94 On At traction and Repulsion accompanying Radiation. 



is blown at the end of a piece of combustion-tubing; and in it 

 is supported a bar of aluminium at the end of a long platinum 

 wire. The whole is attached to the Sprengel pump, and ex- 

 haustion is kept going on for about two days, until a spark will 

 not pass through the vacuum. During this time the bulb and 

 its contents are frequently raised to an incipient red heat. At 

 the end of that time the tube is sealed off, and the bar of alumi- 

 nium is found to behave exactly as it would in a less perfectly 

 exhausted apparatus ; viz. it is repelled by heat. A similar 

 experiment, attended with similar results, has been tried with a 

 glass index. It is impossible to conceive that in these experi- 

 ments sufficient condensable gas was present to produce the 

 effects Professor Reynolds ascribes to it. After the repeated 

 heatings to redness at the highest attainable exhaustion (the 

 gauge and the barometer being level for nearly the whole of the 

 48 hours), it is impossible that sufficient vapour or gas should 

 condense on the movable index to be instantly driven off, by the 

 warmth of the finger, with recoil enough to drive backwards a 

 heavy piece of metal. 



My own impression is that the repulsion accompanying radia- 

 tion is directly due to the impact of the waves upon the surface 

 of the moving mass, and not secondarily through the interven- 

 tion of air-currents, electricity, or evaporation and condensation. 

 Whether the setherial waves actually strike the substance moved, 

 or whether at that mysterious boundary-surface separating solid 

 from gaseous matter there are intermediary layers of condensed 

 gas which, taking up the blow, pass it on to the layer beneath, 

 are problems the solution of which must be left to further 

 research. 



In giving what I conceive to be reasonable arguments against 

 the three theories which have been supposed to explain these re- 

 pulsions, I do not wish to insist upon any theory of my own to 

 take their place. The one I advance is to my mind the most 

 reasonable, and as such is useful as a working hypothesis, if the 

 mind must have a theory to rest upon. Any theory will account 

 for some facts ; but only the true explanation will satisfy all the 

 conditions of the problem, and this cannot be said of either of 

 the theories I have already discussed. 



My object at present is to ascertain facts, varying the condi- 

 tions of each experiment so as to find out what are the necessary 

 and what the accidental accompaniments of the phenomena. 

 By working steadily in this manner, letting each group of expe- 

 riments point out the direction for the next group, and follow- 

 ing up as closely as possible, not only the main line of research, 

 but also the little bylanes which often lead to the most valuable 

 results, after a tin e the facts will sroun themselves together 



