78 Prof. H. L. Oallendar on the Thermodynamical 



not extend below a pressure of 50 atmos at 100" C. The 

 following table exhibits the comparison. 



Table VIII. — Comparison with Amagat. 



Temperature centigrade 0°. 100°. 200 c . 



Values of c—b calculated 3-18 1-44 0-67 



„ ,, observed 3*30 15 072 



There is a small systematic difference which might possibly 

 be explained by surface condensation, but is hardly beyond 

 the limits of uncertainty of the data. 



It is possible by means of formula (54) to make a rough esti- 

 mate of the variation of the specific heat of C0 2 at constant 

 volume for comparison with the experiments of Joly. Neglect- 

 ing the small term nc/V, and putting n = 2, the formula may 

 be written approximately s=s + 2Rc/V, where Y=i)-\-c— b. 

 Joly's observations give for v = Sl c.c, s = -1684; and for 

 v = 27 c.c, s = '1734. These values are in calories, and cor- 

 respond to a mean temperature of 55° C. The mean value 

 of c may be taken as 2*61 c.c, so that 2Rc = 9"8x 10^ c.G.s. 

 The values of V are 89 and 29 c.c. respectively, so that the 

 calculated difference between the values of s comes out 

 0-228 x 10 6 c.G.s., or 0'0054 calorie. The observed difference 

 is seen to be 0*0050 calorie, which agrees quite as well as 

 could be expected with the calculated value. It should be 

 ■observed, however, that if we extrapolate to zero pressure, we 

 find the limiting value of the specific heat s from Joly's 

 observations about 0'1655 calorie per gramme-degree. The 

 ■corresponding value for S from RegnatuYs observations at 

 55° C. is 0*2014. The difference of these is only 0'0359 cal., 

 or 1*51 X 10 6 c.G.s., whereas the value of R is 1*887 x 10 6 c.G.s. 

 The discrepancy is nearly 5 per cent, of the value of S instead 

 of only 1 per cent, as in the case of air. 



16. Application of the Method to Steam. 



The large range of variation of the specific heat of C0 2 

 with temperature shows that the molecule must undergo 

 some fundamental change of structure within the limits of 

 temperature considered. It is possible that this may be 

 associated with the variation of the specific heat of carbon 

 itself. There is no evidence of a similar variation in the 

 case of the diatomic gases. In the case of steam, which is also 

 triatomic, large variations of the specific heat, from S = *387 

 at 100° C. to S = '6t>5 at 160° C, have been found experi- 

 mentally by Grindley, employing the Joule-Thomson method 



