ON ELECTRICAL DISCHARGES THROUGH RAREFIED GASES. 183 
not affect the truth of the statement that the effects of bringing a conductor near to 
the tube consist in all cases of one of the above described types or of a mixture of the 
two; and the doubtful cases can be shown to be cases in which the faintness of the 
effect creates a difficulty in recognizing their true features rather than cases in which a 
foreign element is present in the appearances themselves. We shall presently examine 
the causes which increase sensitiveness, but we wish first to examine the relationship 
in which the two forms of the relief-effect stand to one another. 
Take a tube exhibiting a sensitive discharge, and find a spot where on contact with 
the finger the discharge-effect is manifested. Instead of placing the finger on the 
glass, commence with the finger at so great a distance from the spot that no visible 
effect is produced on the sensitive discharge. Now slowly approach the finger to the 
spot chosen on the tube. The first symptom of sensitiveness' will in all cases be a 
slight repulsion of the luminous column; this will, on nearer approach of the finger, 
become more decided, and at length a slight haze will become visible between the 
repelled luminous column and the sjDot on the glass. This will, if the process be 
continued, rapidly develop into the ordinary discharge-effect. The change from the 
state of greatest repulsion to that of the discharge-effect is usually extremely rapid, so 
much so as to give rise to the idea that there is an absolute discontinuity at. this 
point; but if care be taken to get a steady current, and the approach be very gradual, 
it can be shown that there is a continuous graduation from the one state into the 
other. 
A more accurate method of trying the experiment consists in fixing a piece of tinfoil 
or thin metal, bent to the curvature of the tube, at the end of a glass rod, and con¬ 
necting it to earth by a thin wire running along the rod. If this be made to work 
through a socket in a stand so as to admit of complete control, it will be found that 
the luminous discharge may be made to pass continuously through all gradations from 
almost imperceptible repulsion to the discharge-effect merely by diminishing the 
distance of the tinfoil from the tube. Actual contact is, as may easily be imagined, 
the most favourable arrangement for the production of discharge-effect, but it is by no 
means essential to it, nor is there indeed any discontinuous or even very rapid change 
m the effect when contact commences. The experiment fully bears out the principle 
previously enunciated, that the more -complete the relief the greater the effect on the 
sensitive column, understanding for the future that the discharge-effect must rank 
higher than the repulsion-effect, or, in other words, must be considered as an intensified 
form of it (Plate 16, fig. 8). 
A very slight modification of the arrangement last described will serve for an 
experiment which yet more clearly demonstrates that the intensity of the relief-effect 
depends on the completeness of the relief afforded to the static induction caused by 
the free electricity in the tube, and that for this purpose the discharge-effect must be 
taken as an intensified form of the repulsion-effect. If the tinfoil, instead of being 
connected to earth, be connected to any system of conductors insulated from earth, it 
