30 REPORT— 1902. 



the means of going 100 degrees lower still. The difficulty, it need 

 hardly be said, increases in a geometrical rather than in an arithmetical 

 ratio. Its magnitude may be estimated from the fact that to produce 

 liquid air in the atmosphere of an ordinary laboratory is a feat analogous 

 to the production of liquid water starting from steam at a white heat, 

 and working with all the implements and surroundings at the same high 

 temperature. The problem was not so much how to produce intense 

 cold as how to save it when produced from being immediately levelled up 

 by the relatively superheated surroundings. Ordinary non-conducting 

 packings were inadmissible because they are both cumbrous and opaque, 

 while in working near the limits of our resources it is essential that the 

 product should be visible and readily handled. It was while puzzling 

 over this mechanical and manipulative difficulty in 1892 that it 

 occurred to me that the principle of an arrangement used nearly 

 twenty years before in some calorimetric experiments,^ which was based 

 upon the work of Dulong and Petit on radiation, might be em- 

 ployed with advantage as well to protect cold substances from heat as 

 hot ones from rapid cooling. I therefore tried the effect of keeping 

 liquefied gases in vessels having a double wall, the annular space between 

 being very highly exhausted.'-^ Experiments showed that liquid air 

 evaporated at only one-fifth of the rate prevailing when it was placed in 

 a similar unexhausted vessel, owing to the convective transference of heat 

 by the gas particles being enormously reduced by the high vacuum. But, 

 in addition, these vessels lend themselves to an arrangement by 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 coating. The total 

 eflect 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 effect- 

 ing the desired exhaustion. All that is necessary is to fill completely the 

 space that has to be exhausted with an easily condensable vapour, 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-pump is required, 

 and that theoretically there is no limit to the degree of exhaustion that 

 can be obtained. The action is rapid, provided liquid air is the cooling 

 agent, and vapours like mercury, water, or benzol are employed. It is 

 obvious that when we have to deal with such an exceptionally volatile 

 liquid as hydrogen, the vapour filling may be omitted because air itself is 

 now an easily condensable vapour. In other words, liquid hydrogen, col- 

 lected in such vessels with the annular space full of air, immediately 

 solidifies the air and thereby surrounds itself with a high vacuum. In the 

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



' ' On the Physical Constants of Hydrogenium,' Trans. Hoy. Soc, ed. 1873. 

 ^ It now appears that similar vessels were employed by Professor Violle in a 

 research entitled ' Sur un Calorim^tre par Refroidissement,' Comptet Rendus, 1 882. 



