1875. | The Mechanical Action of Light. 341 
ever, under, or at the side of the source of heat. With 
this apparatus I tried many experiments, to ascertain more 
about the behaviour of the balance during the progress of 
the exhaustion, both below and above the point of no action, 
and also to ascertain the pressure corresponding with this 
critical point. 
In one experiment, which is described in detail in my 
paper on this subject before the Royal Society,* the pump 
was 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 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 attrac- 
tion 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 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 vacuum there appeared to be no room fora change of 
sign. 
The gauge rose until there was only } a millim. between 
it and the barometer. 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 (towards the spiral) in 
the direction of attraction. 
The working of the pump was continued. On next 
making 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. 
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 neutral point had been passed. The sign had 
changed, and the a¢tion was one of faint but unmistakable 
vepulsion. The pump was still kept going, and an observa- 
tion was taken from time to time during several hours. The 
repulsion continued to increase. The tubes of the pump 
* Phil. Trans., 1874, vol. clxiv., p. 501. 
