32 



Mr. W. Crookes on Repulsion 



[Jan. 17, 



such, as pith, mica, charcoal, glass. The complicated nature of these 

 actions is well shown in the results given by three pith disks, the first 

 being plain white, the second lampblacked on the front, and the third 

 lampblacked on the back. The first is repelled with a power of 17 7, 

 the second (the standard) with a power of 100, whilst the third is not 

 moved at all. The repulsion exerted on the white surface must be the 

 same in each case, but the pressure of lampblack behind the pith 

 causes a radiation of heat from the back surface, which produces 

 molecular pressure just sufficient to neutralise the pressure in front. 



Experiments are shown in this table with mica, both plain and 

 roasted, and covered on one side or the other with lampblack. The 

 results cannot well be described in abstract. 



To show that physical condition has more effect in causing repulsion 

 than chemical composition, results are given with various kinds of 

 charcoal. It is shown that the repulsion suffered by cocoa-nut shell 

 charcoal is much less than that by white pith, being only 11 '6 against 

 17*7. At the same time, a radiometer made of cocoa-nut shell char- 

 coal, lampblacked on one side, is only moderately sensitive, instead of 

 being superior to one made of pith lampblacked on one side. The 

 low figure shown by the charcoal is caused by its density enabling it 

 to conduct heat from one surface to the other. Molecular pressure is, 

 therefore, generated on both the back and front surfaces, and the 

 figure given is simply the difference between the two opposing 

 actions. Experiments show that this explanation is correct. 



Besides water, other screens are used to filter the radiation of the 

 candle before it falls on the disks. Water is, however, preferred for 

 several reasons. It is almost perfectly opaque to the invisible heat 

 rays, and, therefore, its employment allows easy discrimination be- 

 tween actions due to heat and to heat and light combined; secondly, 

 it is colourless, and having no selective action on any visible ray of 

 light, it can be used in conjunction with any coloured powder without 

 complicating the results. Alum acts in a similar manner to water ; 

 coloured solutions act as water with a superadded action due to their 

 colour. Very thick plates of glass have less action on the invisible 

 heat rays than a thin layer of water. Sulphate of copper in so weak 

 a solution as to appear only slightly green, has a very strong action 

 when artificial light is used, as it cuts off the lowest visible red rays 

 as well as the ultra red. 



After having given the tabulated results of the examination and dis- 

 cussed the different actions, the author finds that the substances ex- 

 perimented on may be divided into two classes. 



1. In which the repulsion behind water is greater in proportion to 

 the standard than when no screen is present ; and 2, in which the 

 repulsion in proportion to the standard is less behind water than when 

 no screen is present. Amongst class 1 may be mentioned copper 



