40 Mr. W. Crookes on the [Dec. 11, 



rent, 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 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. 



" 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 pump were now washed out with oil of vitriol*, and the 

 working was continued for an hour. 



"48. 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, whilst it would 

 settle the question indisputably, would at the same time be likely to 

 afford information of much interest. 



By chemical means the author obtained in an apparatus a vacuum so 

 nearly perfect that it would not carry a current from a Buhmkorff'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 ener- 

 getic. 



An experiment is next described, in which the rays of the sun, and 

 then the different portions of the solar spectrum, are projected on to the 

 delicately suspended pith-ball balance. In vacua the repulsion 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. 



Experiments are next described in which various substances were used 

 as the gravitating masses. Amongst these are ivory, brass, pith, pla- 

 tinum, gilt pith, silver, bismuth, selenium, copper, mica (horizontal and 

 vertical), charcoal, &c. 



The behaviour of a glass beam with glass ends in a chemical vacuum, 

 and at lower exhaustion, is next accurately examined when heat is applied 

 in different ways. 



On suspending the light index by means of a cocoon fibre in a long 

 glass tube furnished with a bulb at the end, and exhausting in various 

 ways, the author finds that the attraction to a hot body in air, and the 

 repulsion from a hot body in vacuo are rendered still more apparent. 

 * This can be effected without interfering with the exhaustion. 



