66 PHYSICAL PROPERTIES AND COMPOSITION 



On the other hand, the change in the latent heat of 

 evaporation is equal to the differences between the specific 

 heats of the liquid and vapour, so that we find a constant 

 value for the latter quantity also. 



The variation from this rule which must obviously 

 occur when in the liquid, rise of temperature results in 

 decomposition of double molecules, will diminish the fall in 

 the latent heat, or even convert it into a rise ; thus ethyl 

 alcohol shows a minimum in the neighbourhood of 10 : the 

 latent heat of evaporation is 220*9 at o, 211-2 at 10, 

 220-6 at 20. 



4. Specific Heat of Solids. 



Although the solid state lies almost outside the region of 

 theoretical treatment, the specific heat of solids must be 

 discussed, because the numerical values of it are connected 

 with atomic weights by Dulong and Petit's law. We shall, 

 therefore, in this instance put the empirical material in the 

 foreground, and only in concluding touch on its relation 

 to theory. 



(a) The Law of Dulong and Petit. 



It is well known that Dulong and Petit found that the 

 specific heats of the elements in the solid state are, on the 



VYVAHJV**-**! 



whole, about proportional to their atomic weights, and that 

 the product of specific heat into atomic weight the so- 

 called atomic heat is about 6. In physical language it 

 may be said that to raise the atomic quantity in kilograms 

 from o to i requires about six calories. Thus, e.g. copper 

 with atomic weight 63 has a specific heat of 0-095. 



This relation hardly possesses the character of a general 

 rule, but the exceptions are associated with other proper- 

 ties, and are only very marked for elements with small 

 atomic weights ; from potassium (39) onwards the greatest 

 deviation of the atomic heat always greater than 6 is 

 for iodine 6-9. For smaller atomic weights the larger 

 deviations occur, in the form of abnormally small specific 



