514 
MR. W. CROOKES ON ATTRACTION AND 
these cases was to bring the centre of gravity of the brass ball as near as possible to the 
source of beat. 
41. The pump was then worked until the gauge had risen to 5 millims. of the baro- 
metric height. On arranging the ball above the spiral and making contact, the attraction 
was still strong, drawing the ball downwards a distance of 2 millims. 
The pump continuing to work, the gauge rose until it was within 1 millim. of 
the barometer. The attraction of the hot spiral for the ball was still evident, drawing 
it down when placed below it, and up when placed above it. The movement was, 
however, much less decided than before, and in spite of previous experience (30, 31) 
the inference was very strong that the attraction would gradually diminish until the 
vacuum was absolute, and that then, and not till then, the neutral point would be 
reached. Within 1 millim. of a Sprengel vacuum there appeared to be no room for a 
change of sign. 
42. The gauge rose until there was only half a millimetre between it and the baro- 
meter. The metallic hammering heard when the rarefaction is close upon a vacuum 
commenced, and the falling mercury only occasionally took down a bubble of air. On 
turning on the battery-current, there was the faintest possible movement of the brass 
ball in the direction of attraction. 
43. The working of the pump was continued. On next making contact with the 
battery, no movement of the ball could be detected. The red-hot spiral neither 
attracted nor repelled. I had arrived at the critical point. On looking at the gauge 
I saw it was level with the barometer. 
44. The pump was now kept at full work for an hour. The gauge did not rise 
perceptibly ; but the metallic hammering increased in sharpness, and I could see that a 
bubble or two of air had been carried down. On igniting the spiral, I found that the 
critical point had been passed. The sign had changed, and the action was faint but 
unmistakable repulsion. The pump was still kept going, and an observation was 
taken from time to time during several hours. The repulsion continued to increase. 
The tubes of the pump were now washed out with oil of vitriol *, and the working was 
continued for an hour. 
45. The action of the incandescent spiral was now found to be energetically repel- 
lent, whether it was placed above or below the brass ball (figs. 4 c, 4 d). The finger 
Fig. 4 o. Fig. 4 n. 
exerted a repellent action, as did also a warm glass rod, a spirit-flame, and a piece of 
hot copper. 
* This can be effected without interfering with the exhaustion. (See Philosophical Transactions, 1873, 
vol. clxiii. p. 296.) 
