220 HISTORY OF COLD AND THE ABSOLUTE ZERO. 



only one-tifth of the rate i)revailin<4- when it was placed in a similar 

 unexhausted vessel, owing to the convective transference of heat by 

 the gas particles being enormously reduced })y the high vacuum. But, 

 in addition, these vessels lend themselves to an arrangement b}' which 

 radiant heat can also be cut off. It was found that when the inner 

 walls were coated with a bright deposit of silver the influx of heat was 

 diminished to one-sixth the amount entering without the metallic coat- 

 ing. The total effect of the high vacuum and the silvering is to reduce 

 the ingoing heat to about 3 per cent. The efficiency of such vessels 

 depends upon getting as high a vacuum as possible, and cold is one of 

 the best means of effecting the desired exhaustion. All that is iieces- 

 sar}^ is to fill completel}^ the space that has to be exhausted with an easily 

 condensable vapor, and then to freeze it out in a receptacle attached 

 to the primary vessel that can be sealed off'. The advantage of this 

 method^ is that no air pumy) is required and that theoretically there is 

 i\o limit to the degree of exhaustion that can be obtained. The action 

 is rapid, provided liquid air is the cooling agent, and vapors like mer- 

 cury, water, or benzol are employed. It is obvious that when we 

 have to deal with such an exceptionally volatile liquid as hydrogen, 

 the vapor filling may be omitted because air itself is now an easily 

 condensable^ vapor. In other words, liquid hydrogen, collected in 

 such vessels with the annular space full of air, immediately solidi- 

 ties the air and thereby surrounds itself with a high A-acuum. In the 

 same way, when it shall be possible to collect a liquid boiling on the 

 absolute scale at about 5°, as compared with the 20^ 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 soliditica- 

 tion 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 off' from the interior of such a vessel. In the working of 

 regenerative coils such a device becomes all important, and such special 

 vessels can not ])e dispensed with for the liquefaction of hydrogen. 



In the early experiments of Pictet and Cailletet, cooling was i)ro- 

 duced 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 pi'oduction of a temporary mist. Lindc, 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 effect; and he succeeded in making a machine, 



