30 



Mr. W. Crookes on Repulsion 



[Jan. 17, 



The experimental powders are mostly chemical precipitates, laid on 

 the surface of mica or pith disks as a water paint, no cement being 

 nsed to promote adhesion. In other cases the substances are punched, 

 cut, or filed into the shape of -disks, 17'25 millims. in diameter. 



The exhaustion, which has to be effected after each change of the 

 experimental disks, is carefully brought to the same degree both by 

 actual measurement on a McLeod gauge, and by getting the same re- 

 pulsion on the standard black disk. In this way all the different 

 results are fairly comparable one with the other. The presence of 

 aqueous vapour must be specially guarded against by means of tubes 

 containing phosphoric anhydride. 



The effect of residual gas in tending to equalise the amount of 

 repulsion on variously coloured surfaces is shown in an experiment 

 with pith disks, one being lampblacked and the other retaining its 

 natural white surface, the standard candle being at the same distance 

 in each case. When the exhaustion is good enough to cause a fair 

 repulsion, the ratio between the amplitude of swing (measured by the 

 index ray) when the black is exposed and that when the white is 

 exposed is as 100 : 55'5 ; at a little higher exhaustion the ratio is — 



Black : White : : 100 : 42*5 ; 



at a still better exhaustion (at which the experiments are usually tried) 

 the ratio is — 



Black : White : : 100 : 35. 



The results of the quantitative examination of the repulsion resulting 

 from radiation when falling on about 100 different substances are given 

 in fourteen tables. Each table is in three columns, the first consisting 

 of the names of the substances experimented on ; the second the 

 amount of repulsion observed, reduced to the standard of 100 for 

 lampblack ; the third column gives the repulsion observed when a 

 water screen is interposed, reduced to the same standard. 



Table I gives the results of the examination of black powders. Com- 

 pared with lampblack = 100, these have an average value of 92"2, 

 which becomes 99 '1 by the interposition of water. 



Table II contains white powders. These have an average value 

 of 33*5, which is reduced to 8*3 behind water. The powerful ab- 

 sorption for the invisible heat rays which white powders exercise is 

 somewhat remarkable. Assuming that the ultra-red rays from a 

 candle are almost entirely cut off by a water screen, the comparatively 

 strong action (33' 5) produced by the naked flame must be mainly due 

 to the absorption of the invisible heat rays ; and when these are cut off 

 by water the action is diminished nearly fifty times. With black 

 powders the water only diminishes the action about eleven times. 



Table III gives the red powders. Amongst these precipitated 

 selenium is noteworthy. To the naked flame its value is 35'8, but 



