Disappearance of Gas in the Electric Discharge. 703 



liberated by baking at all (unless it is prolonged for many 

 hours), and the pressure of the restored gas measured. Since 

 the gas was in contact with liquid air, only the restored 

 hydrogen, and not water or phosphine, was measured in this 

 way. It was found that, in any given state of the vessel, 

 the ratio of hydrogen re-evolved to that absorbed was remark- 

 ably constant and independent of the amount absorbed ; in 

 the lamps used the normal ratio was 0*6. This ratio could 

 be repeated many times by absorbing and liberating hydrogen 

 in the same vessel, but if the experiment was sufficiently 

 prolonged, the ratio would at some stage always fall suddenly 

 to a lower value, and again remain constant at that value. 

 Later it would fall again until usually a ratio as low as ^ 

 could be reached. It was noticed that any signs of blackening 

 of the walls were immediately accompanied by a fall in the 

 ratio, i. e. by the re-evolution of less gas on "baking. 



An attempt was made to determine whether the hydrogen 

 which did not reappear on baking could be accounted for as 

 water and phosphine (and " active hydrogen ") condensed in 

 liquid air. In a few experiments, which were always the 

 first experiment made on a new lamp, it seemed that, by 

 allowing the products of baking to act upon sodium, prac- 

 tically all the hydrogen could be restored. But when the 

 ratio of re-evolved to absorbed hydrogen fell, there was no 

 rise in the quantity that could 'be restored by action on 

 sodium. The fall is not due to the production of a larger 

 proportion of water ; the hydrogen disappears in a way that 

 cannot be reversed by baking. 



26. As against all these facts tending to show that chemical 

 action is not the cause of the absorption of hydrogen in the 

 presence of phosphorus, we can only set one which supports 

 that view directly. If the lamp in fig. 1 is filled with phospho- 

 rus vapour without other gas, the discharge causes the walls to 

 become yellow. If an amount of gas is present which is 

 near the limit that can be absorbed, then the yellow coloration 

 does not appear. Further, if the walls are first made yellow 

 by the discharge in pure phosphorus and then gas admitted 

 and absorbed, the yellow colour vanishes. These facts might 

 indicate that a colourless compound of phosphorus and the gas 

 is formed. But the general nature of our results leads us rather 

 to believe that the admixture of eas with the verv thin layer 

 of phosphorus on the walls * changes its optical properties. 



* How thick is the thinnest layer which will give a yellow colour, 

 it is impossible to say without a better knowledge of the cross-section of 

 a phosphorus molecule. It may be only one molecule thick, but it may 

 be 2 or 3 thick — the latter alternative fits in best with the views 

 suggested in the following section. 



3 A 2 



