686 



PRINCIPLES OF CHEMISTRY 



ions arrived at by Dulong and Petit with respect to solid elementary 



bodies. 



D Na Mg 



A= 7 23 24 31 



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



AQ= 6-59 6-75 5-88 



Fe 



A= 56 

 Q= 0-112 

 AQ = 6-27 



Pd 



A= 106 

 Q =* 0-0592 

 AQ = 6-28 



Pt 

 A= 196 



Q = 0-0325 

 AQ = 6-37 



Cu 

 63 



0-093 

 5-86 



Ag 



108 

 0-056 

 6-05 



Au 



198 

 0-0324 

 6-41 



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

 element into the atomic weight is an almost constant quantity, 

 which is nearly 6. Hence it is possible to determine the valency 

 by the specific heats of the metals. Thus, for instance, the specific 

 heats of lithium, sodium, and potassium convince us of the .fact that 

 their atomic weights are indeed those which we chose, because by 



gives 5-6, the same as for magnesium and aluminium. I may here direct the reader's 

 attention to the fact that for solid elements having a small atomic weight, the specific 

 hea,t varies considerably if we take the average figures for 'temperatures to 100 : 



Li=7 Be=9 B = ll C = 12 



Q = 0-94 0-42 0-24 0'20 



AQ= 6-6 8-8 2-6 2'4 



It is therefore clear that the specific heat of beryllium determined at a low temperature 

 cannot serve for establishing its atomicity. On the other hand, the low atomic heat of 

 charcoal, graphite, and the diamond, boron, &c., may perhaps depend on the complexity 

 of the molecules of these elements. The necessity for acknowledging a great complexity 

 of the molecules of carbon was explained in Chapter VIII. In the case of sulphur the 

 molecule contains at least S 6 and its atomic heat =82 x 6-163 = 5-22, which is distinctly 

 below the normal. If a large number of atoms of carbon are contained in the molecule 

 of charcoal, this would to a certain extent account for its comparatively small atomic 

 heat. With respect to the specific heat of compounds, it will not be out of place to 

 mention here the conclusion arrived at by Kopp, that the molecular heat (that is, the 

 product of MQ) may be looked on as the sum of the. atomic heats of its component 

 elements ; but as this rule is not a general one, and can only be applied to give an approxi- 

 mate estimate of the specific beats of substances, I do not think it necessary to go into 

 the details of the conclusions described in Liebig's ' Annalen Supplement-Band,' 1864 J 

 which includes a number of determinations made by Kopp. 



