along positive ray parabolas of atoms and molecules of hydrogen 319 



in a neutral atom striking the screen at the central undeflected 

 spot while the capture of two will cause the faint negative 

 parabolic streak a^ as has already been described (I.e., p. 39). 



But besides these forms of collision by which the velocity of 

 the atom is practically unaffected there is distinct evidence that 

 it may collide with and capture another hydrogen atom. If the 

 atom struck is negatively charged the resulting molecule will 

 strike the central spot but if it is neutral and the collision is 

 inelastic the resulting positive ray will have the same momentum 

 (the^ atom struck being relatively at rest) but double the mass so 

 that it will strike the molecular parabola at a point the same height 

 above the JT-axis as would the atom which generated it. Molecular 

 rays formed in this manner will therefore form the streak b^ 

 which, allowing for the geometrical difference in the curves will 

 show a similar distribution of intensity to a^. Collision with a 

 positively charged atom wiU obviously be unlikely to result in 

 capture and those with heavier atoms will be referred to later. 

 It is to be noted in connection with the brightness of these 

 secondary streaks a^ and 63' which may conveniently be called 

 'satellites' to distinguish them from the 'secondary lines' already 

 fully described {I.e., p. 32), that a^ is always very much fainter than 

 its primary but b^ can be equally bright. 



This atomic type of discharge with its pendant bright arc on 

 the molecular parabola corresponding to similar momentum and 

 half normal energy is most beautifully illustrated in Fig. 29 of 

 Plate III already referred to. It was this photograph which 

 suggested the above theory of its explanation. 



Molecular type of discharge. 



The extreme form in which the whole discharge is carried up 

 to the cathode by ions of molecular mass is unattainable so far 

 in practice and is probably impossible but its share in the illumina- 

 tion of the screen can be deduced by eliminating the superimposed 

 atomic type and is indicated in Fig. 2, 



The principal feature is a short and very bright spot of light b^ 

 on the molecular parabola at the point corresponding in energy 

 to a fall through the full potential of the discharge. It will be 

 shown that all the ions causing this are probably generated in the 

 negative glow. Besides this there are two symmetrical and equally 

 bright positive and negative satellite patches ag ^^^ (^2 on the 

 atomic parabola but of half the normal energy. The proposed 

 explanation of these is somewhat similar to that considered by 

 Sir J. J. Thomson {I.e., p. 94) and is as follows. The collision with 

 and capture of a single negative electron by a positively charged 

 molecule will not necessarily merely neutralise it and cause it to 



VOL. XIX. PART VI. 22 



