THE LIQUEFACTION OF GASES 63 



filled with oxygen and carbonic acid at low 

 pressures, gave correct temperatures as low as 

 the boiling-points of those gases at the normal 

 atmospheric pressure. He used therefore a 

 constant volume hydrogen thermometer, working 

 at low pressure, to determine the boiling-point of 

 liquid hydrogen itself, and confirmed the result 

 obtained, — 252° C, by experiments with a similar 

 thermometer filled with helium. 



Some very remarkable effects are obtained 

 with liquid hydrogen. A vessel containing it is 

 so cold that the air in contact with it immediately 

 freezes. A snow-shower of solid air is thus 

 produced. This process may be applied to the 

 production of very high vacua. If the vessel to 

 be exhausted be sealed to a long tube, one end 

 of which is plunged into liquid hydrogen, the air 

 in the vessel is frozen out almost completely. 

 The air in the cooled end of the tube first con- 

 denses, but, as it is removed, the residual air in 

 the vessel expands, again fills the whole tube, 

 and again that portion of it in contact with the 

 cold part of the tube is frozen. This process 

 continues till the pressure within the tube falls to 

 the millionth of an atmosphere or less, a pressure 

 so low that an electric discharge will only pass 

 through the vessel with extreme difficulty. A 

 vacuum nearly complete may also be obtained by 

 using charcoal cooled by liquid air in place of the 

 hydrogen. . 



The liquefaction of air and hydrogen has 

 led to the making of many experiments on the 

 influence of low temperature on chemical action, 

 and it is found that the rate of change is very 



