ON THE SENSITIVE STATE OP VACUUM DISCHARGES. 
591 
But there is another point of view from which the results given above are important. 
They show that no conclusions can be drawn from the length of the path of these 
molecular streams as to the average free path of the molecules of the gas or the 
frequency of collisions between them. We know enough of gases to be certain that at 
a pressure of a quarter of an inch of mercury the ordinary laws of gases are in full 
force; that the average free path of the gaseous molecules is infinitesimal; and the 
number of collisions between them in any finite time inconceivably great. And yet 
at that pressure we can get phosphorescence at a distance of at least two inches. 
Although, therefore, we can no longer regard phosphorescence as so exceptional a 
phenomenon as has been generally supposed, we are far from intending to underrate 
its importance as a characteristic phenomenon of electric discharge. But this impor¬ 
tance is due to the fact that it becomes more and more prominent as the degree of 
exhaust increases, and not to its specially appertaining to any type of exhaust. And 
this increase of importance is greatly enhanced by the consideration that the other 
characteristics of the discharge, such as positive luminosity and the like, become 
gradually less and less marked as the degree of exhaust increases, till at length 
almost the sole visible phenomenon of the discharge is the phosphorescence* in the 
tube caused by the streams of molecules which its passage excites. And in one 
respect the indications given to us by phosphorescence are more definite than those of 
any other of the luminous phenomena, because it always speaks to the existence of a 
negative discharge ; and if it is possible, by the method of shadows or otherwise, to 
determine the direction of the streams of molecules, it tells us with considerable 
accuracy the position of the source of that discharge. And this renders it, as we 
shall presently see, of the greatest value in researches which have for their object the 
discovery of the mechanism of the discharge, and indeed constitutes it the main source 
from which we derive information in the matter. 
must retain its normal state of intermolecular motion. Tins gas must be inextricably mixed up with that 
which has undergone the direct influence of the electrode, so that it is well nigh inconceivable on any 
hypothesis that there can be anything like order or directed motion in the molecular stream. And yet it 
is found to produce all the effects of a molecular stream produced by a more continuous discharge in 
a higher vacuum. 
From these considerations and from the entire absence of anything which points to the suppression of 
the lateral motions, we conclude that the molecular streams furnish no evidence that the gas of which they 
are composed is in any other than its ordinary state. [July, 1880.] 
* It is needless to repeat that the colour of this phosphorescence depends upon the substances used in 
the manufacture of the glass. The most convenient and easily distinguishable kind of phosphorescence 
is the green phosphorescence of German glass, and all the experiments for this paper have been performed 
with tubes of this glass. We shall therefore speak of phosphorescence as being green, although, as we 
have said, it is not necessarily so. 
4 G 2 
