THE MECHANICAL ACTION OF LIGHT. 687 



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 rare- 

 faction is close upon a vacuum commenced, and the falling mercury 

 only occasionally took down a bubble of air. On turning on the bat- 

 tery-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 cai*- 

 ried 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,' and the working was continued for an 

 hour. 



The action of the incandescent spiral was now found to be ener- 

 getically 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 aiford 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 RuIimkorft"'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 w;hich the rays of the sun, and then the 

 different portions of the solar spectrum, were projected on to the deli- 

 cately-suspended pith-ball balance. In vacuo the repulsion by a beam 

 of sunlight is so strong as to cause danger to the apparatus, and resem- 

 bles that which would be produced by the physical impact of a mate- 

 rial 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 h (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 

 1 This can be effected without interfering with the exhaustion. 



