120 DR. K. H. GRIFFITHS AND MR. EZER GRIFFITHS ON THE 



derived therefrom are based on the assumption that the relation between the specific 

 heat and the temperature is of a linear order. 



Again, some, in fact a large majority, are comparative determinations and 

 dependent on the capacity for heat of other bodies, as, for example, those which 

 assume REGNAULT'S values for the capacity for heat of water at ordinary tempe- 

 ratures values which we now know to be inaccurate. 



The experimental difficulties connected with the method of mixtures are considerable 

 and that method has probably been pushed to its extreme limits of accuracy. The 

 agreement between the results obtained by different observers, and also between those 

 resulting from repetition by the same observer, is rarely satisfactory. The tempe- 

 rature changes have, as a rule, been measured by means of mercury thermometers, 

 without a proper appreciation of the difficulties attendant upon the use of those 

 instruments for accurate work. 



A further possible source of uncertainty is the effect of the sudden chilling of a 

 metal when rapidly cooled from a high temperature. 



The experimental conditions have not been varied sufficiently to demonstrate the 

 absence of unsuspected causes of error and, according to the chemists, sufficient care 

 has not been devoted to the detection and elimination of the impurities present in the 

 samples used. 



It is true that there are determinations of a high order of accuracy which may not 

 justly be subject to this criticism, but such examples are few and it is difficult, 

 when comparing the evidence, to assign to each determination its due weight. 



As an illustration of the divergences which exist, we append the values given by 

 leading authorities in the case of copper, a metal which does not appear to present 

 any peculiar difficulties and one in which the values obtained by different observers 

 are, on the whole, in better agreement than is the case with other metals. 



All the observers agree with the conclusion that the capacity for heat of copper is 

 a function of the temperature, but they differ markedly as to the value of the function. 

 For example TOMLINSON ('Roy. Soc. Proc.,' 1885) gives 



S, = G'09008 + 0'0000648. 



LOREXZ gives values at C., 50 C. and 75 C., from which the Mowing expression 

 is obtained : 



S, = G'0898 + 0' 



an expression which denotes that the specific heat increases more rapidly than the 

 temperature. 



GAEDE ('Phys. Zeitschr.,' 4, 1902) gives values at various temperatures between 

 J. and 92" C., from which the following expression can be deduced : 



S, = 0' 

 This would give a maximum capacity for heat at a temperature of 341 C. 



