Pays of Positive Electricity. 571 



of e/m obtained with hydrogen was the same as in air, 

 1*2 x 10 4 , the velocity was 1*8 xlO 8 cm./sec. The presence 



Fig. 7. Fig. 8. 



.;oJ 



Fiir. 





of the second band indicates that mixed with these we have 

 another set of carriers, for which the maximum value e/m is 

 half that in the other band, i. e. 5 x 10 3 . The curvature of 

 the boundary generally observed is due to the admixture of 

 these two rays. 



Positive Rays in Helium. 

 In helium the phosphorescence is bright and the deflected 

 patch has in general the curved outline observed in hydrogen. 

 I was fortunate enough, however, to find a stage in which 

 q the deflected patch was split up into two distinct 

 bands, as shown in fig. 9. The maximum value of 

 e/m in the band a was 1*2 x 10 4 . the same as in air 

 and hydrogen, and the velocity was 1*8 x 10 s ; while 

 the maximum value of e/m in band b was almost 

 exactly one quarter of that in a (i. e. 2*9 x 10 3 ) . As 

 the atomic weight of helium is four times that 

 hydrogen, this result indicates that the carriers which 

 produce the baud b are atoms of helium. This result 

 is interesting because it is the only case (apart from 

 hydrogen) in which I have found values of e/m corresponding 

 to the atomic weight of the gas ; and even in the case of 

 helium, when the pressure in the discharge-tube is very low 

 and the electric field very intense, the characteristic rays 

 with e/7n — 2'9 x 10 3 sometimes disappear and, as in all the 

 gases I have tried, we get two sets of rays, for one set of 

 which e/m = 10 4 and for the other 5 X 10 3 . 



Although the helium had been carefully purified from 

 hydrogen, the band a (for which e/m — lO 4 ") was generally 

 the brighter of the two. The case of helium is an interesting 

 one: for the class of positive rays, known as the a rays, which 

 are given off by radioactive substance?, would a priori seem 

 to consist most probably of helium, since helium is one of the 

 products of disintegration of these substances. The value of 

 e/m for these substances is 5 X 10 3 , where we have seen that 

 in helium it is possible to obtain rays for which e/m = 2' ( .) x 10 3 . 

 It is true that, at very low pressures 



and with strong electric 



