364 Prof. H. E. Armstrong on Low-Temperature Research 



and purifying it from carbon compounds and oxygen. The remaining 

 mixture of nitrogen with krypton and xenon is separated into its 

 constituents by condensation and fractionation. 



No more effective demonstration of the extraordinary absorptive 

 power of charcoal could well be imagined than that given at the 

 Friday evening lecture in June 1908, when liquid hydrogen (sur- 

 rounded by liquid air) was solidified in the course of very few minutes 

 by the cold produced by its own evaporation, this being brought 

 about by means of charcoal cooled by liquid air. The arrangement 

 was that of a cryophorus in which the one bulb contained liquid 

 hydrogen the other charcoal. 



Evolution of Heat on Absorption of Gases by Charcoal. — When 

 gases are condensed by charcoal the amount of heat liberated is con- 

 siderably in excess of that which necessarily attends the passage from 

 the gaseous into the liquid state. The first values deduced by Sir James 

 Dewar are as given below, those in the middle column of figures being 

 the amounts of heat (in gramme calories) liberated by the mere lique- 

 faction of gramme molecular proportions of the gases, those in the 

 last the amounts liberated when the same quantities of gas are 

 condensed by charcoal at the temperature of liquid air : — 



Molecular Heat Molecular Heat 

 of Liquefaction, of Absorption. 



The surprising fact brought out by these figures is that hydrogen, 

 although the least condensable gas, is that which has the greatest 

 affinity for charcoal. The behaviour of helium, however, is even 

 more remarkable. When hydrogen is absorbed at - 185"^, it is at 

 a temperature about four and a half times its boiling-point (20° abs.) ; 

 but helium is being absorbed at a temperature between fifteen and 

 twenty times its boiling-point (d'^abs.). To make a fair comparison, 

 hydrogen should be taken at a temperature at least fifteen times its 

 boiling-point, so that the absorption of helium at - 185° C. should be 

 contrasted with that of hydrogen at 0° C. It was therefore to be 

 inferred that at 15° to 20° absolute helium would be condensed to a 

 more remarkable extent than hydrogen at - 185° ; the following 

 figures showing the number of volumes of the two gases condensed 

 by the charcoal are proof of the accuracy of this conclusion : — 



Temperature. 



— 185'^ C. (boiling-point of liquid air) . 



— 210- C. (liquid air under exhaustion) 



— 252^ C. (boiling-point of liquid hydrogen) 



— 258" C. (solid hydrogen) . 



