::s-i Messrs. AV. Makowc-v and 11. L>. Xoblc. [June LV, 



E = intrinsic energy of fluid per unit mass. 



p = pressure ; v = volume of unit mass. 



F = E +pv = total heat of fluid per unit mass. 

 H = heat supplied per unit mass from external sources. 

 <Q = (dO/dp) F = " cooling effect" or fall of temperature per unit fall 

 of pressure in adiathermal expansion at constant F. 



S = (dH/dO) p = specific heat of fluid at constant pressure. 



T = (pv/R) = temperature by gas thermometer. 



= temperature on thermodynamic scale. 



Ati the point in the neighbourhood of 50 C., the degrees on the 

 scale of the gas thermometer are of the same size as those on the 

 absolute scale, dT/dd = 1, and as is shown in the paper quoted 

 above, the difference between the temperature on the gas scale and on 

 the absolute scale is given approximately by the formula 



The term (d (pity/dp^ can be evaluated, as shown by Callendar, in the 

 following manner : Assuming the characteristic equation of the gas 

 to be 



< 2) - 



where 



a 



C == K0-' = r 

 it may be shown that 



SQ = (n + I)r-b (4), 



and that 



The index n depends on the nature of the gas, and may be taken as 

 1*5 for oxygen. 



From the Joule-Thomson results,* we have for oxygen at 92 C., 

 SQ = 1-41 c.c., and at 8*7 C., SQ = 2'88 c.c. ; S being expressed in units 

 of 10 6 ergs. 



Hence from equations (3) and (4), 



C Q = 1-92 c.c. and b = 1*70 c.c. 

 At 50C., 



c = 1-49, 

 substituting in equation (5) 



d (pv)/dp at 50 C. = - (1-49 - 1-70) = +0'21 c.c. 

 * JouVs Collected Papers, vol. 2, p. 348. 



