150 Miss E. W. Hobbs on the Change in the 



-accompanying the sorption of hydrogen which involves a 

 marked change in the nature of the film ; and (3) a fall in 

 resistance due to the combination with evolution of heat 

 between the hydrogen and any traces of oxygen present in 

 the film under the catalytic action of the finely divided metal. 

 The relative values of these three effects may vary greatly 

 with the condition of the film and with its previous history. 

 Evidence of (2) is given in the ageing curves of films depo- 

 sited in hydrogen, shown as dotted curves in figs. 1 and 3. 

 It will be noticed that in the hydrogen curves, both for 

 platinum and palladium, the initial portions are much steeper 

 than those of the air curves, which often show a lag in the 

 ageing rate at the beginning (e. g. curve C, fig. 1). 



Still greater changes have been observed in the short 

 interval of time that elapsed before the pump was put on 

 •after the cessation of the discharge. For instance, in 

 curve H (fig. 1) the film was deposited in hydrogen at a 

 pressure of \5 to *6 mm., and was left for about 1 minute 

 at this pressure. During that time its resistance fell from 

 something greater than 30,000 to 9300 ohms. 



Striking examples of the effect wrought in a film by 

 hydrogen are given in the curves of fig. 3. They refer to 

 palladium films, aged in vacuo, to which dry hydrogen was 

 admitted, the point of admission being marked (h). In the 

 •case of curve Q, the film was previously supplied with air at 

 the point (a) and the pump was put on at the point (p). 

 Films P and Q were deposited in air ; films T, W, and Y in 

 hydrogen. In the films P and Q the initial rise in resistance 

 at (K) may be taken to represent the direct effect of the gas. 

 The subsequent rapid fall, which sometimes almost swamps 

 the initial rise, is attributable to the changes produced in the 

 iilm by the heat evolution which accompanies the large 

 sorptiou of hydrogen in palladium. Further complications 

 •may be introduced by the combination between hydrogen 

 and traces of oxygen present in the film. This effect in the 

 case of the curves T, W, and Y, deposited in hydrogen, will 

 he absent. Some hydrogen will have already been taken 

 up by the films, and the consequent evolution of heat will 

 have taken effect ; so that on admission of hydrogen at the 

 point (h) the ordinary rise in resistance due to the sorption 

 ■of gas alone is shown. 



In most instances, when hydrogen is admitted to a 

 palladium film, the percentage rise in resistance due to the 

 direct effect of the gas is much greater than that produced 

 by letting air into the discharge-vessel. This might be 



