THE ATTAINMENT OF VERY LOW TEMPERATURES 1 3 



by merely increasing the velocity of the flow of the expanded gas, than 

 are any of the methods by which I have attempted to increase the sur- 

 face exposed to the gas, or its tendency to form eddies over the surface 

 of the coil. It is to this type that I have now reverted. 



VIII. An Attempt to Employ a Metal Vacuum Vessel in the 

 Liquefaction of Hydrogen. 



As there appeared to be many obvious advantages in employing a 

 metal vacuum vessel in place of the glass vessel which usually forms 

 part of the apparatus, I determined to make one, and to carry out some 

 experiments with it. The vessel was constructed of drawn brass tube 

 with spun copper connection, as in the figure. After the parts had been 

 soldered together the whole was silver-plated with the idea of render- 

 ing the metal absolutely non-porous. The vacuum vessel was secured 

 by means of a flange and screws to a horizontal plate, which forms part 

 of the annular space surrounding the liquid air chamber in the lique- 

 fier. A spiral copper tube connected the inside with the outside, and 

 terminated below in a valve similar to that one used in the Hampson 

 air liquefier. 



The vacuum vessel was exhausted through a piece of fine copper 

 tube, hard-soldered into the outer tube. When the exhaustion was 

 complete this tube was melted in the oxyhydrogen flame and securely 

 sealed. When the vacuum vessel was half filled with liquid air the 

 outside did not become cold, but it was noticed that the liquid evap- 

 orated more quickly than from a glass vessel on account of the heat 

 supplied to it by conduction down the inner wall. 



An attempt to liquefy hydrogen with a machine fitted with the 

 vacuum vessel proved a failure. This was probably due to the reason 

 mentioned above, and also to the evaporation of the liquid in passing 

 through the valve. Only a trace of liquid was collected, and the ex- 

 periment was not repeated. 



IX. On the Preparation of Hydrogen for Use with the 



Liquefier. 



In my first memoir on the liquefaction of hydrogen I pointed out 

 that the difficulty introduced by the presence of impurity in the gas 

 was due rather to the separation of solid at the expansion valve than 

 to the actual blocking of the coil. I suggested that this was probably 

 due to the so-called " solvent " properties, which gases under high 

 pressure and in the neighborhood of their critical temperature are 

 known to possess. 



I have come to the conclusion that while some impurities are highly 

 detrimental to the working of the apparatus, others are comparatively 



