264 Dr. J. W. Nicholson on the Number of 



shown fairly definitely that the negative ions emitted by hot 

 metals are electrons, and that positive ions emitted even by 

 alkaline sulphates and fluorides are atoms of the metallic base 

 which have lost electrons *. 



Finally, a few remarks must be made as to the significance 

 of K, and this may be most conveniently done by a com- 

 parison with the theory as set forth in Drude's treatise. 

 This theory divides the current into three portions, 

 (I) ^Ethereal polarization current, (2) Current carried by 

 positive ions, and (3) Current carried by negative ions. 

 These ions, positive or negative, may be of more than one 

 u class." Equations of motion of these classes are given in 

 a form which is unsatisfactory in its introduction of a viscous 

 frictional term. When there are only two classes, one of 

 negative electrons moving freely and one of positively charged 

 ions with a natural period of their own, Drude gives, in terms 

 of the constants (v. k) we have used, 



-•I ■& 47rN?ft 



v z —k-— l-l- 



1 — (t\/t) 2 &{f + m*\p 

 2tt^N 



where, if a group d~N of electrons can be held to constitute a 

 class, and if the frictional coefficient r is otherwise interpreted, 

 the value of vk and the last term of v 2 — k 2 become, on summa- 

 tion by Maxwell's law, identical with the values we have 

 used. 



Drude's 3 is a positive constant determining the force of 

 restitution of an ion when it vibrates under the action of the 

 electric force, t x is the natural period of the ion, and t that 

 of the light. Thus the sympathetic vibration of an atom 

 affects this term. If it is important the denominator is 

 affected, and vk is altered to a smaller extent in consequence 

 of the resulting absorption. This alteration is relatively 

 unimportant if ^ is not nearly equal to t. 



The current analysis above is not complete. Vibrations 

 will take place ordinarily within the atom, and they will 

 include the vibrations of doublets in which the positive and 

 negative charges approach and recede. If d is a component 

 of their distance apart, along any direction, a polarization 

 current %ed is formed in that direction, the summation being 

 for all doublets in a unit volume. The convection of such 

 polarization is also effective to some extent. It will not be 

 correct, in such a case, to suppose that in a non-metallic 



* Vide Phil. Map;. December 1910. 



