360 Prof. H. E. Armstrong on Loiv -Temperature Research 



one out of every five of the molecules of oxygen and nitrogen 

 (3 + 2) is displaced by hydrogen, giving rise to the system repre- 

 sented by Fig. 3. When charcoal saturated with oxygen is exposed 

 in hydrogen, the composition of the mixture finally occluded is 

 about that represented by Fig. 4 ; whilst if it act on charcoal 

 saturated with nitrogen it gives rise to the system represented in 

 Fig. 5. Lastly, if air be passed over charcoal saturated with 



(Sk® 



Fig. 1. 



(Sk® 



Fig. 2. 



hydrogen, the whole of the latter is ultimately displaced, the 

 occluded gas being of the composition represented by Fig. 2. 



It is clear from these observations that the more volatile or less 

 condensable the gas, the less it is absorbed and retained in the charcoal. 

 Charcoal may therefore be made use of most effectively in separating 

 the constituents of a mixture of gases of different degrees of volatility. 



In the perfectly gaseous state, at and above the critical tempera- 

 ture, the molecules of a substance are free from mutual control. 

 In the liquid state the molecules are in a state of w^hat may be termed 





Fig. 3. 



® 



Fig. 5. 



shifting association, their vibratory activity being so much reduced 

 that they are able to cling together during an appreciable interval 

 of time in virtue of the surplus or residual affinity with which they 

 are endowed. In the passage from the one state into the other, an 

 increasing proportion passes from the condition of freedom into that 

 of limited association or vice versa, according as the temperature falls 

 or rises. A liquid, therefore, always has a certain attraction for the 

 gaseous molecules of its own kind with which it is surrounded ; 

 the passage from the one state into the other may be regarded not 



