646 
MR. W. CROOKES OK MOLECULAR PHYSICS IN HIGH VACUA. 
time, and contact is again made the same way as before (b being negative), the spot does 
not now start from position e, but falls on cl, in the first instance. This can be repeated 
any number of times. 
Fig. 6. 
596. If now the pole b be made positive even for the shortest possible interval, and 
it then be made negative, the original phenomenon occurs, and the spot of light starts 
from e and rapidly travels to cl. After this it again falls on cl, ah initio, each time 
contact is made, so long as b is kept the negative pole. There seems no limit to the 
number of times these experiments can be repeated. The explanation of this result 
appears to depend on a temporary change in the condition of the wall of the glass tube 
when positively electrified molecules beat against it, a change which is undone by sub¬ 
sequent impact from negative molecules. This phenomenon is closely connected with 
some shadow and penumbra experiments described further on (601 to 605), and as the 
same explanation will apply to both I will defer any theoretical remarks for the 
present. 
Fig. 7. 
597. A suggestion was made by Professor Maxwell that I should introduce a third, 
idle, electrode in a tube between the positive and negative electrodes so that the 
molecular stream might beat upon it, so as to see if the molecules gave up any electrical 
charge when impinging on an obstacle. A tube was therefore made as shown in fig. 7; 
a and b are the ordinary terminals ; c and d are large aluminium disks nearly the 
diameter of the tube, connected with outer terminals. The poles a and b were con¬ 
nected with the induction coil, an earth wire was brought near the idle pole c, and a 
gold leaf electroscope was brought near d. 
On passing the current at inferior exhaustions, when the dark space is about 
