256 Dr. H. F. Weber on the Specific Heat of Carbon. 



its temperature 1°. This real specific heat 7^ can be derived 

 from Co-t in the following manner : — 



i=J^V..^/. 



t . Co- 



We obtain 



7^=0-0947 + 0'000994^-0-00000036^2^ . . (2) 

 According to this^ for 



O'' 7=00947 ISO'-' 7=0-2357 



50° 7 = 0-1435 200'' 7 = 0-2791 



100° 7=0-1905 



Want of snow prevented me from investigating in an equally 

 comprehensive manner the remaining allotropic modifications of 

 carbon. With the rest of my stock of snow I made only two 

 more determinations, upon a piece of very pure native graphite 

 of 951 milligrammes. The graphite was heated to 34°, and gave 



Co_34= 0-1439 J 

 heated to 100° it gave 



co-ioo=0-1937. 

 From these two determinations it would follow that 



co_i = 0-1167 + 0-0008if,~l 



and yt =0-1167 + 0-0016^. J 



Although the constants of these equations may not have the 

 utmost accuracy, still it is perfectly evident as the result of these 

 two experiments that the specific heat of graphite also consider- 

 ably increases with rising temperature. By this the numbers 

 hitherto found for graphite, so widely diverging from one another, 

 are brought into almost perfect accordance. 



Lastly, when we consider that for wood charcoal De la Rive 

 and Marcet found C3°-i4° = 0*2009, while Regnault obtained 

 Cgo_g80 = 0-2415, it appears hardly doubtful that also the porous 

 form of carbon exhibits, in relation to specific heat, the same 

 behaviour as graphite and diamond. 



Now this great variability of the specific heat of carbon with 

 the temperature makes the hitherto observed anomalous beha- 

 viour of carbon towards Dulong and Petit^s law perfectly expli- 

 cable. If we might assume that the validity of the relation (2) 

 extends up to 500° (which, of course, is most probably not 

 strictly the case), then would the specific heat of the diamond at 



about 525° have the value 0*52, i. e. ^o ^ which Dulong and 



Petit's law requires. But with the extremely high, not yet 

 reached melting-temperature of carbon, this behaviour might 



