412 Dr. M. W. Travers on the 



adiabatic, the fall of temperature inside the apparatus might 

 be calculated approximately from the formula 



T 2 - W ' 



where p x and p 2 are the initial and final pressures, T x and" 

 T 2 the initial and final temperatures, and k the ratio of the 

 specific heats. The failure to produce any quantity of liquid 

 is chiefly to be attributed to the great difference between the 

 thermal capacities of the gas and of the vessel into which it 

 is compressed. 



Lord Rayleigh and Kammerlingh Onnes have suggested 

 independently that it might be possible to liquefy hydrogen 

 by allowing the gas to do work in driving a heat-engine.. 

 The cylinder of Onnes's engine is supposed to be constructed 

 of a non-conducting substance of low specific heat and to be 

 enclosed in an insulated space ; a long piston-rod transmits 

 the energy of the system to some mechanism placed outside 

 the apparatus. A mixture of liquid and saturated vapour 

 would escape from the cylinder, and this alone adds to the 

 difficulties to be overcome in constructing the machine. 



Lord Rayleigh's suggestion of applying a turbine in a 

 similar manner could be more easily realized. 



By either of these processes, if conducted adiabatically, it 

 should be possible to liquefy & perfect gas; and we now come 

 to a method which can only be applied to gases w 7 hich are 

 imperfect and show a divergence from the simple gas law. 



In the case of a " perfect " gas we may write the equation 



p x v x = p 2 v 2 , 



where pv represents the total energy of the gas. If such a 

 gas were allowed to expand either without doing work or in 

 doing work in such a manner that the whole of the heat 

 generated were absorbed by the gas, no temperature-change 

 would take place in the system. These conditions could be 

 partially realized by allowing the gas to enter a vacuous 

 space through a large orifice, as in Gay-Lussac's experiment, 

 or by forcing it through a porous plug so constructed that 

 the velocity of the escaping gas is reduced to a minimum. 

 The latter method was adopted by Joule and Lord Kelvin, 

 and the results of their experiments show that for all known 

 gases the equation must be written 



where Q is the quantity of heat absorbed or generated in 



