594 
MESSRS. W. SPOTTISWOODE AND J. FLETCHER MOULTON 
state of discharge in high vacua is dependent on intermittence. The arguments in 
favour of this are precisely identical with those that have been previously adduced in 
the case of discharges in low vacua, and the evidence is just as conclusive. As in the 
former case, sensitiveness is never found except in the presence of circumstances 
which render it extremely probable if not certain that the discharge is intermittent; 
and on the other hand, whenever the circumstances are such as to cause an inter¬ 
mittence of the proper type, the resulting discharge is found to possess sensitiveness. 
The telephone gives exactly the same indications of intermittence when placed in 
circuit between the earth and a piece of tinfoil laid upon a tube containing a sensitive 
discharge, and the revolving mirror gives exactly the same direct evidence of the inter¬ 
mittence of such discharges. In short, so far as has been observed, the whole of the 
evidence in favour of the connexion between intermittence and sensitiveness that can 
be adduced in the case of tubes of low exhaust is equally applicable to the case of 
tubes of high exhaust, excepting so far as instrumental difficulties or special peculiari¬ 
ties of the discharge (as, for instance, the extremely faint luminosity of the positive 
haze) make it impossible to apply the same tests. But although the amount of evi¬ 
dence is somewhat diminished by the limitation of our methods of producing inter¬ 
mittence, yet the nature of the evidence remains the same, and it is sufficient to show 
conclusively that in discharges through high vacua sensitiveness is just as much the 
invariable accompaniment of sharp intermittence and just as inseparable from it as is 
the case in the discharges of which we treated in our former paper. And, further, all 
the considerations which render the examination of the intermittent discharge of im¬ 
portance in the analysis of ordinary vacuum discharges exist and if possible possess 
yet greater force in the case of discharges in high vacua. There is the same identity 
of phenomena in the continuous and the discontinuous discharges, and there is the 
same ground for seeking in the discontinuous discharge the explanation of the various 
phenomena of the continuous discharge. No excuse will therefore be necessary for 
subjecting the sensitive state of discharges in high vacua to an investigation of the 
same type as that which is contained in our former paper. 
We shall, for the sake of simplicity of language, assume during the remainder of this 
paper that the intermittence and consequent sensitiveness is produced in all cases in 
the simplest and most convenient way, i.e., by an air-spark situated either in the 
positive or negative portion of the external circuit. 
XIX.— When the air-spark is in the positive the discharge passes through the tube in 
the shape of positive electricity, and vice versa. 
It will be seen that this amounts to saying that the general results of our former 
paper hold good for tubes of high vacua. 
The importance of demonstrating this is very great. For it signifies that there is 
