18 REPORT — 1901. 



Again, according to accepted theory, when a current of electricity 

 flows through a gas some of the atoms are divided into parts which 

 carry positive and negative charges as they move in opposite directions, 

 and unless this breaking-up occurs a gas does not conduct electricity. 

 But a gas can be made a conductor merely by allowing the Rontgen rays 

 or the radiation given ofi" by uranium to fall upon it. A careful study of 

 the facts shows that it is probable that some of the atoms have been 

 broken up by the radiation, and that their oppositely electrified parts are 

 scattered among their unaltered fellows. Such a gas is said to be 

 ionised. 



Thus by these two distinct lines of argument we come to the conclu- 

 sions : — 1st, that the presence of electrified particles promotes the forma- 

 tion of mist, and 2nd, that in an ionised gas such electrified particles are 

 provided by the breaking-up of atoms. 



The two conclusions will mutually support each other if it can be 

 shown that a mist is easily formed in ionised air. This was tested by 

 Mr. Wilson, who showed that in such air mist is formed as though nuclei 

 were present, and thus in the cloud we have visible evidence of the 

 presence of the divided atoms. If then we cannot handle the indi- 

 vidual molecules we have at least some reason to believe that a method 

 is known of seizing individuals, or parts of individuals, which are in a 

 special state, and of wrapping other matter round them till each one is 

 the centre of a discrete particle of a visible fog. 



I have purposely chosen this illustration, because the explanation is 

 based on a theory — that of ionisation — which is at present subjected to 

 hostile criticism. It assumes that an electrical current is nothing more 

 than the movement of charges of electricity. But magnets placed near 

 to an electric current tend to set themselves at right angles to its direc- 

 tion ; a fact on which the construction of telegraphic instruments is based. 

 Hence if the theoi'y be true, a similar effect ought to be produced by a 

 moving charge of electricity. This experiment was tried many years ago 

 in the laboratory of Helmholtz by Rowland, who caused a charged disc 

 to spin rapidly near a magnet. The result was in accord with the theory ; 

 the magnet moved as though acted upon by an electric current. Of late, 

 however, M. Cremieu has investigated the matter afresh, and has obtained 

 results which, according to his interpretation, were inconsistent with that 

 of Rowland. 



M. Cremieu's results are already the subject of controversy,' and are, 

 I believe, likely to be discussed in the Section of Physics. This is not the 

 occasion to enter upon a critical discussion of the question at issue, and I 

 refer to it only to point out that though, if M. Cremieu's result were 

 upheld, our views as to electricity would have to be modified, the founda- 

 tions of the atomic theory would not be shaken. 



' See Phil. Mat/., July 1901, p. Hi; and Joh>is Hopkins University Circulars, 

 XX. No. 162, May- June 1901, p. 78. 



