THOMSON'S METHOD OF CHEMICAL ANALYSIS 57 



there may be no possible doubt as to the identity of the H 

 and H 2 parabolas, the absolute value of tn/e for these lines 

 has been determined and found to correspond to the values 

 obtained by electrolytic and other methods for the hydrogen 

 atom and molecule. 



To return to the diagram (fig. 5), since OX is an imaginary 

 line and has no existence on the photograph, in order that 

 measurements may be made with greater convenience and 

 accuracy the magnetic field is reversed during the second half 

 of the exposure when — in the case we are considering— two 

 new parabolas will appear at r r 1 , s s 1 , due to m and m 1 respec- 

 tively ; the masses can be compared by the equation : 



m 1 flr 3 



m ~ qrs 3 



p, q, r, s being any straight line cutting the curves approxi- 

 mately parallel to the magnetic axis. The measurements of 

 these lines is independent of zero determination and if the 

 curves are sharp can be carried out with considerable accuracy. 



It has been shown that the electrical displacement is 

 inversely proportional to the kinetic energy of the particle. 

 Since this kinetic energy is simply dependent on and propor- 

 tional to the electrical potential through which the charged 

 particle fell before it reached the cathode and not upon its 

 mass, in general there will be a definite maximum kinetic 

 energy corresponding to the whole potential drop across the 

 Crookes' Dark Space in the discharge tube, with a correspond- 

 ing minimum displacement on the plate ; so that the parabolas 

 will end fairly sharply at points p, q, etc., equidistant from the 

 magnetic axis O Y. From the same reasoning it follows that, 

 the farther the parabola extends away from this limiting tip, 

 the larger must be the range of voltage through which the 

 particles forming it have fallen. 



Such true parabolic curves as we have considered are 

 caused by positive rays which have retained their charge 

 unaltered throughout both the electric and magnetic field and 

 are termed Primary Positive Rays. Unfortunately a simple in- 

 terpretation of these is impossible, as the pressure in the 

 camera is seldom and in the canal-ray tube never entirely negli- 

 gible, so that owing to the intense ionising effect of the rays 

 on the small amount of gas present in these localities free cor- 



