1914] Further Researches on Positive Rays 263 



WEEKLY EVENING MEETING, 



Friday, April 3, 1914= 



The Right Ho\. Lord Rayleigh, O.M. P.O. D.C.L. LL.D. 

 D.Sc. F.R.S., in the Chp-ir. 



Professor Sir J. J. Thomson, O.M. LL.D. D.Sc. Pres.R.S. 

 M.R.I. , Professor of Natural Philosophy, R.I. 



Further Researches on Positive Rays. 



As I have on several occasions described the apparatus used to apply 

 positive rays to chemical analysis, it will not be necessary this 

 evening to enter more fully into the details of the method tlian to 

 say that it consists : — • 



1. In electrifying ihe gases to be analysed by passing an electric 

 discharge through them vvhen at a low pressure. 



2. A fine stream of positively electrified particles is obtained by 

 inserting a long and very fine tube in the cathode, 



3. The different kinds or particles forming this stream are 

 separated by exposing them to strong electric and magnetic forces, 

 arranged so that the deflection due to the magnetic force is at right 

 angles to that due to the electric. The lighter particles are more 

 deflected than the heavie:- one, and the original stream is split up into 

 as many secondary streams as there are different kinds of particles. 



These particles when they strike against a photographic plate 

 leave their record, and can thus be photographed. The trace left by 

 any particular kind of particle on the plate is the arc of a parabola, 

 and arcs of the different parabolas on the plate correspond to different 

 kinds of particles. [Examples of these photographs were shown ; 

 one of these is given in Fig. 1.] 



I will begin this evening by referring to some questions of 

 technique. A great deal depends, as might be expected, on the 

 character and qualities of the photographic plates used in these 

 investigations. The effects these plates have to record are not due 

 to light, but to the impact of particles against them ; these particles 

 cannot penetrate beyond a very small distance, so that the front of 

 the film is the only part which is brought into action. Thus, to get 

 the best effects the film should be exceedingly thin and contain the 

 maximum amount of silver. The old Daguerreotype (a silver 

 plate exposed to iodine vapour) vould satisfy these conditions, but 



