ON THE SENSITIVE STATE OF VACUUM DISCHARGES. 
581 
state in its physical qualities, and especially in the length of the free path of its 
molecules and the frequency of the collisions between them. As we are of opinion 
that there are no sufficient grounds for such a supposition, but that, on the contrary, 
the phenomena are compatible with the ordinary molecular theory of gases, we shall 
proceed to state our views upon the subject, and the experimental facts upon which 
these views are based. 
It is by no means an unusual phenomenon to find streams of particles driven off 
from the surface of bodies highly charged with electricity. The very familiar 
phenomenon known as the “electric wind'’ is an instance of this kind. In this case 
particles of air are driven off from the pointed terminal of an electric machine or a 
highly charged Leyden jar with such force that they produce a perceptible wind ; and 
their reaction can be made to turn a vane much in the same way as Crookes’ electric 
radiometer is turned by the electric discharge in a vacuum. This phenomenon is 
admitted to be due to the repulsion between the highly charged conductor and the 
neighbouring particles of air which have become charged in a like sense by coming in 
contact with it. 
There are, it is true, many peculiarities of this “ electric wind ” which prevent our 
accepting it as an exact analogue of the molecular streams which produce phosphor¬ 
escence in high vacua. In the first place it is common to both the positive and the 
negative poles, and is indeed more easily produced at the former than at the latter. 
Then, again, the velocities of the particles seem to be much less than in the case of phos¬ 
phorescence, although the whole pressure produced by them is relatively considerable 
owing to the greater density of the medium affected. But these are not differences 
which weigh very heavily in the consideration of the matter as a whole. The typical 
peculiarities of the negative terminal only begin to manifest themselves as the pressure 
of the surrounding gaseous medium is lessened, and it is only in very high vacua that 
they attain their full proportions. Moreover, the lower velocity of the particles is 
exactly what we should expect to find in a medium so much denser than the high 
vacua in which phosphorescence is usually observed. 
These are not the only cases in which, at the ordinary atmospheric pressure, we 
have phenomena of this nature. It is well known that in the electric arc there is a 
constant stream of particles of carbon from the positive to the negative pole. And of 
late a method of coating glass with platinum has, we believe, been invented and 
carried out, as a commercial process, which depends on a like principle. The platinum 
is deposited from an electrode of that metal held near the glass and connected with 
some source of high tension electricity. And doubtless if experiments were made upon 
the streamers which are seen between the two poles of an electric machine when they 
are beyond striking distance some very closely analogous phenomena might be observed. 
But if these phenomena fail in being strictly analogous to the molecular streams 
that produce phosphorescence in that they show no special preference for the negative 
pole, or are even characteristic of the positive pole rather than of the negative at 
4 F 
MDCCCLXXX. 
