ON THE SENSITIVE STATE OF VACUUM DISCHARGES. 
565 
electricity. It is neither more nor less than the examination of the electric-spark. 
Each of the separate discharges was shown to pass through the tube in a time so 
short that it was negligible in comparison with the interval between two such dis¬ 
charges, so that each may be taken as a separate and absolutely isolated discharge ; 
the eftect of the rapid repetition of these individual discharges being only to render 
visible the phenomena which each of them would have presented had it been possible to 
observe it separately. Thus we are in reality observing the phenomena of the passage 
of an electric spark through rarefied gas, under circumstances, it is true, which cause 
great uniformity of conditions throughout the whole series of sparks, but which do 
not subject any one of the series to any peculiarity of attendant circumstances which 
would unfit it to be taken as a representative of such electric sjoarks. 
The interest of the investigation rises still higher when we consider it in relation to 
the continuous discharge. The latter bears precisely the same relation to the electric 
arc that the sensitive discharge bears to the electric spark ; and when we see that 
these two types of discharge have such striking resemblance, if not identity, of 
phenomena, we see that a study of them will probably give to us an insight into the 
relationship of the electric arc and the electric spark which would otherwise be 
unattainable. Indeed, it is doubtful whether the read intimacy of their relationship 
would have passed beyond the stage of conjecture but for the light thrown upon 
them by observing the modifications which each undergoes when the medium through 
which it passes becomes rarefied. So closely do these modifications resemble one 
another that there is no doubt that the careful and exhaustive examination of the 
electric arc in its process of development into the continuous vacuum discharge which 
we owe to Mr. Warren De La IIue might without much error be taken as giving 
the development of the electric spark into the intermittent or sensitive vacuum 
discharge. 
Such being the case, we have in the following paper continued our researches into 
the phenomena of sensitive discharges with the aim of establishing conclusions as to 
the structure and mechanism of the electric spark. But before so doing it was 
necessary to extend the conclusions of our former paper to the case of high vacua, the 
experiments upon which they were based having been exclusively performed with tubes 
containing gaseous media at a pressure of from about 1 millim. of mercury upwards. 
The special peculiarities of discharges in high vacua have also required separate 
examination in order that the phenomena of vacuum discharges might be viewed as 
a continuous whole. These matters have necessarily occupied much of our attention, 
but they have not wholly prevented our arriving at certain general conclusions as to 
electric discharge which we hope may in the future turn out to be of importance. 
As we have seen no reason to alter any of the views expressed in our last paper we 
shall take them as our starting point. So far as the immediate purposes of this paper 
are concerned, they may be taken to be as follows : that the intermittent discharges 
which give the sensitive vacuum discharge pass into the tube under the influence of 
MDCCCLXXX. 4 D 
