542 Prof. A. S. Mackenzie on the Deflexion of 



possible in this measurement, and it will be seen from the 



7YIV 



above table that the values obtained for — - agree to this 



e 



approximation for the very different arrangements used. In 

 figure 2 (PL VIII.) is given a copy of the negative used in 

 measurement IT. of the table, where the dispersion of the 

 beam is about 3/8 of the mean deviation. 



The agreement of the values obtained for — points to the 



conclusion that the ^particles, in a very rare gas of this kind 

 at least, retain their charges unaltered along their path (that 

 is, assuming that their velocities and masses remain unchanged, 

 which is probable at low pressures). As one can mark by the 

 use of a magnifying-glass the position of the scintillations, it 

 is further proved that these scintillations are a ways deflected 

 by the field, and that they are caused only by the a rays and 

 cannot be due to uncharged particles, if such exist. The 

 fact that with this apparatus one can make eye measurements 

 is of great assistance in roughly checking up one's results 

 and in arranging the conditions for the particular determina- 

 tion in hand. 



mv 



The mean value of the average — found was 3 "00 x 10 5 . 



e 

 on v 

 The values of the extremes of — as given by fig. 2 (PL VIII.) 



are 2*5 x 10 5 and 3*7 x 10 5 approximately. The value 



of — for radium in radioactive equilibrium found by 



Rutherford in his earlier experiment was 3'9 X 10 5 , and by 

 Des Coudres 2*56 X 10 5 . The value I have found is between 

 the two, and not far from that of Des Coudres. In his paper 

 in this Magazine for July, Rutherford shows the amount of 

 decrease in velocity of the a rays due to their passage through 

 successive layers of aluminium '00031 cm . thick. The decrease 

 in the present experiments due to the thin plate of mica 

 through which the a. rays have to pass before entering the 

 vacuum box would be equal to that due to two or three of 

 Rutherford's aluminium sheets, and so amount to about 



071 



6 per cent. Assuming that — is constant this would make 



the value of the average — for the rays as they left the 



surface of the radium 3' 18 x 10 5 , and of the extreme ones 

 2*65 Xl0 5 and 3*92 xlO 5 . Since the fastest rays are those 

 from radium C the value 3*92 x 10 5 must belong to them, 



