582 
MESSRS. W. SPOTTISWOODE AND J. FLETCHER MOULTON 
ordinary atmospheric pressures, the analogy becomes much more strict when we come 
to discharges in rarefied gas. It has long been known that if small metallic particles 
are lying loose on the negative terminal of an exhausted tube, a strong electric current 
will drive them along the tube towards the positive terminal. A convenient form of 
the experiment is obtained by enclosing some platinum black in the tube. If this be 
shaken down to the negative end of the tube, so as to lie upon the terminal, a shock 
from a coil of fairly large size will drive it along the tube in spite of the great specific 
gravity of the particles of which it is composed. No such phenomenon will be seen if 
the platinum black be placed on the positive terminal.* And all who have used 
vacuum tubes with platinum terminals will remember how commonly it is the case 
that the portion of the tube round the negative terminal becomes coated with a thin 
film of platinum due to the small particles of the metal that are driven off from the 
terminal by the electric discharge. 
In the opinion of the authors of this paper, there are no sufficient grounds for 
looking upon the molecular streams which produce phosphorescence in vacuum tubes 
as anything other than or different from the phenomena above referred to.t 
The hypothesis that the existence of the molecular streams that produce phos¬ 
phorescence depends upon some special modification of the gaseous structure of the 
medium through which the discharge passes, apparently owes its origin primarily to 
the belief that this phosphorescence is peculiar to tubes of high exhaust, and 
secondarily to the belief that in the tubes in which it is found to occur the particles 
which cause it (and which are presumably molecules of the gas within the tube) are 
exempted from the usual interference which gas molecules exercise upon one another 
in their motions through the space which contains them. Both these beliefs we 
consider to be unsupported. So far as our observation goes, phosphorescence can be 
produced in almost any vacuum tube. The sole condition is that the violence of 
the discharge from the negative terminal should be sufficiently great, taking into 
consideration the form and size of the tube, and of its negative terminal and its 
degree of exhaust. This degree of violence is attained in the case of tubes of very 
high exhaust without any special arrangements, and a continuous current sufficiently 
* Plumbago, lampblack, and finely divided steel have been used with success in this experiment. 
Lycopodium and sand, and apparently non-conductors in general, are not similarly affected. These 
experiments were suggested some years ago by Mr. Wakd. 
t A very remai'kable confirmation of the theory that these molecular streams are identical in their 
nature with the phenomena above described is obtained from the fact that, as Pluckee has shown, the 
metallic deposit in the neighbourhood of the negative terminal will follow the magnetic curves if the 
deposit be allowed to take place in a magnetic field, thus showing that the particles of platinum are 
affected by a magnet in precisely the same way as the particles in these molecular streams. Other experi¬ 
mental facts which in the opinion of the authors of this paper conclusively demonstrate the substantial 
identity of the two phenomena will be given in a subsequent portion of this paper (see page 648), but it 
is not convenient to insert them here as it would require us to anticipate in some measure the results of 
several of the sections that follow. 
