Oil Attraction and Repulsion resulting from Radiation. 245 



them ; from these substances, indeed, the most important part of 

 the results were derived. 



" On Attraction and Eepulsion resulting from Radiation." — Part 

 II. By William Crookes, F.R.S. &c. 



This is the second part of a paper which the author sent to the 

 Royal Society in August 1873. The author commences by de- 

 scribing improvements which he has made in the Sprengel pump, 

 and in various accessories which are necessary when working at 

 the highest rarefactions. 



Continuing the description of apparatus, the author describes dif- 

 ferent new forms which enable the phenomena of repulsion by ra- 

 diation to be observed and illustrated. A bulb 3 inches in diameter 

 is blown at the end of a glass tube 18 inches long. In this bulb a 

 fine glass stem, with a sphere or disk of pith &c. at each end, is 

 suspended by means of a cocoon-fibre. The whole is attached to 

 the Sprengel pump in such a way that it can be perfectly exhausted 

 and then thermetically sealed. Besides pith, the terminals may be 

 made of cork, ivory, metal, or other substance. During exhaustion 

 several precautions have to be taken, which are fully entered into 

 in the paper. To get the greatest delicacy in an apparatus of this 

 kind, there is required large surface with a minimum of weight. 

 An apparatus constructed with the proper precautions is so sensi- 

 tive to heat, that a touch with the finger on a part of the globe 

 near one extremity of the pith will drive the index round over 90°, 

 whilst it follows a piece of ice as a needle follows a magnet. "With 

 a large bulb, very well exhausted and containing a suspended bar of 

 pith, a somewhat striking effect is produced when a lighted candle 

 .s placed about 2 inches from the globe. The pith bar commences to 

 oscillate to and fro, the swing gradually increasing in amplitude 

 until the dead centre is passed over, when several complete 

 revolutions are made. The torsion of the suspending fibre now 

 offers resistance to the revolutions, and the bar commences to turn 

 in the opposite direction. This movement is kept up with great 

 energy and regularity as long as the candle burns. 



The author discusses the action of ice, or a cold substance, on 

 the suspended index. Cold being simply negative heat, it is not 

 at first sight obvious how it can produce the opposite effect to 

 heat. The author, however, explains this by the law of exchanges, 

 and shows that attraction by a cold body is really repulsion by 

 radiation falling on the opposite side. According to the same 

 law, it is not difficult to foresee what will be the action of 

 two bodies, each free to move, if they are brought near to one 

 another in space, and if they differ in temperature either from 

 each other or from the limiting walls of the space. The author 

 gives four typical cases, with experiments, which prove his reason- 

 ing to be correct. 



Experiments are described with the object of ascertaining whether 

 the attraction by heat, which, commencing at the neutral point, 

 increases with the densitv of the enclosed air, will be continued in 



