592 
MESSRS. W. SPOTTISWOODE AND J. FLETCHER MOULTON 
XYIII .—The sensitive state'’’ exists in discharges through tubes of high exhaustion when 
the current has the sharp intermittence which is the essential condition of its exist¬ 
ence in tubes of lower exhaustion. 
When we examine a discharge in a tube of high vacuum which gives phosphorescence, 
we usually find that there is present an ill-defined column of haze of a greyish or 
purple colour, extending from the positive end of the tube. This must be taken as 
the representative of the positive column of the ordinary discharge, and it can be 
shown experimentally that such is the case by exhausting a tube while a discharge is 
passing through it, when it will be found that the positive luminosity passes continu¬ 
ously into the haze of which we are speaking. 
So long as there is no interruption in the circuit external to the tube, these luminous 
appearances mayt be non-sensitive, i.e., may be indifferent to the approach of a con¬ 
ductor to the tube. 
But if an air-spark be inroduced, the green phosphorescence becomes decidedly 
more brilliant, and the haze is found to be highly sensitive. With a positive air-spark 
the haze behaves on the approach of a conductor in all respects like the positive 
column in an ordinary sensitive discharge with a positive air-spark, excepting that its 
sensitiveness is usually more intense. When the air-spark is in the negative it is 
more difficult to establish the identity of behaviour of the haze and the ordinary 
sensitive luminous column under similar circumstances, but this does not affect the 
question of whether it is sensitive or not. As to this there can be no doubt, for the 
faint luminosity changes its conformation in a very marked way on the approach of a 
conductor to the tube. 
If the vacuum be very high the tube appears almost wholly destitute of the haze of 
which we have spoken, and of course it then becomes difficult to demonstrate the 
sensitiveness of the discharge in the manner which we have just described. The very 
term itself seems to require an extended meaning, inasmuch as the true luminous dis¬ 
charge to which it was originally applied no longer exists. But it is not difficult to 
decide on the meaning which must now be given to it, for it will be found that the 
only luminous phenomenon that still remains, viz.: phosphorescence, undergoes changes 
when a conductor is brought near to or in contact with the tube so that we may fairly 
apply to it the same term “sensitive” that we have used with regard to luminous dis¬ 
charges in tubes in which the vacuum is not so perfect. It is true that in one class of 
cases (viz.: those in which the air-spark is in the negative) the sensitiveness of the 
* It will be remembered that tbe definition given in our former paper of the sensitive state is “ the state 
in which the discharge is affected by the presence or approach of a conductor .” This definition will be adhered 
to throughout. 
t We say that the discharge may be non-sensitive when there is no interruption in the external circuit 
because we shall see that it is not necessarily so, just as in the case of tubes of lower exhaust a tube of 
high vacuum may itself cause the discharge passing through it to become intermittent and sensitive. 
