Nipher—On the Nature oj the Electric Discharge. 67 
nal, the electrical particles contained in the molecules 
within the shadow, or between the tube and plate. The 
positive column shown in Figs. A and C, Plate XXIV, 
and all the figures of Plate XXV at any instant show to 
the eye the same details which ¢re shown in Fig. A, Plate 
XXI. A time exposure in a camera shows, of course, 
no such detail of the discharge. 
It appears that the insulated metal plate CC of Fig. 2 
serves to separate the luminous positive column from the 
Faraday dark space, to such an extent that they may 
become apparent in discharges through air at ordinary 
pressure. When the metal plate is removed, the mole- 
cules at the boundary of these two regions mingle with 
each other. Electrically they are friendly, and they are 
being urged in opposite directions, by the compression 
and rarefaction t rminals of the machine. 
The Critical Spark-Length. 
The Faraday Dark-Space. 
The metal plate CC of Fig. 2 was removed. A minute 
spark-gap was made at a’. The contact at a was made 
as complete as possible, so that no luminous point is seen 
at this contact. The discharge then swept through the 
entire spark gap of about 15 em. The photograph of 
this luminous column as taken by the camera is shown in 
Fig. B, Plate XX V. The exposure was about five minutes. 
The mica wind-mill shows a feeble wind from the positive 
terminal. If the gap at a! is made somewhat larger, the 
discharge is then filled throughout with small disruptive 
sparks, and the windmill will not operate. If the gap a 
is made still longer, as seen in Fig. D, Plate XXV, a 
strong positive wind causes the windmill to rotate so 
rapidly that its vanes are invisible. This’ wind sweeps 
through the entire gap. The discharge is not then disrup- 
tive in character. If the gap at a‘ is closed and that at a 
is opened, the luminous streamers forming the positive 
column are beaten back by a blast of air from the nega- 
tive terminal. The mica windmill shows that the nega- 
tive wind now sweeps the entire gap. Fig. C, Plate XXV, 
