Repulsion accompanying Radiation. 93 



Professor Reynolds further says, " Mr. Crookes only obtained 

 his results when his vacuum was nearly as perfect as the 

 Sprengel pump would make it. Up to this point he had nothing 

 but the inverse effects, viz. attraction with heat and repul- 

 sion with cold." In the abstract of my paper published in the 

 Proceedings of the Royal Society, I describe an experiment with 

 a pith-ball apparatus in which the neutral point is 7 millims. 

 (about J inch) below the vacuum, repulsion by heat taking place 

 at higher exhaustions. At the Royal Society Soiree, April 22, 

 1874, I showed, and fully described in print, the apparatus now 

 before you, consisting of a pith bar suspended by a cocoon fibre 

 in a glass bulb, from which the air is exhausted until the baro- 

 metric gauge shows a depression of 12 millims. below the ba- 

 rometer. Neither attraction nor repulsion results when radiant 

 light or heat falls on the pith. Exhaustions of 7 and 12 mil- 

 lims. are certainly very inferior vacua for a Sprengel pump. 



As a matter of fact, however, I have obtained repulsion by 

 radiation at far higher pressures than these. The true effect of 

 radiation appears to be one of repulsion at any pressure, over- 

 balanced when a gas is present by some cause — possibly air- 

 currents, but probably not. I have already explained that the 

 barometric height of this neutral point dividing attraction from 

 repulsion varies with the density of the substance on which ra- 

 diation falls, on the relation which the mass bears to the surface, 

 and on the intensity of radiation. By modifying the conditions 

 it is not difficult to get repulsion by radiation when the appa- 

 ratus is full of air at nearly the normal pressure of the atmo- 

 sphere. 



Professor Reynolds again says, " The reason why Mr. Crookes 

 did not obtain the same results with a less perfect vacuum was 

 because he had then too large a proportion of air, or non- con- 

 densing gas, mixed with the vapour." On this I may remark 

 that the writer, before he explained how it was I could not get 

 certain results, should have made sure that what he assumed to 

 be the case was really so. I have not the least difficulty in 

 showing repulsion by heat in imperfect vacua when mixed va- 

 pours and gases are present. 



In my arguments against the air- current theory, I have shown 

 that the best results are obtained when the vacuum is so nearly 

 perfect that an induction-spark will not pass through it. This 

 is an equally good argument against the presence of a conden- 

 sable vapour as it is against that of air. 



From the construction of my Sprengel pump I am satisfied 

 that the vapour of mercury is absent from the apparatus. 



The following experiments have been specially tried with the 

 object of testing this theory. A tolerably thick and strong bulb 



