Joule-Thomson Thermal Effect. 107 



(2) Notation. 



The following scheme of notation is adhered to in this 

 paper : — 



Pressure of a substance p 



Volume of unit mass v 



Absolute temperature T 



Density of substance p 



Specific heat at constant pressure .... K P 



Specific heat at constant volume .... K„ 



Intrinsic energy of unit mass U 



Joule-Thomson cooling effect per atmosphere } a 



rise of pressure J 



Standard pressure of 1 atmosphere .... II 



Mechanical equivalent of heat J 



Coefficient of dilatation a 



Quantity of heat Q 



The letters A, B, C, R, a, b, a, ft, 7, 8 denote constants 

 occurring in various equations ; subscribed letters indicate 

 that the quantities denoted by them are supposed constant. 



(3) The Joule-Thomson Effect and the Gas-Equation. 



The reasoning of this section is based throughout on 

 Lord Kelvin's well-known equation 



where dT denotes the rise of temperature in a " porous plug " 

 experiment corresponding to a rise of pressure dp on passing 

 the plug. 



(1) If we adopt the ordinary Boyle-Charles equation of a 

 perfect gas 



pv=BI, 



then we can show at once by differentiation that 



(S)t— ' 



in other words, the Joule-Thomson thermal effect is zero, a 

 result which the experiments conclusively disprove. 



(2) A very different result is obtained if we adopt van 

 der Waals's equation; as we shall see, it leads directly to the 

 expression recently* formulated by Rose-Innes. We start 



* Phil. Mag, March 1898. 



