648 
MR. W. CROOKES OK MOLECULAR PHYSICS IN HIGH VACUA. 
suffered any deflection in passing an idle pole when it was suddenly uninsulated by an 
earth contact. For this purpose I used the tube described in a former paper,'" where 
the shadow of an aluminium star was projected on a plate of phosphorescent glass. 
So long as the aluminium star is insulated the shadow is sharp, as already described ; 
but on touching the star to earth, the shadow widens out forming a tolerably well- 
defined penumbra outside the original shadow, which can still be seen unchanged in 
size and intensity. On removing the earth connexion, the penumbra disappears, the 
umbra remaining as before. The same penumbra is produced by connecting the idle 
pole with the negative pole through a very high resistance, such as a piece of wet 
string, instead of connecting it with earth. On bringing a magnet near the negative 
pole, the shadow of the (insulated) star is much increased in definition, the adjacent 
luminous parts of the screen becoming more luminous. Touching the star now brings 
a large, somewhat blurred, penumbra round the original image. The penumbra obeys 
the magnet the same as the umbra. 
603. The aluminium star was now made the positive pole, the other pole remaining 
unchanged. The shadow of the star was projected on the phosphorescent plate of the 
same sharpness and almost the same intensity of light and shade as if the positive pole 
had been the one ordinarily used as such. The image obeyed the magnet as usual. 
With this arrangement the penumbra! action could not be tested. 
604. This, therefore, confirms the above-described results—that the idle pole, the 
shadow of which is cast by the negative pole, has strong positive charge. Now the 
stream of molecules must be assumed to carry negative electricity; when they actually 
strike the idle pole they are arrested, but those which graze the edge are attracted 
inwards by the positive electricity, and form the shadow. When the idle pole is con¬ 
nected with earth, its potential would ‘become zero were the discharge to cease; but, 
inasmuch as a constant positive charge is kept up from the passage of the current 
through the tube, we must assume that the potential of the uninsulated idle pole 
is still sufficiently positive to neutralise the negative charge which the impinging 
molecules would give it, and leave some surplus of positive. The effect of alternately 
uninsulating and insulating the idle pole is therefore to vary its positive electricity 
between considerable limits, and consequently its attractive action on the molecules 
which graze its edged 
Let a (fig. 9) be the negative pole, b the idle pole, whose shadow falls at c; and let 
b be at first supposed to be insulated. Molecular rays impelled from a in a slightly 
divergent direction,! on passing the positively electrified idle pole b, are rendered 
much less divergent, and bending inwards take the directions of the lines a d d', 
* Phil. Trans., 1879, Vol. 170, p. 147. 
f I am aware that the theory which makes these effects of deflection depend on electrostatic attractions 
and repulsions is open to some grave objections; still it was that which in a great measure guided me in 
my experiments, and it could not well he omitted without reducing the description of them to a dry record 
of apparently unconnected facts. 
1 The divergence in the figure is purposely exaggerated in order to better illustrate the argument. 
