THE ATTAINMENT OF VERY LOW TEMPERATURES 3 



and with the valve below. The doubling of the coil was intended to 

 diminish any tendency on the part of the pipe to become blocked with 

 impurity separated from the gas, and to reduce friction in the pipe, 

 and so increase the pressure at the expansion valve. These precau- 

 tions are, as a matter of fact, totally unnecessary. 



The results obtained with this liquefier were far from satisfactory, 

 and a careful study of its behaviour showed that the fault lay entirely 

 in the regenerator coil. It was evident that the preliminary cooling 

 of the compressed gas was as complete as in the case of my first ma- 

 chine, probably even more so ; but that the heat-interchange between 

 the compressed and expanded hydrogen in the regenerator coil was 

 unsatisfactory. Careful consideration of the problem soon led to an 

 explanation of the inefficiency of the second regenerator coil, and it is 

 this point I shall next discuss. 



The phenomena connected with the interchange of heat between 

 fluids in motion has been studied chiefly with a view to applying the 

 results to steam boiler problems. Osborne, Reynolds, Stanton, and 

 others have shown that when currents of water are made to flow in 

 opposite directions through concentric tubes, the change of tempera- 

 ture is independent of the velocity of the streams, or that the heat- 

 interchange is proportional to the relative velocities. The same re- 

 marks apply to the change of temperature and loss of heat by furnace 

 gases in passing through boiler tubes ; and to show how closely the 

 phenomena of heat regeneration in gas liquefiers resembles those I 

 have just referred to, it is sufficient to state that in the Hampson air 

 liquefier the difiference between the temperatures of the air which enters 

 and leaves the apparatus shows the same dift'erence of temperature 

 even when the velocity of the stream of air passing through it is greatly 

 increased. 



It is also well known that the loss of heat that furnace gases un- 

 dergo is considerably diminished if the absolute velocity of the gas be 

 decreased. It can be shown, for instance, that by increasing the diam- 

 eter of the tubes of an ordinary boiler, or by spacing the tubes of a 

 water-tube boiler more widely the heat interchange between the gases 

 and the water may be considerably reduced. 



Were it possible to consider the phenomenon of heat interchange 

 between two fluids moving in opposite directions on different sides of 

 a conducting surface one of conduction of heat through the fluid, the 

 study of it would be a comparatively simple matter. As a matter of 

 fact, however, this is the case only when the fluids were travelling 

 slowly, and on account of the low conductivity of gases the heat inter- 

 change is, under such circumstances, very small. When, however, the 

 velocity of the fluid bears a certain relationship to the dimensions of 

 the space through which it is flowing, the longitudinal components into 



