146 Professor Sir J. J. Thomson [April 7, 



is as great as that corresponding to the positive, whereas when the 

 discharge is passing with great difficulty, and the velocity of the 

 neutral particles is very high, the negative part is very faint compared 

 with the positive. 



The particles which have been observed on the negative side are 

 the hydrogen atom, the carbon atom, the oxygen atom, and the 

 chlorine atom. The presence of the oxygen and chlorine atoms might, 

 perhaps, have been expected, as these are universally regarded as 

 strong electro-negative elements, i.e. as elements which have a strong 

 affinity for negative electricity. The presence of the hydrogen atom 

 is more remarkable, for hydrogen is generally considered to be a 

 strongly electro-positive element, yet on these photographs we find 

 it more persistently on the negative side than any other particle : 

 often when no other line on the negative side is strong enough to be 

 detected, the line corresponding to the hydrogen atom is distinctly 

 visible. This is all the more remarkable, because the hydrogen atom, 

 being the lightest of all the particles, is moving with the greatest 

 velocity relatively to the corpuscles, and therefore would, other cir- 

 cumstances being the same, be the least likely to capture them. The 

 heavier the particle, the slower is its velocity, and the greater chance 

 it has of capturing the corpuscles ; the fact that heavy complicated 

 particles are conspicuous by their absence on the negative side shows 

 that the attraction of these for the corpuscles must be exceedingly 

 small compared with that exerted by a neutral atom of hydrogen. 

 It will be seen that the atom of carbon, also regarded as an electro- 

 positive element, is also conspicuous on the negative side. 



On looking at the list of the particles which occur on the negative 

 side, we are struck by the fact that they are all atoms : there is not a 

 molecule among them. Thus, although the curve corresponding to 

 the negatively electrified hydrogen atom occurs on every plate, there 

 is not a single plate which shows a trace of a curve corresponding to 

 a negatively electrified hydrogen molecule, although that corresponding 

 to the positively electrified molecule is always present, and on some 

 of the plates is stronger than that due to the positive hydrogen atom. 

 Again, on some plates the positive oxygen molecule shows stronger 

 than the oxygen atom, but on the negative side only the atom is 

 visible. 



Thus neutral atoms, but not neutral molecules, can exert on the 

 negative corpuscles those enormous attractions which, under the con- 

 ditions of these experiments, are required to bind the corpuscles to 

 these rapidly moving particles. We may compare this result with the 

 properties ascribed by chemists to bodies when in the nascent con- 

 dition, i.e. when they have only recently been liberated from chemical 

 combination, and when they are likely to be partly in the atomic 

 state ; for atoms, as we have seen, exert forces on electric charges in 

 their neighbourhood vastly greater than those exerted by molecules. 



We may compare the forces exerted by a neutral atom on the 



