576 PRINCIPLES OF CHEMISTRY 



cite several figures 4 proving the truth of the conclusions arrived at 

 by Dulong and Petit. 



Li Xa Mg P 



A= 7 23 24 31 

 Q= 0-9408 0-2934 0-245 0-202 

 AQ= 6-59 6-75 5-88 6-26 



Fe Cu Zn Br 



A= 56 63 65 80 

 Q= 0-112 0-093 0-093 0-0843 

 AQ= 6-27 5-86 6-04 6-74 



Pel Ag Sn I 



A= 106 108 118 127 

 Q^= 0-0592 0-056 0-055 0-541 

 AQ= 6-28 6-05 6-49 6-87 



Pt Au Hg Pb 



A= 196 198 200 206 



Q= 0-0325 0-0324 0-0333 0-0315 



AQ= 6-37 6-41 6-66 6-49 



It is seen from this that the product of the specific heat of the 

 element into the atomic weight is an almost contant quantity, which 

 is nearly 6. Therefore the possibility arises of judging the valency 

 with a sufficient degree of exactitude, by the specific heats of the 



4 The specific heats here given refer to different limits of temperature, but in the 

 majority of cases between and 100 ; only in the case of bromine the specific heat is 

 taken (for the solid state) at a temperature below 7, according to Regnault's deter- 

 mination. The variation of the specific heat with a change of temperature forms a 

 very complex phenomenon, the consideration of which I think would here be out of place. 

 I will only cite certain figures as an example. According to Bystrom, the specific heat of 

 iron at = 0-1116, at 100 = 0-1114, at 200 = 0'1188, at 300 = 0'1267, and at 1400 = 

 *0"4081. Between these last limits of temperature a change takes place in iron (a spon- 

 taneous heating, recalescence), as we shall afterwards see. For quartz SiO 2 , Pionchon 

 gives Q = 0-1737 + 394210- 6 - 27 2 10~ 9 up to 400 ; consequently, as a rule, the specific 

 'heat varies with the temperature. Still more remarkable are H. E. Weber's observations 

 on the great variation of the specific heat of charcoal, the diamond and boron : 



100 200 600 900 



Wood charcoal 0'15 0'23 0'29 0'44 0'46 



Diamond 010 0'19 0-22 0'44 0'45 



Boron 0'22 0'29 0'85 



These determinations (they have been verified by Dewar) are of especial importance as 

 confirming the universality of Dulong and Petit's law, because the elements mentioned 

 above form exceptions to the general rule when the mean specific heat for temperatures 

 between and 100 is taken. Thus in the case of the diamond the product of Ax Q 

 at = 1-2, and for boron =2'4. But if we take the specific heat towards which there is 

 ^evidently a tendency with a rise of temperature, then we obtain a product approaching 



