788 Prof. A. LI. Hughes on Dissociation oj 



Discussion. 



It is necessary to give reasons for believing that the 

 disappearance of the gas is due to dissociation of hydrogen 

 into atoms which condense on the walls. 



Langmuir* found that pure hydrogen at a low pressure 

 in a bulb containing an incandescent filament disappeared. 

 On carrying out the experiment with the bulb, or a side 

 tube, immers d in liquid air, a peculiar effect w.is noticed. 

 After the pressure decrease had been in progress for some 

 time, the liquid air was removed (the filament heating- 

 current being cut off) and the pressure rose. On replacing 

 the liquid air, the pressure fell a little, but not to its original 

 value. There was, therefore, as Langmuir terms it, a non- 

 recondensible gas. Langmuir accounted for this on the 

 supposition that hydrogen molecules on impact with the very 

 hot filament were dissociated into atoms, and that these 

 atoms, if they had a clear run to the walls, would condense 

 on them, the effect being the more marked at low temper- 

 atures. On removing the liquid air, some of the atoms 

 would come off and re-combine with other atoms to form 

 molecules which could not be condensed. Thus Langmuir 

 gives a natural explanation for the "non-recondensible " gas. 

 The more atoms already on the surface, the greater the 

 chance of an atom impinging on the surface, striking one of 

 them and recombining to form a molecule, which leaves the 

 surface and so reduces the apparent rate of dissociation of 

 the gas. From experiments on the transfer of heat through 

 hydrogen from an incandescent filament, Langmuir calcu- 

 lated the amount of hydrogen dissociated. He states that, 

 under the most favourable conditions, the observed decrease 

 in pressure corresponds to only 1/7 of the amount actually 

 dissociated, and may often be much less. 



Non-recondensible gas was obtained in these experiments. 

 The following is an illustration from one set of observations. 

 Hydrogen was admitted to a pressure of 548 x 10~ 5 mm. and 

 cleaned up by electron impact to a pressure of 100 x 10~° mm. 

 The liquid air was then removed, the pressure rose, the 

 liquid air was replaced and the pressure fell to 263 x 10 ~° mm. 

 Hence the amount of non-recondensible gas was proportional 

 to (263 — 100) X 10 -5 mm., and the amount of hydrogen 

 which originally disappeared was proportional to (548 — 100) 

 X 10~° mm. Thus the amount of non-recondensible gas 

 formed 37 per cent, of the amount of hydrogen cleaned up. 

 Then, without out</assiny, more hydrogen was admitted until 



* Langmuir, Journ. Ainer. Chem. Soc. xxxvii. p. 451 (191o). 



