27G l'n>I'. 11. L < 'ullrmhir. On the T/n-rtin/iini/i;- 



plotting is analogous to that rendered familiar by Amagat and others 

 in the case of the deviation of gases from Boyle's law. It is usual to 

 plot the product pv against p, but it seems to me to l>e preferable to 

 plot />v[R6 instead of pv, because the pv method confuses the diagram 

 by introducing the effects of the variation of temperature, so that the 

 different isothermals cannot be so well compared, and their relations 

 til.-erved. 



In the diagrams of Amagat and others, who have adopted the direct 

 method of measuring the whole specific volume instead of the differ- 

 ential method of observing only the deviation from the ideal volume, 

 the isothermals are not accurately straight, but always bend down- 

 wards more steeply as saturation is approached, so that they are 

 concave to the axis of pv. There seems reason to believe that this 

 peculiarity may be partly due to the effect of surface condensation so 

 well established by the observations of Ramsay and Young.* It is 

 true that a similar though smaller increase in the slope results from the 

 work of Xatanson on the Joule-Thomson effect for CO., at pressures up 

 to 26 atmospheres, at a temperature of 20 C. But in that case also 

 the effect may be explained by condensation in the pores of the porous 

 plug, as is indicated in some of the work of Joule and Thomson. I 

 was not able to find by the differential method, which would eliminate 

 any error of this kind, any trace of this effect at moderate pressures. In 

 fact, the cooling effect appeared to diminish very slightly with increase 

 of pressure, as it should, on account of the small increase in the value 

 of the specific heat with increase of pressure. If this is generally true 

 for other vapours, it would appear possible that many of the complica- 

 tions which have been introduced in current forms of empirical equa- 

 tions of the fluid state, may serve only to represent errors inherent in 

 the experimental methods on which they are founded. 



Values of the Specific Heats of Steam. 



The values of the specific heats at any temperature and pressure are 

 easily calculated from their limiting values at zero pressure, by means 

 of the formulae (9), (10), and (11) already given. The actual value of 

 the specific heat at a pressure of one atmosphere was experimentally 

 determined by the electrical method to be subsequently described. The 

 value so found, though slightly larger than Regnault's, agreed so well 

 with the theoretical value deduced from the characteristic equation (6) 

 by means of Maxwell's assumption, that there can be little doubt that 

 the method of deduction employed is valid. The value of the constant 

 K for steam is readily found from the value of the ideal volume already 

 assumed, we thus obtainf 



* Phil. Trans.,' A, 1892. 



t Assuming that the pressure due to a column of mercury 760 mm. in height at 

 0C. and sea-level in latitude 45 is equal to 1*0133 megadynes per sq. cm. 



