metre of a vacuum there appeared to be no room for a change of sign.
The gauge rose until there was only half a millimetre 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 (toward 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 action was one of 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,[1] and the working was continued for an hour.
The action of the incandescent spiral was now found to be energetically repellent, whether it was placed above or below the brass ball. The fingers-exerted a repellent action, as did also a warm glass rod, a spirit-flame, and a piece of hot copper.
In order to decide once for all whether these actions really were due to air-currents, a form of apparatus was fitted up which—while it would settle the question indisputably—would at the same time be likely to afford information of much interest.
By chemical means I obtained in an apparatus a vacuum so nearly perfect that it would not carry a current from a Ruhmkorff's coil when connected with platinum wires sealed into the tube. In such a vacuum the repulsion by heat was still found to be decided and energetic.
I next tried experiments in which the rays of the sun, and then the different portions of the solar spectrum, were projected on to the delicately-suspended pith-ball balance. In vacuo the repulsion by a beam of sunlight is so strong as to cause danger to the apparatus, and resembles that which would be produced by the physical impact of a material body.
A simpler form of the apparatus for exhibiting the phenomena of attraction in air and repulsion in a vacuum consists of a long glass tube, a b (Fig. 3), with a globe, c, at one end. A light index of pith, d e, is suspended in this globe by means of a cocoon fibre.
When the apparatus is full of air at ordinary pressure, a ray of
- ↑ This can be effected without interfering with the exhaustion.
