346 Prof. J. J. Thomson on the Charge of 



present as well as short ones; but supposing that a violent 

 disturbance, lasting an indefinitely short time, leaves the 

 inner side of a Rontgen tube, where it is generated, it will 

 quickly become modified. Before it has traversed the glass 

 walls, the impulse has already spread out owing to the absorp- 

 tion of some of the periods in the glass, and if this modified 

 impulse is further sent through a screen like a thin sheet of 

 aluminium or black paper, all the visible rays are cut off, and 

 the invisible ones down to at any rate very short lengths. 

 The Rontgen radiation that affects a fluorescent screen or a 

 photographic plate therefore can only contain waves which 

 are either exceedingly short or very long, provided it con- 

 sists of transverse waves at all. 



That an impulsive motion of ?ether cannot possibly remain an 

 impulsive motion, after traversing media with selective absorp- 

 tion, or media in which dispersion takes place, should be clearly 

 understood. I may, perhaps, in connexion with this, quote 

 a sentence from a letter I wrote to ' Nature ' a few weeks 

 after Rontgen' s discovery was first announced. At that time 

 there was no theory put forward, except the original one of 

 Rontgen, that the rays were due to longitudinal vibrations. 

 The absence of interference was considered to be an objection 

 to any undulatory theory. 



" The absence of interference would not, however, be 

 sufficient to show that the radiation is not of the nature of 

 ordinary light, but only that it does not possess sufficient 

 regularity, or, in other words, that the disturbance is not 

 sufficiently homogeneous. That this is the case is not at all 

 impossible, for the radiation is produced by an impact which, 

 in the first instance, may be an impulsive motion propagated 

 outwards, and, after passing through the screen, would only 

 possess such regularity as is impressed upon it by the absorp- 

 tion of the longer waves "■*. 



1 think that the theory of impulses, as well as its limita- 

 tions, is clearly expressed in this passage. 



XXXIII. On the Charge of Electricity carried by a Gaseous 

 Ion. By J. J. Thomson, Cavendish Professor of Experi- 

 mental Physics, Cambridge^. 



IX the Philosophical Magazine for December 1898, I 

 gave a determination of the electric charge carried 

 by an ion in a conducting gas. The method employed in 

 this investigation was as follows : — If n is the number of 

 charged positive and negative ions per unit volume of the 

 * ' Nature.' vol. liii. p. 208. 

 i Communicated by the Author. 



