ADDRESS, 31 



absolute scale at about 5 degrees, as compared with the 20 degrees 

 of hydrogen, then you might have the annular space filled with the latter 

 gas to begin with, and yet get directly a very high vacuum, owing to the 

 solidification of the hydrogen. Many combinations of vacuum vessels can 

 be arranged, and the lower the temperature at which we have to operate 

 the more useful they become. Vessels of this kind are now in general 

 use, and in them liquid air has crossed the American continent. Of the 

 various forms, that variety is of special importance which has a spiral 

 tube joining the bottom part of the walls, so that any liquid gas may be 

 drawn otf from the interior of such a vessel. In the working of regenera- 

 tive coils such a device becomes all -important, and such special vessels 

 cannot be dispensed with for the liquefaction of hydrogen. 



In the early experiments of Pictet and Cailletet, cooling was produced 

 by the sudden expansion of the highly compressed gas preferably at a low 

 temperature, the former using a jet that lasted for some time, the latter 

 an instantaneous adiabatic expansion in a strong glass tube. Neither 

 process was practicable as a mode of producing liquid gases, but both gave 

 valuable indications of partial change into the liquid state by the 

 production of a temporary mist. Linde, however, saw that the continuous 

 use of a jet of highly compressed gas, combined with regenerative cooling, 

 must lead to liquefaction on account of what is called the Kelvin-Joule 

 efiect ; and he succeeded in making a machine, based on this principle, 

 capable of producing liquid air for industrial purposes. These experi- 

 menters had proved that, owing to molecular attraction, compressed 

 gases passing through a porous plug or small aperture were lowered in 

 temperature by an amount depending on tlie difference of pressure 

 and inversely as the square of the absolute temperature. This means 

 that for a steady difference of pressure the cooling is greater the lower 

 the temperature. The only gas that did not show cooling under such 

 conditions was hydrogen. Instead of being cooled it became actually 

 hotter. The reason for this apparent anomaly in the Kelvin-Joule 

 effect is that every gas has a thermometric point of inversion above 

 which it is heated and below which it is cooled. This inversion 

 point, according to van der Waals, is six and three-quarter times the 

 critical point. The efficiency of the Linde process depends on working 

 with highly compressed gas well below the inversion temperature, and in 

 this respect this point may be said to take the place of the critical one, 

 when in the ordinary way direct liquefaction is being effected by the use 

 of specific liquid cooling agents. The success of both processes depends 

 upon working within a certain temperature range, only the Linde method 

 gives us a much wider range of temperature within which liquefaction can 

 be effected. This is not the case if, instead of depending on getting cool- 

 ing by the internal work done by the attraction of the gas molecules, we 

 force the compressed gas to do external Avork as in the well-known air 

 machines of Kirk and Coleman. Both these inventors have pointed out 

 that there is no limit of temperature, short of liquefaction of the gas ia 



