) be employed a somewhat higher 

 value, approximating to 6'6, will be found for the atomic heat. 



As is well known, the specific heat, and therefore the atomic 

 heat likewise, increases, though very slowly, with the tem- 

 perature. If c, therefore, really represents the atomic heat, 

 it is not strictly but only approximately a constant, being a 

 function of the temperature. Consequently, in each of "the 



