Repulsion accompanying Radiation. 8* 



arrangement, which effectually prevents air getting into the 

 pump from the reservoir A when the mercury has completely 

 run out. In this case no harm whatever is done to the opera- 

 tion : the vacuum is not injured; and the exhaustion proceeds 

 immediately on retransferring the mercury from the reservoir H 

 to the reservoir A, and raising A again into its place. The ap- 

 paratus, as thus arranged, is readily manageable with certainty 

 of obtaining a barometric vacuum. 



The mercury fall-tube of a pump in constant use frequently 

 wants cleaning. I find the most effectual means of doing this 

 is to put oil of vitriol into the funnel h, and then, by slightly 

 loosening the glass stopper, allow a little of the strong acid to 

 be carried down the tube with the mercury. With care this 

 can be effected without interfering with the progress of exhaus- 

 tion. The residual acid adhering to the walls of the chamber 

 N does good rather than harm. When sufficient sulphuric acid 

 has run into the fall-tube, the funnel-stopper can be perfectly 

 closed by pressing it in with a slight twist and then filling up 

 with mercury. 



Many physicists have worked on the subject of attraction and 

 repulsion by heat. In 1792 the Rev. A. Bennet recorded the fact 

 that a light substance delicately suspended in air was attracted 

 by warm bodies ; this he ascribed to air-currents. When, by 

 means of a lens, light was focused on one end of a delicately sus- 

 pended arm, either in air or in an exhausted receiver, no motion 

 could be perceived distinguishable from the effects of heat. 

 After Mr. Bennet the subject has been more or less noticed 

 by Laplace, Libri, Fresnel, Saigey, Forbes, Baden Powell, and 

 Faye ; but the results have been unsatisfactory and contradictory. 



My first experiments were performed with apparatus made on 

 the principle of the balance. An exceedingly fine and light arm 

 is delicately suspended in a glass tube by a double-pointed 

 needle ; and at the ends are affixed balls of various materials. 

 Amongst the substances thus experimented on I may mention 

 pith, glass, charcoal, wood, ivory, cork, selenium, platinum, 

 silver, aluminium, magnesium, and various other metals. The 

 beam is usually either of glass or straw. 



The apparatus, consisting of a straw beam and pith-ball ends, 

 being fitted up as here shown attached to the pump, and the 

 whole being full of air to begin with, I pass a spirit-lamp across 

 the upper part of the tube just over one of the pith-balls. The 

 ball rises. The same effect is produced when a bulb of hot water, 

 or even the warm finger is placed over the pith-ball. 



On working the pump and repeating the experiment, the 

 attraction to the hot body gets less and less, until it becomes nil ; 

 and after a certain barometric pressure is passed, the attraction 



G2 



