68 Royal Society : — Mr. W. Crookes on the 



vacuum-tube instead of outside it as before ; and the pith balls of 

 the former apparatus were replaced by brass balls. By careful ma- 

 nagement and turning the tube round, the author could place the 

 equipoised brass pole either over, under, or at the side of the source 

 of heat. With this apparatus it was intended to ascertain more 

 about the behaviour of the balance during the progress of the ex- 

 haustion, both below and above the point of no action, and also to 

 ascertain the pressure corresponding with this critical point. 



After describing many experiments with the ball in various po- 

 sitions with respect to the incandescent spiral, and at different 

 pressures, the general result is expressed by the statement that 

 the tendency in each case was to bring the centre of gravity of the 

 brass ball as near as possible to the source of heat, when air of or- 

 dinary density, or even highly rarefied air, surrounded the balance. 

 The author continues : — 



" 44. The pump was then worked until the gauge had risen to 

 within 5 millims. of the barometric height. On arranging the ball 

 above the spiral (and making contact with the battery), the attrac- 

 tion 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, however, was 

 much less decided than before ; and in spite of previous experience 

 (33, 34) 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 one 

 millimetre of a vacuum there appeared to be no room for a change 

 of sign. 



" 45. The gauge rose until there was only half a millimetre be- 

 tween it and the barometer. The metallic hammering heard when 

 the rarefaction is close upon a vacuum commenced, and the fall- 

 ing 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 (towards the spiral) in the direction of 

 attraction. 



"46. The working of the pump was continued. On next ma- 

 king contact with the battery, no movement 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. 



" 47. The pump was now kept at full work for an hour. The 

 gauge did not rise perceptibly ; but the metallic hammering sound 

 increased in sharpness, and I could see that a bubble or two of air had 

 been carried down. On igniting the spiral, I saw 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 



