Discharge from an Electrified Point. 599 



electric wind was sometimes even reversed. It is, therefore, 

 impossible to say what the actuil velocity of the negative ions 

 was, but in the light of Franck's work it is almost certain to 

 have been high. It was found that the wind pressure rapidly 

 rose with the addition of slight traces of air, that is to say, 

 the calculated velocity of the negative ions rapidly fell, as 

 was found by Franck in argon and nitrogen. On Franck's 

 view this is due to the tendency possessed by the negative 

 ions, when oxygen is present, to attach themselves to mole- 

 cules and become clustered; in oxygen-free inert gases they 

 remain for a longer time in a corpuscular state. 



This is supported by the results obtained for z in pure 

 and impure hydrogen. In pure hydrogen for negative dis- 

 charge, z was about 4 centimetres, but with the a idition of 

 2 per cent, of oxygen it fell to about 0'3 cm. If it is assumed 

 that 4 centimetres is the distance that negative ions travel in 

 pure hydrogen before clustering, one would expect a rapid 

 decrease in this distance when small percentages of oxygen 

 are added ; the negati/e ion in pure hydrogen would thus 

 cluster within a distance depending on the amount of oxygen 

 present. The growing ion view as an explanation of z may, 

 therefore, be retained for the particular case of hydrogen, 

 either pure or containing not more than a few traces of 

 oxygen. 



It is not so easy to explain the form of the p-z curves for 



different percentages of oxygen. If for negative discharge 



in hydrogen, curves of the type of curve 5 are taken, it is 



found that, in general, whatever the percentage of oxygen 



present the part AB is straight but that its slope for a given 



current rapidly increases with the addition of oxygen. Thus 



at "015 per cent, oxygen z w r as found to be 2*4 centimetres, 



but the slope of the curve was constant and about l/6th of 



that in ordinary impure hydrogen. The fact that at a given 



d d 

 percentage -^ is constant, implies that outside the distance 



z Q no further clustering takes place, and yet the ions appear 

 to move much faster than in impure hydrogen. At the 

 time the facts were explained by the theory, that in pure 

 hydrogen there was a considerable amount of back discharge 

 from the positive plate, which gradually ceased with the 

 addition of oxygen. Now the slightest reversal of the electric 

 wind in pure hydrogen shows that back discharge was un- 

 doubtedly present ; but if the negative ions are corpuscular 

 in nature they will not contribute any appreciable wind, and 

 the amount of back discharge which is necessary to give the 

 observed reversal need, therefore, onlv be very slight. 



