iments on Positive Rays. 223 



deflexion, which might be sufficient to drive them off the plate 

 and prevent their detection. I have never been able to detect 

 any increase in the brightness of the negative oxygen line 

 when hydrogen was added to the gas ; indeed, the brightest 

 negative oxygen lines I ever observed were in an exceptionally 

 pure specimen of oxygen. It is certainly true that in some 

 compounds of oxygen the negative lines are much more pro- 

 nounced than in others. I am inclined to think that the 

 exceptional brightness of the lines is due to the presence of 

 the hydroxyl radicle,, and that the bright lines are due to 

 OH and not to 0. 



The theory which on the whole seems to me most in 

 accordance with the observations, is that the negatively 

 electrified particles are due to systems which are positively 

 charged when they are in the discharge-tube ; then, after 

 passing through the cathode, get neutralized by combining 

 with a corpuscle, and when in the neutral condition have so 

 strong an affinity for a negatively charged corpuscle that, in 

 spite of the high velocity with which they are moving past 

 it, they are able to capture a corpuscle and thus become 

 negatively electrified. This attraction for the negative 

 corpuscle depends on the chemical properties of the atom. 

 The atoms of some elements do not exert sufficient attraction 

 to enable them to become charged in this \a ay : for example, 

 I have never observed any indications of negativelv charged 

 helium, argon, nitrogen, or mercury, however strong the lines 

 corresponding to the positively charged atoms may have been. 

 On the other hand, the atoms of hydrogen, carbon, oxygen, 

 sulphur, chlorine are conspicuous for the readiness with which 

 under ordinary conditions they acquire a negative charge. 

 The case of oxygen is especially interesting. In very pure 

 oxygen, carefully dried by liquid air, the oxygen occurs on 

 the negative side solely in the atomic condition : an example 

 of this is shown in fig. 14, where the only line on the negative 

 side is one due to atomic oxygen. If, however, a small 

 quantity of hydrogen, say from 1 to 3 per cent, by volume, 

 is added to the oxygen, another line appears on the negative 

 side with a value of m/e about 33 times that for the 

 hydrogen atom ; an example of this line is shown in 

 fig. 15 (Pi. IX.). 



It is remarkable that this line disappears again when the 

 quantity of hydrogen is increased, with 20 per cent, hydrogen 

 it is no longer visible. I have a series of about 40 photo- 

 graphs of the discharge through mixtures of hydrogen and 

 oxygen in varying proportions ; in all of these, where the 



