THE ATTAINMENT OF VERY LOW TEMPERATURES II 



published by Alessrs. Bradley and Rowe. The results of their experi- 

 ments are tabulated above. 



They observe that when these results are plotted (figure 50) the 

 curve cuts the pressure axis at a point corresponding to a pressure 

 between 700 and 800 atmospheres, below which no liquid would be 

 formed. They remark that this may be in some measure due to the 

 increase in the interchange temperature with fall of pressure, but make 

 no attempt to connect the figures in the second and third columns. 



Now if we take the heat of evaporation of liquid air to be 50 calories 

 per gram, and its specific heat at constant pressure to be 0.237 calories 

 per gram, we can assume that a fall of temperature of one degree in 

 the escaping air is equivalent to the evaporation and warming up to 

 the room temperature of ^ of its mass of liquid air, where 



(50 + 0.237 X 200) A- = ( I — -v) 0.237, 

 x = 0.25 per cent, per dyne. 



If then the interchange temperature were zero over all ranges of 

 pressure the quantity of liquid air produced in the machine would be : 



In figure 5 the quantities of liquid air actually produced in the 

 liquefier, and the quantities which would be produced if the difference 

 between the temperature of the air as it enters and leaves the liquefier 

 were zero, are plotted against the pressure. In neither case do 

 the points lie on a straight line, but in the second case x they deviate 

 from the straight line passing through the origin of the axis by amounts 

 which are of the order of the errors of experiment. 



I have not repeated these experiments myself, for I do not think 

 that it would be possible without considerable elaboration to obtain 

 more accurate results. Further, it appears to me that when the obser- 

 vations are corrected in the way I have suggested, they point to the 

 conclusion under normal conditions of working the quantity of liquid 

 air produced in the machine is very nearly a linear function of the 

 temperature fall of pressure in the machine. If this is the case the 

 production of liquid may be entirely attributed to Joule-Thomson 

 cooling. 



I have observed, however, that when the apparatus is worked at a 



