ON THE SENSITIVE STATE OF VACUUM DISCHARGES. 
651 
In the next place, it seems abundantly probable that they are quite left behind by the 
negative discharge when it is in the tube. The time they take to pass along the tube 
is of the same order of smallness as the time that is occupied by the emission negative 
discharge that produces them. But the time that negative electricity takes to pass 
along the tube is, as we have seen, of a higher order of smallness than this, for when 
the intermittence is negative the discharge can pass along the tube in time to stop the 
production of phosphorescence at a piece of tinfoil near the positive terminal, which is 
connected with a piece near the negative terminal. Thus it appears that it out-runs 
these molecular streams, and cannot depend on them for its propagation. 
Further, we now know that a discharge may be effected by positive passing through 
the tube and deriving its satisfaction by a response from the negative terminal. Or it 
may be effected by the negative passing throughout the tube and meeting with a 
response at the positive terminal. Now, on the supposition that these molecular 
streams are the carriers of the discharge, or that they have any special function to 
perform in its propagation, it is very difficult to understand the first of these modes 
of effecting the discharge. Moreover, it is admitted that there is not the slightest 
necessity that any of these molecular streams should strike or even pass near the 
positive terminal, so that the latter of the two modes of effecting the discharge seems 
equally incomprehensible on the above theory. 
The most attractive hypothesis relating to their functions is that they officiate at 
the birth of the discharge and enable it to get into the gaseous medium, where it has 
modes of propagation which are independent of these molecular streams. 
There is much to recommend this theory which views molecular streams as a necessary 
attendant phenomenon of negative discharge, but having no share in its propagation. 
We are not in a position to pronounce upon this hypothesis. The fact that resistance 
rises higher with increased degree of exhaust, after a certain point is passed, seems to 
favour it, as this law would then admit of the simple explanation that the resistance was 
greater because of the lack of carriers to carry the electricity into the gaseous medium. 
But this increase of resistance may come from other causes, and this single considera¬ 
tion does not seem to be sufficient ground for assigning to the molecular streams such 
a special function in the absence of other evidence that they possess it. 
On the whole, then, we are inclined to doubt whether molecular streams have any 
necessary function in the discharge. This does not, of course, imply that the molecules 
that compose them are not charged. On the contrary, it seems very probable that 
they are, as it would otherwise be difficult to account for their being shot off at so great 
a velocity or for their obeying a magnet. But the fact that in this way some small 
portion of the negative discharge is convectively carried along the tube would no more 
entitle them to be looked upon as having a function to perform in the discharge than 
it would entitle the particles of lamp-black to be looked upon in that light. 
