238 Sir J. J. Thomson : Further 



distance from the vertical of the head of the parabola, 

 showing that the particles in the first beading have fallen 

 through a potential difference just half of that fallen through 

 by the particles at the head of the parabola. This is not, 

 however, always the case ; if it were, we should have to look 

 for an explanation of this beading to some other cause than 

 that just given. If, for example, aggregates of two mole- 

 cules were formed possessing a single charge, and if, after 

 passing through the cathode, the two molecules separated, 

 one retaining the charge and the other going on uncharged, 

 then the charged molecule w T ould have half the energy due 

 to a fall of the atomic charge through the electric field in 

 the dark space, and would therefore be at the point often 

 occupied by the first beading. If this were the explanation 

 we should expect to find traces of the aggregates which had 

 not broken up on passing through the cathode : these would 

 be on a parabola corresponding to a value of e/m half that of 

 the molecule. I have never been able to find traces of such 

 parabolas. Again, it is not clear on this view why the 

 existence of this beading should be so dependent on the 

 shape of the cathode and its position in the discharge-tube. 

 For this reason I am inclined to regard the ratio of 2 to 1 

 which sometimes exists between the horizontal distances of 

 the first bead and the head of the parabola as accidental, 

 and to prefer the first explanation of this beading. 



Beading of the Mercury Line. — The beading on the mer- 

 cury line is especially w<ell marked since, as mercury is 

 monatomic, the atomic as well as the molecular headings 

 occur on this line. As the mercury atom occurs with three 

 charges, we get on the mercury line (m/e = 200) beads corre- 

 sponding to the atom which was highly charged when in the 

 dark space, but lost two of the charges after passing through 

 the cathode. As the kinetic energy acquired by the triply 

 charged atom in the dark space is three times that acquired 

 bv the singly charged atom, the head of the bead corre- 

 sponding to the triple charge will only be g of the distance 

 from the vertical of the head of the main portion of the 

 parabola. This would bring the mercury parabola much 

 nearer to the vertical and to the origin than the other 

 parabolas. Now it is a very remarkable feature of the mer- 

 cury parabola that it very often seems to reach right up to 

 the origin, as, for example, in fig. 14. Measurements show 

 that this is a parabola up to quite close to the origin : if this is 

 so, then some of the particles must have acquired an amount 



