±96 Mr. A. B. Meservey on the Potentials required 



above the critical pressure are the direct opposite of what 

 would result from the expulsion of gas. In the second 

 method '(Table II.), so far as expansion is concerned, the gas 

 should be in exactly the same condition at all three times. 

 The valves are closed before the current is started, and are 

 kept closed, so that no gas can escape ; and after the current 

 has stopped, and sufficient time elapsed to allow the tempera- 

 ture to regain its original value, the gas should be exactly 

 the same, with regard to temperature, pressure, and actual 

 amount present, as it is at the start. And since it makes no 

 difference at the start whether the sparking potential is found 

 before or after the valves are closed, so at the end it should 

 make no difference whether it is found before or after the 

 valves are opened. That is, all the values should be the 

 same, so far as any expansion of the gas originally between 

 the plates is concerned, and the value of R should be zero. 

 But in Table II., which gives the results of this method, R 

 is never equal to zero. Table II., therefore, like Table I., 

 shows plainly that the effects obtained in these experiments 

 are due to some other cause than the expulsion of gas from 

 between the electrodes. 



In considering what may be the real cause of the dif- 

 ferences in sparking potentials observed in these experiments, 

 the first thing to be noticed is that, whatever its nature, it is 

 confined to the gas. If the valves are left open during and 

 after the passage of the current, a proceeding which leaves 

 the gas free to circulate but which cannot have any effect 

 upon the electrodes, running the current does not affect the 

 sparking potential. This fact, and the fact that as soon as 

 the gas is allowed to pass, by the opening of the valves, in ihe 

 methods used in obtaining Tables I. and II., the potential 

 drops to practically its original value, show that the cause of 

 the effect is to be found in the gas. 



The first thing which suggests itself as a cause is the 

 driving out of gas from the electrodes themselves by the 

 passage of the current. The gas which is most frequently 

 met with in such cases is hydrogen, but the addition of a 

 little hydrogen to that already present does not suffice to 

 explain the results obtained. With the spark chamber closed, 

 an increase in the amount of hydrogen should increase the 

 sparking potential above the critical pressure, that is, the 

 value of R should be positive ; while below the critical pres- 

 sure, R should be negative, and numerically greater because 

 of the steepness of the sparking-potential curve in this region. 

 Table II., taken by itself, is more or less in accord with this 

 supposition, since the value of R is positive in each case 



