On the Specific Heat ofCarhon at High Temper attires. 461 



III. AVe novv^ by a perfectly natural deduction, arrive at the 

 law of cooling. Indeed, since the gas when it has attained its 

 stationary temperature 6-\-d6 takes from the wire a quantity of 

 heat q' equal to that which it yields to the outer wall, it is only 

 necessary to make q and q' equal. Now the quantity q is 

 known : according to our previous memoir it is equal to 



/id' 



^^' ijd'-c' } ^^} taking logarithms, 

 H 



log^ = log;8^' + ^nogA-(^'-c')logH. . . (4) 



But we have just found by experiment 



log (r' — r)=log^ + alogA — iSlogH. . (3) 



Eliminating h between these two equations, we shall have g as a 

 function of r'—r and of H — that is, the law of cooling. This 

 elimination conducts to an equation of the form 



log ^ = log 71 + ^ log (r' — r) + clog H, ... (5) 



and, passing again to numbers, 



q = n(7''-rYR', (6) 



which is precisely the law of Dulong and Petit, found again by 

 an entirely different process. These are the values of the con- 

 stants : — 



Carbonic acid. Air. Hydrogen. 



A . . . 0-922 2-57 26-70 



c . .. . 0-37 0-44 0-42 



d . . . 1-17 1-28 1-30 



The exponents differ little from those found by Dulong and 

 Petit. We shall not now insist on this point, as we believe that 

 they are variable; we purpose to return to it in an early com- 

 munication. 



LV. On the Specific Heat of Carbon at High Temperatures. 

 By James Dewar, F.R.S.E., Lecturer on Chemistry, Edinburgh. 



To the Editors of the Philosophical Magazine and Journal. 

 Gentlemen^ Edinburgh, October 25, 1872. 



THE following paper contains a few details of some of the 

 experiments on which I founded the communication to 

 the British Association at Brighton, on the Specific Heat of Car- 

 bon at High Temperatures, that has seemingly caused some un- 

 necessary annoyance to Dr. H. F. Weber, of Berlin. 



So far as I am aware, very few observations have been made 

 in this direction since Pouillet^s well-known research on the Spe- 

 cific Heat of Platinum. It was with the object of filling up this 



