July 13, 1900.] 



SCIENCE. 



45 



ory that the rays did not really penetrate 

 Lenard's aluminium window, but that they 

 made of it a secondary kathode, which sent 

 out new rays of its own into the region be- 

 yond.* But the objections to this view are 

 numerous. For example, it is remarkable 

 that the secondary rays should be exactly 

 similar in their properties to the rays which 

 produced them, regardless of whether the 

 secondary kathode is thick or thin, a conduc- 

 tor such as aluminium, or an insulator such 

 as glass. Again, Lenard obtained these rays 

 both in air at ordinary pressures, and in a 

 vacuum so high that no discharge could be 

 made to pass. In neither case can kathode 

 rays be produced by any other known 

 method. Is it not strange that a secondary 

 kathode, forming part of a grounded metal 

 inclosure, should not only develop these 

 rays under conditions where all other 

 methods fail, but that it should also pro- 

 duce rays of the same kind and intensity 

 under such widely dififerent conditions? 

 These and other objections make it seem 

 highly unlikely that the Lenard rays can 

 be satisfactorily explained by treating the 

 aluminium window as a secondary kathode. 

 In fact, I think that this view has now been 

 very generally abandoned. But even if it 

 were accepted as correct, the difficulties in 

 the way of the Crookes theory still re- 

 mained. For if the kathode rays consisted 

 of charged atoms, as had been indicated by 

 the work of Schuster and J. J. Thomson, 

 the fact that they were able to pass through 

 air is scarcely less surprising than that they 

 should penetrate thin sheets of metal. f 



Lenard himself interpreted his results as 

 offering additional support to the ether 

 theory, and called attention to the fact that 

 in order to explain the observed phenom- 



* J. J. Thomson, ' Eecent Researches in Electricity 

 and Magnetism,' p. 126. 'Discharge of Electricity 

 through Gases,' p. 190. 



t See J. J. Thomson, ' Discharge of Electricity 

 through Gases,' p. 196. 



ena the wave-length must be small com- 

 pared with the dimensions of a molecule. 

 At the close of his first article in 1894 he 

 says, " Judging by the observed behavior of 

 the gases " (viz, diffusion and absorption 

 of the rays) " the ether phenomena that 

 constitute the kathode rays must be of such 

 extraordinary fineness that dimensions as 

 small as those of molecules have to be 

 taken into consideration. Even toward 

 light of the shortest known wave-length, 

 matter acts as though it were continuous. 

 But toward kathode rays, even the ele- 

 mentary gases behave like non-homoge- 

 neous media ; each individual molecule 

 seems to form an obstacle to their propaga- 

 tion. Analogous phenomena are observed 

 when ordinary light passes through a me- 

 dium made turbid by suspended particles." 

 When we consider the condition of the 

 subject at that time, Lenard's conclusion 

 that the rays must consist of something anal- 

 ogous to wave motion seems most natural. 

 From our present standpoint, however, it 

 is seen that his results might be equally 

 well explained by a modification of the 

 Crookes theory. The same difficulties that 

 are surmounted by the assumption of ex- 

 tremely short waves can also be removed 

 by the assumption of extremely small par- 

 ticles. If the kathode ray particles are only 

 small enough, they might pass for a con- 

 siderable distance through air, or even 

 through metal films ; upon colliding with 

 the molecules of a gas they would rebound 

 in all directions, and diffusion would re- 

 sult ; and both diffusion and absorption 

 would be roughly proportional to the den- 

 sity of the medium. But this requires that 

 particles of matter should exist which are 

 small as compared with atoms. The sug- 

 gestion is a startling one, and so violently 

 contradicts our ordinary views of the con- 

 stitution of matter that it cannot be ac- 

 cepted without strong support. It is not 

 surprising, therefore, that several years 



