140 Professor Sir J. J. Thomson [April 7, 



WEEKLY EVENING MEETING, 



Friday, April 7, 1911. 



His Grace the Duke of Northumberland, K.G. P.C. D.C.L. 

 LL.D. F.R.S., President, in the Chair. 



Professor Sir J. J. Thomson, M.A. LL.D. D.Sc. F.R.S. M.R.I. 



A Neiv MetJiod of Chemical Analysis. 



I have had on several occasions the privilege of bringing before the 

 Members of the Royal Institution some of the results of the ex- 

 periments on the positive rays on which I have been engaged for the 

 last few years. I wish this evening to draw your attention to some 

 applications of these to various chemical problems. 



The first application I shall consider is the use of these rays to 

 determine the nature of the gases present in a vacuum tube, to show 

 how they can be used to make a chemical analysis of these gases — an 

 analysis which, as we shall see, will enable us to determine not merely 

 whether an element, say, for example, oxygen, is present in the tube, 

 but will tell us in what form it occurs, whether, for example, it is 

 present in the atomic as well as the molecular condition, and whether 

 there are allotropic modifications present, such as ozone, O3, and other 

 still more complex aggregations. 



The method is as follows : the positive rays after passing through 

 a fine tube in the cathode are exposed simultaneously to magnetic 

 and electric forces, the magnetic field being arranged so as to produce 

 a vertical deflection of the rays, while the electric field produces a 

 horizontal deflection. Thus, if when neither electric nor magnetic 

 fields are present, the rays strike a screen placed at right angles to 

 their direction at a point 0, they will, when both electric and magnetic 

 forces are at work, strike it at a point P, where the length of the 

 vertical line P N is equal to the deflection produced by the magnetic 

 field, and the horizcnt-il line ON to that produced by the electric field. 



We know from the theory of the action of electric and magnetic 

 fields on moving electrified particles, that 



PN = A~ 0-N= B %, 



m V " mv 



where A and B are constants depending on the strength of the 

 magnetic and electric fields and the geometrical data of the tube, e is 

 the charge on the particle, m its mass, and v its velocity. 

 From these relations we see that 



m ^ Ar ON 

 ~e B PN- 



