244 ANITUAL REPOET SMITHSONIAN INSTITUTION, 1910. 



valent ions in air, the number of such ions formed can not exceed 

 1 or 2 per cent of the number of univalent ions formed. At the 

 present time therefore it seems probable that, despite the contrary- 

 evidence presented by Townsend ^ and Franck and Westphal,- the 

 process of gaseous ionization by both radium and X rays always 

 consists in the detachment from a natural molecule of one single 

 elementary electrical charge. 



MECHANISM OF THE CHANGE OF CHARGE OF A DROP. 



It has been tacitly assumed thus far that the only way in which a 

 drop can change its charge is by the capture of ions of one sign 

 or the other from the air. When a negative charge increases there 

 seems to be no other conceivable way by which the change can be 

 produced. But when it decreases there is no a priori reason for 

 thinking that the change may not be due as well to the direct loss 

 of a j^ortion of the charge as to the neutralization of this same 

 amount by the capture of a charge of opposite sign. Table I shows 

 conclusively, however, that if direct losses occur at all they take place 

 with exceeding infrequency as compared with the frequency with 

 which ions are captured from the air, even when there is no ex- 

 ternal source of ionization whatever. For if there were two com- 

 parable processes tending to diminish the charge (viz, direct loss 

 and capture of opposite ions) and only one tending to increase it 

 (viz, capture of ions of the same sign) and that one of approxi- 

 mately the same efficiency as one of the first two, the drop, instead 

 of maintaining as it did in these experiments for three and one-half 

 hours after the radium was removed from the room, essentially 

 the same mean charge despite its repeated changes, would have 

 quickly lost its charge and gone to the lower plate. The fact that 

 it did not do this furnishes perhaps the most convincing evidence 

 which has yet been brought forward that the process of evaporation, 

 which must have been going on continuously at the surface of the 

 drop, does not have the power of removing at all an electrical charge 

 which resides upon an evaporating surface.^ 



There is but one more comment to be made upon Table I. At a 

 point indicated in the table by the remark " change forced with 

 radium," it will be noticed that the charge was suddenly changed 

 from eleven negative units to five negative units — i. e., that six posi- 

 tive units were forced upon the drop. This sort of a change was 

 one which, after the phenomenon had once been got under control, 

 we could make at will in either direction — i. e., we could force charges 



1 J. Townsend, Proc. Roy. Soc, 80, p. 207, 1908. 



« J. Frank u. Westphal, Verb. d. D. Phys. Ges., vol. 2, pp. 146 and 276, 1909. 



^ This question has been considerably discussed in the past and the experiments of 

 Henderson (Phil. Mag.. 50, p. 489, 1900) and at Schwalbe (Ann. de Phys., i, p. 295, 

 1900) strongly support the conclusions here reached, despite the opposite evidence brought 

 forward by Pellat (Jour, de Physique, 8, p. 225, 1899). 



