[satterly] partial CONDENSATION 117 



many instances, it halts and kicks down suddenly to the extent of 

 many centimetres. It rests in its new position for some time and then 

 gradually climbs up to somewhere near its old position. A possible 

 explanation of the down kick is that as the contents of G cool, nitrogen 

 re-dissolves in the liquid. The pressure in G then becomes smaller 

 and smaller until less energy is required to keep the nitrogen in the 

 gaseous state than in the liquid state. It accordingly suddenly leaves 

 the liquid. Often, when such a down kick has occurred, I have heard 

 a noise in the condenser like a splash or a mild explosion, and on re- 

 moving the liquid air the condenser has been seen to have frozen 

 hard and to have a very jagged crust on it as if at the instant of 

 freezing a violent outburst occurred. Sometimes pieces of the frozen 

 condensate have been scattered over the walls of the tube. After 

 having once removed the liquid air and then replaced it as above 

 described, the final pressure is usually greater than the initial pressure, 

 showing that some of the gas which before was dissolved in the con- 

 densate at liquid air temperature has now gone into the space above. 



In many cases I have put on a vacuum pump to P and exhausted 

 all the uncondensed gases from G. The pressure has been brought 

 down as low as 3 or 4 cm. so that not only have the uncondensed 

 gases been removed, but much of the methane, some of the methane 

 even boiling away at the lower temperature induced by the pressure. 

 Even in these cases I have observed the upward kick in the gauge 

 after removal of the liquid air, and the downward kick after the 

 application of the liquid air, though the extents of these kicks dimin- 

 ish gradually if the vacuum pump is applied several times. 



The following are examples : 



November 17, 1917. Making Helium in Quantity from PetroUa 

 Gas. Twenty-five "collectors-ful," i.e. 30 litres of natural gas had been 

 passed into G and the pump attached to P had been in operation some 

 time in order to transfer the uncondensed gas from G to the next part 

 of the apparatus. The pressure in G had been reduced to 7 cm. of 

 mercury (the vapour pressure of methane) when suddenly there was 

 a violent ebullition of gas and the mercury fell in the gauge to a pres- 

 sure of 34 cm., i.e. the gas pressure had risen 27 cm. All this occurred 

 M^thout any tampering with the liquid air. The dissolved nitrogen 

 (and possibly helium, too) had evidently reached an unstable state 

 and much of it was forced to leave the liquid for the space above. An 

 increase of gauge pressure of 27 cm. means an outburst (measured 

 under atmospheric pressure and temperature of well over 400 cc. of 

 nitrogen — including helium). After this outburst, the pressure was 

 allowed to rise to atmospheric pressure and the tap O was opened. 



