254 Prof. A. P. Chattock and Mr. A. M. Tyndall on the 



fall. This also implies a falling field ; and it thus appears 

 that at a distance from their origin of less than a centimetre 

 the N ions possess the power of ionizing air in fields which 

 steadily decrease as this distance decreases. 



This is precisely what we should expect if the N ions take 

 time to grow to their full size, and it is interesting to 

 consider it in connexion with other facts bearing on the 

 growth of ions. 



Franck * has shown that when discharge occurs in air 

 from the sides of a fine wire in a strong field, it is extremely 

 probable that the ions, whether positive or negative, do not 

 reach their full size while travelling a distance of 7 mm. 

 We t have shown, by a different method of experimenting, 

 that when discharge occurs from a fine point, both the 

 positive and the negative ions probably travel about 3 mm. 

 before they are fully formed — >a result which is consistent 

 with Franck's, when it is remembered that the average 

 field in the 3 mm. was probably lower than in Franck's 

 7 mm., and the ions consequently travelled slower. With 

 the relatively blunt point of the present experiments the 

 distance should be greater, and we now find that the distance 

 of growth in the case of the negative ions seems to have 

 increased to a centimetre or so. 



All these facts thus hang well together, as far as they go, 

 and so afford support to the view that the negative ions do 

 really take time to grow after leaving N. 



The lower limit to the size of a negative ion is the 

 corpuscle. If the second drop in the field curve is really 

 due to a growth of the ions, the curve ought either to become 

 horizontal again when the still lower field is reached in 

 which corpuscles can ionize the air, or else to cut the 

 vertical axis at this field. We have made a number of 

 experiments on the starting of the glow in this critical 

 region, the mean of the results being given in Curve V. It 

 was found impossible to bring N nearer to P than 0*14 cm. 

 on account of sparking, but down to this distance P could 

 be made to glow in what seemed to be the normal manner. 

 The numbers obtained were rather irregular, and it was 

 only by making many observations that we were able to 

 obtain so smooth a curve. We do not therefore wish to 

 press conclusions drawn from them until we have studied 

 this part of the field more carefully. As the curve stands, 

 however, it certainly does show a tendency to cut the vertical 

 axis at a field of about 75 E.S. units. 



* Franck, Ann. der Physik, Vierte Folge, Bd. xxi. p. 984. 

 f. Chattock & Tyndall, Phil. Mag. [6] vol. xix. p. 449 (1910). 



