Repulsion accompanying Radiation* 85 



first sight to understand how it can produce attraction. The 

 law of exchanges, however, explains this perfectly. The pith 

 index and the whole of the surrounding bodies are incessantly- 

 exchanging heat-rays ; and under ordinary circumstances the 

 income and expenditure of heat are in equilibrium. A piece of 

 ice brought near one end of the index cuts offthe influx of heat 

 to it from that side, and therefore allows an excess of heat to 

 fall upon it from the opposite side. Attraction by a cold body 

 is thus seen to be only repulsion by the radiation from the op- 

 posite side of the room. 



Instruments of the kind just described are perhaps the best 

 for exhibiting large and striking movements of attraction or 

 repulsion. Two glass globes 4 inches in diameter, fitted up with 

 bars of pith 3J x \ inch, are now before you. One is full of air 

 at ordinary pressure, whilst the other is completely exhausted. 

 A touch with a finger on a part of the globe near one extremity 

 of the pith will drive the bar round over 90°, in the vacuum. 

 In air the attraction is not quite so strong. 



If I place a lighted candle an inch or two from the vacuous 

 globe, the pith bar commences to oscillate. The swing gradu- 

 ally increases in amplitude until one or two complete revolu- 

 tions ar» made. The torsion of the suspending fibre here inter- 

 feres, and the vibrations proceed in the opposite direction. The 

 movement continues as long as the candle burns. This con- 

 tinued movement ceases if the source of radiation is removed 

 some distance off; the pith index then sets equatorially. The 

 cause of the continued vibration when the radiant body is 

 at a particular distance from the pith is easy to understand on 

 the supposition that the movement is due to the direct impact 

 of waves on the suspended body. 



For more accurate experiments I prefer making the apparatus 

 differently. Fig. 4 represents the best form, ab is a glass 

 tube, to which is fused at right angles another, narrower tube, c d; 

 the vertical tube is slightly contracted at e so as to prevent the 

 solid stoper d, which just fits the bore of the tube, from falling 

 down. The lower end of the stopper de is drawn out to a point ; 

 and to this is cemented a fine glass thread about 0*001 inch 

 diameter, or less, according to the torsion required. 



At the lower end of the glass thread an aluminium stirrup 

 and a concave glass mirror are cemented, the stirrup being so 

 arranged that it will hold a beam fg having masses of any de- 

 sired material at the extremities. At c in the horizontal tube 

 is a plate-glass window cemented on to the tube. At b is also 

 a piece of plate glass cemented on. Exhaustion is effected 

 through a branch tube h projecting from the side of the upright 

 tube. This is sealed by fusion to the spiral tube of the pump, 



