MR. W. CROOKES ON REPULSION RESULTING FROM RADIATION. 
129 
towards it till the index ray of light marks —140, when the disk and cup touch; 
a positive movement to the right signifying repulsion, and a negative movement to the 
left signifying attraction. Electrical contact was made by pressing down a key. The 
current was kept on during the whole time of each experiment. 
464. Pressure 260 millims. —When the current is turned on the index ray moves to 
+ 2 ; it then swings back to —140, the disk touching the cup, and being held there 
for some time after contact is broken. 
465. Pressure 240 millims .—On making contact the movements of the index are 
the same as at 260, the first movement, however, being +3 divisions. 
466. Pressure 210 millims. —The same as above, but the first movement being +4. 
467. Pressure 160 millims.—On. making contact the first movement of the index is 
+ 4-5° repulsion. It then swings back to —139, the plate just failing to touch the 
cup by one division. There was no permanent attraction, as in former cases, but the 
beam swung quite freely. 
468. Pressure 125 millims. —The index moved on making contact +6°, repulsion ; it 
then returned to —136°, attraction, being 4 degrees short of contact. 
469. Pressure 110 millims. —When contact was made the first movement was +9, 
repulsion ; the return movement was to — 135°, attraction, or 5 degrees short of contact. 
470. Pressure 90 millims. —The first movement was +15? repulsion; the return 
movement was —131°, or 9 degrees short of contact. 
471. Pressure 60 millims. —The first movement was +22, repulsion; the return 
movement was to —115, or 25 short of contact. 
472. Pressure 55 millims. —The first movement of the index on heating the spiral 
was +34, repulsion ; the return swing was to —94. 
473. Pressure 15 millims. —On heating the spiral the index ray showed repulsion, 
going to +145. It then oscillated a little to and fro, but did not show any attraction. 
474. Pressure 10 millims. —As soon as battery contact was made, strong repulsion 
ensued, the index ray going off the scale (250° + ), and the mica disk being driven 
against the side of the tube and remaining there. This being the case, no further 
experiments could advantageously be tried with the apparatus. 
Between each of the latter experiments the index took a long time to return to zero, 
owing to the mass of heated metal inside, which had to get quite cold. 
475. Fig. 32 shows the results of the above experiments drawn as curves. The 
attraction, which is very strong at pressures between atmospheric and 210 millims., 
begius to decline after that degree of exhaustion is passed, until it disappears at 
15 millims. At the same time, the repulsion, which begins to be apparent at 250 
millims., increases as the attraction diminishes. It is probable that the attraction is 
the effect of air currents caused by the permanent heating of the surface in front of the 
mica disk, and that if the first action of the molecular pressure, which I assume is 
practically instantaneous, could be still further separated from the more slowly acting 
air currents, we should succeed in getting decided repulsion at still higher pressures, 
mdccclxxix, s 
