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ni; K. H. GRIFFITHS AND MR. EZER GRIFFITHS ox THE 



APPENDIX I. 



The hypothesis of DULONG and PETIT has undoubtedly been of great service to 

 chemists ; nevertheless, it is acknowledged that, at best, it is but approximately true 

 and that whatever value of the constant is assumed, the number of exceptions at 

 ordinary temperatures, especially in the case of elements of small atomic weights, 

 entitles us to regard it as an indication of a probability rather than as a valid 



generalization. 



Let us consider the values it would yield, at C., if we apply it to the metals 

 whose specific heats we have dealt with in this communication, arranged in order of 

 their atomic weights, assuming that 



Atomic weight x specific heat = 6'25. 

 TABLE XXVIII. 



The increase in the numbers in column IV., as the atomic weights diminish, is very 

 noticeable. 



If we plot the experimental values (column III., supra) as ordinates and the atomic 

 weights as abscissae, the points lie very evenly about a smooth curve of an exponential 

 type ; Cu being rather low, Zn rather high, and Sn decidedly high. 



In order to obtain an expression for the curve, assume (column III.) the following 

 values : 



Al = 0'209G ; mean of Cu and Zn = 0'09132 ; and Pb = 0'03020. 



Then the curve drawn through these three points will be found to follow closely a 

 mean path through the above experimental values. 

 The expression for this curve is 



S = 4'804xa- u ' ( *, 



