igh value, then continually diminished
towards the bright side of the next striation when it again increased.
This distribution of electric force implies that there is great excess
of negative electricity at the bright head of the striation, and a small
excess of positive everywhere else. The temperature of the gas is higher
in the bright than in the dark parts of the striations. Wood (_Wied.
Ann._ 49, p. 238), who has made a very careful study of the distribution
of temperature in a discharge tube, finds that in those tubes the
temperature varies in the same way as the electric force, but that this
temperature (which it must be remembered is the average temperature of
all the molecules and not merely of those which are taking part in the
discharge) is by no means high; in no part of the discharge did the
temperature in his experiments exceed 100 deg. C.

[Illustration: FIG. 17.]

_Theory of the Striations._--We may regard the heaping up of the
negative charges at intervals along the discharge as the fundamental
feature in the striations, and this heaping up may be explained as
follows. Imagine a corpuscle projected with considerable velocity from a
place where the electric field is strong, such as the neighbourhood of
the cathode; as it moves towards the anode through the gas it will
collide with the molecules, ionize them and lose energy and velocity.
Thus unless the corpuscle is acted on by a field strong enough to supply
it with the energy it loses by collision, its speed will gradually
diminish. Further, when its energy falls below a certain value it will
unite with a molecule and become part of a negative ion, instead of a
corpuscle; at this stage there will be a sudden and very large
diminution in its velocity. Let us now follow the course of a stream of
corpuscles starting from the cathode and approaching the anode. If the
speed falls off as the stream proceeds, the corpuscles in the rear will
gain on those in front and the density of the stream in the front will
be increased. If at a certain place the velocity receives a sudden check
by the corpuscles becoming loaded with a molecule, the density of the
negative electricity will increase at this place with great rapidity,
and here there will be a great accumulation of negative electricity, as
at the bright head on the cathode side of a striation. Now this
accumulation of negative electricity will produce a large electric force
on the anode side; this will drive