Correction of the Gas- Titer mo meter. 79 



(Phil. Trans. A, 1900), and assuming Regnault's formula for 

 the total heat of saturated steam. The following values have 

 also been deduced by other writers on theoretical grounds : 

 Zeuner, S = *568; Gray, 0*385; Tumlirz, 0*536 to 0*475; 

 Perry, 0*306 to 0*463. I find, however, by direct expe- 

 riment, employing the continuous electrical method with a 

 vacuum-jacket calorimeter, the value S = 0*497 at 1 atmo 

 and 108 D C, which agrees fairly with Regnault's value 0*475 

 at 175° C, allowing for the variation due to the coaggregation 

 by formula (52). I have endeavoured to show (Proc. R. S. 

 lxvii. p. 266, 1900) that all the properties of steam may be 

 consistently calculated on the assumption that the limiting 

 value of the specific heat is constant, employing the same 

 type of equation as for C0 2 , but leaving the value of n to be 

 determined from observations of the cooling-effect at various 

 temperatures. If we adopt this type of formula, it appears 

 from the observations of Grindley (Phil. Trans. 1900) that 

 the value of n for steam should be about 3*8 instead of 2. 

 My own observations on the cooling-effect and the specific 

 heat of steam would give the values n = 3*3, and <? = 2(r3 c.c. 

 at 100° C. In calculating the properties of steam by this 

 formula in the paper above referred to, I adopted the mean 

 value 3*5 for the index, partly to facilitate calculation and 

 partly in consequence of an hypothesis (doubtfully attributed 

 to Maxwell) that the number of degrees of freedom of a 

 molecule containing m atoms is 2m + 1. This hypothesis 

 would make the ratio of the specific heats S/s at constant 

 pressure and volume, 5/3 for a monatomic gas, 7/5 for a 

 diatomic gas, 9/7 if or atriatomic gas, and so on ; values which 

 agree very fairly with the ratios of the specific heats actually 

 observed in many cases. Later and more accurate expe- 

 riments on the specific heat of steam have shown that the 

 ratio s/R should be more nearly 3*3, and have so far confirmed 

 the value of the index given by my experiments on the 

 cooling- effect. 



Adopting the experimental value S = 0*497 at 1 atmo and 

 108° C. we find by applying formula (52) the limiting value 

 S = 0*478 at zero pressure. If we employ this value in place 

 of the value 45R adopted on Maxwell's hypothesis in the 

 paper above referred to, we find that the agreement with 

 experiment in the values of the total heat and the saturation 

 pressure is somewhat improved, but the general nature of the 

 conclusions remains unaltered. Since the value of the index 

 n cannot be determined very accurately from the cooling- 

 effect, it is better in this case to take it equal to s/R for fche 

 sake of simplifying- the equation of the isentropics, which 



