ON ELECTRICAL DISCHARGES THROUGH RAREFIED GASES. 
225 
discharge. There may be many discharges from each terminal into the tube in the 
space of time that it takes the discharge to pass through the tube, and the alterations 
of tension at any point outside the glass may be infinitesimal, so as to be wholly 
unequal to producing any relief-effect within. 
It may be objected that this theory supposes something like a discontinuity in the 
range of rapidity of pulsations in electrical discharge. Either they are of the 
comparatively slow period that gives the sensitive state, or of the indefinitely more 
rapid period that gives the non-sensitive state. This difficulty is, however, only 
parallel to the difficulty with which we are met when we see an air-spark interval 
enlarged to beyond the striking distance, when the discharge across it at once becomes 
silent, and the current that passes is found, when conducted through the tube, to have 
all the characteristics of a continuous discharge. No one can believe that the discharge 
across the interval has suddenly become continuous—the real nature of the change 
must be that the intermittent pulses (whether carried by convection on particles of air 
or not) have become so numerous that the average of the discharge, when taken over 
an extremely short space of time, is constant. And the appositeness of the comparison 
is shown by the fact that the discharge through the tube is non-sensitive.* 
We have seen that the discharges in the sensitive discharge are sudden and of 
sufficient quantity to affect to a considerable extent the electric tension in the space 
around. This suddenness is an essential. And it is to this that we must attribute 
the indifference of the discharge to the potential of the body affecting it and the 
dependence of the effects on the change of electric state alone. If an electrified body 
be surrounded by air, it must of course form an electric field around it, which will or 
will not alter with time according as the air can or can not get at the body and convey 
away its free electricity. But when this electric field has been formed, it is affected 
just as easily by a new electric force brought suddenly into the field as though 
the original body were not there. The change is to all intents and purposes the same. 
We do not know what material changes accompany the formation of such an electric 
field, but for the sake of argument assume that there is a certain driving off of gaseous 
particles until equilibrium is attained. Then the new force suddenly brought in will 
send away a like stream of particles whether or not the field on which it operates has 
already been subjected to a similar operation. 
The true analogue to these effects is to be found in the consideration of impulsive 
and continuous forces in dynamics. It is a well-known principle that in calculating 
the effect of impulsive forces all continuous forces, whether they be of the nature 
of pressures, strains, external attractions, or otherwise, may be neglected. And 
the same is true of the changes of electric tension with which we are dealing in 
* In this case, as in the other, there is probably no discontinuous change in the character of the dis¬ 
charge. No doubt much of the discharge is carried in the silent form as we approach striking distance, 
and the apparent discontinuity is due to the fact that the increase in the proportion so carried is very 
sudden as we get near that point. 
2 G 
MDCCCLXX1X. 
