92 Mr. W. Crookes on Attraction and 



periments appear to show that evaporation from a surface is 

 attended with a force tending to drive the surface back, and 

 condensation with a force tending to draw the surface forward." 



It does not appear that Professor Reynolds has tried more 

 than a few experiments ; and he admits that they were in reality- 

 undertaken to verify the explanation above quoted. I have 

 worked experimentally on this subject for some years : and the 

 last experiment recorded in my notebook is numbered 584. 

 From the abundant data at my disposal, I can find many facts 

 which will, I think, convince you that this hypothesis has been 

 arrived at on insufficient evidence. 



In the first place, I will show that the presence of moisture or 

 of a condensable vapour is not necessary. Besides pith, which from 

 its texture and lightness might be supposed to absorb and con- 

 dense considerable quantities of vapour, I have used glass, mica, 

 and various metals ; and with a proper amount of exhaustion 

 they all act in the same manner. The fact that the neutral 

 point for platinum is close upon a vacuum, whilst that for pith 

 is so much lower, tends to show that the repulsion is not due to 

 any recoil caused by condensable vapour leaving the surface 

 under the influence of heat. Were it so, it would certainly 

 require more vapour to be present when platinum had to be 

 driven backwards than when pith had to be moved ; but the 

 contrary obtains in all cases. The rule seems to be, the greater 

 the density the higher the neutral point. 



I have worked with all kinds of vacua ; that is to say, I have 

 started with the apparatus filled with various vapours and gases 

 (air, carbonic acid, water, iodine, hydrogen, &c.) ; and at the 

 proper rarefaction I find no difference in the results which can 

 be traced to the residual vapour. A hydrogen vacuum seems 

 neither more nor less favourable to the phenomena than does a 

 water vacuum, or an iodine vacuum. 



If moisture be present to begin with, I find it necessary to 

 allow the vapour to be absorbed by the sulphuric acid of the 

 pump, and to continue the exhaustion, with repeated heating of 

 the apparatus, until the aqueous vapour is removed. Then and 

 then only do I get the best results. 



When pith is employed as the index, it is necessary to have it 

 thoroughly dried over sulphuric acid before using it, and during 

 the exhaustion to keep it constantly heated to a little below its 

 charring-point, in order to get the greatest sensitiveness. 



Professor Reynolds says, " In order that these results might 

 be obtained, it was necessary that the vapour should be free from 

 air." On the contrary, I find the results take place with the 

 greatest sharpness and rapidity if the residual gas consists of 

 nothing but air or hydrogen. 



