148 PROFESSOR W. A TILDEN ON THE SPECIFIC HEATS OF METALS AND 



the gas all are equally free. In the liquid state there is reason to believe that there 

 is a mixture of clusters or aggregates of molecules having different degrees of 

 complexity, and that the effect of rise of temperature upon these is to cause 

 dissociation of the more complex into simpler groups, a process which necessarily 

 implies work done. 



Notwithstanding the validity of NEUMANN'S law, the attempts which have been 

 made to deduce the atomic heats of elements, such as oxygen, which do not admit of 

 experiment in the solid state, cannot, however, be regarded as satisfactory. It is 

 obvious that iii such calculations whatever change in the molecular heat of the 

 compound is induced by slight alteration of density, or of structure in the solid, is 

 concentrated upon one element in the compound of any two, when it is assumed that 

 the other enters into combination with the atomic heat it possesses in the elemental 

 state. Taking the figures for the compounds containing silver, for example, the value 

 deduced for the atomic heat of silver is found to vary considerably according to the 

 nature of the compound selected. To calculate the atomic heat of silver from the 

 mean molecular heat of the telluride which is '0672 X 340'8 = 22'90 at the 

 usual temperature of experiment, to 100 C. the value for tellurium is deducted 

 and the remainder divided by 2. The result is 8 '39. 



Similarly the atomic heat of silver in the silver-aluminium, Ag 3 Al, comes out as 

 G - 19, and in the aluminium-silver, AgAlp, as 9'67. The variations are still greater if 

 a comparison is made over different ranges of temperature. Hence it appears probable 

 that the values which have been calculated for hydrogen, oxygen, nitrogen and 

 chlorine in the solid state are very far from the truth. 



KOPP estimated the atomic heats of these elements in the solid state to be as 

 follows : 



Hydrogen 2 '3 Nitrogen 6 '4 



Oxygen 4'0 Chlorine 6 '4 



From its various compounds no approach to a uniform value for the atomic heat of 

 carbon is to be found, but KOPP preferred the value 1'8, which is deduced from the 

 specific heat of diamond. 



Without here entering into a discussion of all these elements, it may be mentioned 

 that in the case of hydrogen gas at constant volume the atomic heat is practically 

 identical with that deduced from the specific heat of solid compounds, while that of 

 oxygen is less, and that of carbon in the form of carbon dioxide gas is greater than 

 the estimates thus made. JOLY found* the specific heat of air between 15 and 

 100 C. to be '172, that of carbon dioxide gas '173, and that of hydrogen 2*41, when 

 under approximately equal pressures. The atomic heat of hydrogen gas is therefore 

 2 '41. Assuming the specific heat of oxygen at constant volume very near to that of 



* ' Phil. Trans.,' A, vol. 182, p. 73 (1892). 





