CARBON DIOXIDE AT DIFFERENT TEMPERATURES. 237 



(29) Variation of the Specific Heat at Constant Pressure with Pressure. From 

 the equation OH p. 230 it can be shown the (ds/dp\ = n (n+ 1) c/0. Prof. CALIENDAR 

 has shown that* for CO 2 , n = 2, c = 4'56. Hence at 20 C. (3*/3p) = O'OSll, or if 

 S be expressed in calories per gr. degree C., and 1 cm. of Hg be taken as the unit of 

 pressure (3s/3p)ao = 0'000026, corresponding to a change of O'Ol per cent, in the 

 value of S for an alteration of pressure of 1 cm. of mercury. At 100 C. the 

 effect is even smaller, and thus it is not necessary to take account of the effect of 

 fluctuations of the atmospheric pressure on the specific heat. 



(30) Further Corrections. To the above mean values of the specific heat the 

 corrections mentioned on pp. 229 and 230 remain to be applied. Correction (1) amounts 

 to 6 parts in 10,000 at 20 C. and is additive. The value at 100 C. is 1'5 parts per 

 1000 and is additive. Correction (2) in the case of CO 2 amounts to an additive 

 correction of 4"8 parts in 10,000. Corrections (3), (4), and (5) form the same per- 

 centage of the value of S as in the case of air, and amount to a subtractive correction 

 of 0"2 parts in 10,000. Hence the corrected values of the specific heat in terms of 

 the calorie at 20 C. are 



S = 0-20202 cal. per gr. degree at 20 C., S = 0-22141 cal. per gr. degree at 100 C. 



It is interesting to observe that, although the above values are so much greater 

 than those found by REGNAULT and WIEDEMANN, the percentage variation of the 

 specific heat with temperature works out to a value intermediate between the values 

 found by those investigators. This is only what might be expected, for the criticism 

 given on p. 231, though affecting the absolute values of the specific heats, would not 

 greatly affect the temperature coefficient. 



(31) Comparison of the Results ivitli the Theoretical Values. It is possible to 

 make a comparison of the above results with JOLY'S value for s in a manner similar 

 to that adopted on p. 230 for air. JOLY performed two sets of experiments. The 

 first of these gave s as a function of the absolute density (p) in the form 

 s = 0-16577 + 0-2064/3. The second gave .s- = 0'1650 + 0-2125/3 + 0-340/3 2 ; the experi- 

 ments referring to a mean temperature of 55 C. In order to obtain the true value 

 of s a it will, perhaps, be advisable to take the mean of the two values given by the 

 above equations, for, although JOLY considered his second set of experiments the 

 more reliable, the first set applied more particularly to the lower pressures. 



Thus we obtain for s the value 0'16538 cal. per gr. degree at 55 C. Correcting 

 this value for the fact that JOLY used the value 5367 cal./gr. for the latent heat of 

 steam, we obtain s = 0'16645, on the assumption that L = 540*2 cal./gr. This value 

 is expressed in terms of the calorie at 20 C. If we assume a linear law for the 

 variation of S with temperature over the range from 20 C. to 100 C., my own 

 observations give, for the value of S at 55 C., S = 0-21050 in terms of the calorie at 



20 C. 



* ' Phil. Mag.,' S. 6, vol. 5, p. 77, January, 1903. 



