248 Royal Society: — Mr. W. Crookes on Attraction 



The paper concludes with a discussion of the various theories 

 which have been adduced in explanation of these phenomena. The 

 air-current and electrical theory are considered to have been 

 abundantly disproved. The following experiment is given by the 

 author to show that Prof. Osborne Reynolds's hypothesis of the 

 movements due to evaporation and condensation at the surface 

 Mall not account for all the facts of the case, and that therefore he 

 has not hit upon the true explanation. A thick and strong bulb 

 was blown at the end of a piece of very difficultly fusible green 

 glass, specially made for steam-boiler gauges. In it was supported 

 a thin bar of aluminium at the end of a long platinum wire. The 

 upper end of the wire was passed through the top of the tube and 

 well sealed in, for electrical purposes. The apparatus was sealed by 

 fusion to the Sprengel pump, and exhaustion was kept going on for 

 two days, until an induction-spark refused to pass across the vacuum. 

 During this time the bulb and its contents were several times raised 

 to a dull red heat. At the end of two days' exhaustion the tube 

 was found to behave in the same manner as, but in a stronger degree 

 than, it would in a less perfectly exhausted apparatus, viz. it was 

 repelled by heat of low intensity and attracted by cold. A similar 

 experiment was next tried, only water was placed in the bulb be- 

 fore exhaustion. The water was then boiled away in vacuo, and 

 the exhaustion continued, with frequent heating of the apparatus 

 to dull redness, for about 48 hours. At the end of this tim© 

 the bar of aluminium was found to behave exactly the same 

 as the one in the former experiment, being repelled by radiation. 



It is impossible to conceive that in these experiments sufficient 

 condensable gas or vapour was present to produce the effects Prof. 

 Osborne Reynolds ascribes to it. After the repeated heating to 

 redness at the highest attainable exhaustion, 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. 



While objecting to the theories already advanced as not ac- 

 counting for all the facts of the case, the author confesses that he 

 is not as yet prepared with one to put in their place. He wishes 

 to avoid giving any theory on the subject until a sufficient number 

 of facts have been accumulated. The facts will then tell their own 

 tale. The conditions under which they invariably occur will give 

 the laws, and the theory will follow without much difficulty. 



Supplement. Received April 20, 1875. 



Since the experiments mentioned in the foregoing Abstract were 

 concluded, the author has examined more fully the action of radi- 

 ation on black and white surfaces. At the highest exhaustion 

 heat appears to act almost equally on white and on lampblacked 

 pith, repelling them in about the same degree. 



The action of the luminous rays, however, is different. These 



