at the Royal Institution, 1 900-1 907, 



391 



properties and also render it a solvent of oxygen : probably these 

 junctions become localised in their action at low temperatures, owing 

 to some rearrangement of the molecules more or less akin to that 

 involved in the formation of ice from liquid water. The effect 

 produced by cold on india-rubber, it may be pointed out, is very 

 different from that noticeable in the case of charcoal, the activity of 

 which is also attributable to its polyethenoid structure ; but in the 

 latter case, being rigid, the ordinary molecule is probably insusceptible 

 -of rearrangement, so that the ethenoid junctions remain operative 

 even at low temperatures. The power of condensing gases — more- 

 over of acting as a selective " solvent " — which is exhibited Ijy 

 ■charcoal in so marked a degree, may be regarded as incipient in 

 india-rubber, since this is a solvent of oxygen but not of other 

 gases to any marked extent. 



The manifestation of so high a degree of attraction for clia-rcoal 

 by hydrogen gas, as pointed out above, is proof that although it is 

 a univalent element the biatomic molecules of the gas are possessed 

 of residual affinity, which comes into operation when their rate of 

 motion is sufficiently reduced by cooling. 



A similar argument is applicable to helium. 



GAL0RI3IETRIC STUDIES. 



When a volatile liquid is evaporated by passing a current of air 

 through it, the temperature falls owing to the escape of heat in the 

 vapour. Sir James Dewar has made a most interesting application 

 •of this principle by utilising it in solidifying liquefied nitrogen and 

 hydrogen. If, as he has pointed out, the limit of temperature 

 reached on evaporating a volatile liquid be contrasted with the critical 

 temperature of the substance on the absolute scale, its value is about 

 half the critical temperature, as shown in the following table : — 



It therefore follows that if hydrogen be bubbled through liquid 

 nitrogen, the critical temperature of which is - 146° C. or 127° 

 absolute, the temperature should be reduced to about 278 - 63 

 = - 210°C. and consequently the nitrogen should freeze, as it melts 



