Action of Heat on Gravitating Masses. 69 



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 Ruhm- 

 korff'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. 



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 vacuo 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, platinum, gilt pith, silver, bismuth, selenium, copper, mica 

 (horizontal and vertical), charcoal, &c. 



The behaviour of a glass beam with glass ends in a chemical va- 

 cuum, 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. 



Speaking of Cavendish's celebrated experiment, the author says 

 that he has experimented for some months on an apparatus of this 

 kind, and gives the following outline of one of the results he has 

 obtained :— 



"A heavy metallic mass, when brought near a delicately sus- 

 pended light ball, attracts or repels it under the following circum- 

 stances : — 



" I, When the ball is in air of ordinary density. 



a. If the mass is colder than the ball, it repels the ball. 



b. If the mass is hotter than the ball, it attracts the ball. 

 " II. When the ball is in a vacuum. 



a. If the mass is colder than the ball, it attracts the ball. 



b. If the mass is hotter than the ball, it repels the ball." 

 The author continues: — " The density of the medium surround- 



* This can be effected without interfering with the exhaustion. 



