ON THE SENSITIVE STATE OF VACUUM DISCHARGES. 
583 
strong to pass through such tubes appears always to excite phosphorescence to a 
greater or less distance from the negative terminal. In tubes of a less degree of 
exhaust this is not the case, but the difficulty can be got over by intensifying the 
violence of the negative discharge by means of the introduction of an air-spark. 
This divides the current into isolated individual discharges of great violence ; and if 
the air-spark be taken long enough this process generally results in the production of 
phosphorescence in the neighbourhood of the negative terminal. If this does not 
succeed, yet more violent methods must be resorted to ; and if even all means fail to 
produce phosphorescence we trust to be able to show that it is solely on account of the 
interference of the surrounding gas ; and that the absence of phosphorescence is not 
due to the non-existence of the requisite molecular streams, but to their not travelling 
with sufficient velocity to enable them to impinge on the glass with the requisite 
violence. In other words, we hope to establish experimentally that these molecular 
streams are present in all vacuum discharges, and that their behaviour under various 
conditions of vacuum and discharge, so far from pointing to any unusual state of 
the gaseous medium in which they occur, shows a perfect continuity of variation 
throughout the whole of the wide range of circumstances under which they appear. 
It will be convenient in demonstrating these propositions to show first of all that 
phosphorescence can be produced in vacuum tubes, in which it would not otherwise 
occur, by increasing the violence of the discharge. By far the best example of this is 
obtained in the way to which we have just referred, viz.: by introducing an air-spark. 
If a tube of very moderate exhaust be placed in circuit with a large Holtz machine, 
and an air-spark of considerable size be introduced into any part of the circuit, it will 
generally be found that phosphorescence appears in the neighbourhood of the negative 
terminal, even though there was not the slightest appearance of it while the discharge 
was passing continuously. This is clearly due to the fact that intermittent discharges 
are necessarily much more violent during the very short period of time which they 
occupy than are continuous discharges ; and hence the velocity imparted to the mole¬ 
cular streams is sufficient to make them impinge on the glass with the velocity requisite 
to produce phosphorescence. It will be noticed that we have here no change in the 
degree of exhaustion, but only in the violence of the discharge; and it is further to be 
remarked that this* increased violence can be obtained, either directly as in the case 
of the negative air-spark, or by way of response to a violent positive discharge in 
the tube, as in the case of the positive air-spark.'" 
But it is not only thus that the introduction of an air-spark can be made to produce 
phosphorescence in a vacuum tube. It is possible to obtain similar phosphorescence 
in other portions of the tube than those immediately surrounding the negative 
terminal, and it is by these methods that we can show most clearly that the pheno- 
* This fact alone is a sufficient warning against viewing the emission of these molecular streams as 
indicating in any way a special direction of the electric discharge. It is, in our opinion, fatal to the idea 
that these molecular streams prove that an electric discharge in a vacuum tube is a “ negative flow.” 
4 F 2 
