1877.] 



on Crookes's Force. 



557 



With the improved apparatus repulsion at first appeared to exist at 

 all tensions in hydrogen, the distance between the disks being 1 millim. 

 This repulsion was not perceptibly increased by alterations in tension, 

 until the latter was reduced to about 200 millims., when a slight increase 

 of repulsion took place, and further reductions of tensions were in each, 

 case followed by an increase of repulsion. 



It was observed that when repulsion w?is detected at ordinary tensions 

 the glass disk occupied the upper portion of the containing tube, and 

 that when the disk was made to swing in the lower portion of the tube, 

 instead of being repelled it was attracted towards the pith when illumi- 

 nated. These results are obviously due to convection-currents. By 

 carefully balancing the glass disk as nearly as possible in the centre of 

 the tube the effects are greatly reduced ; but it would be very difficult, if 

 not impossible, to balance the disk in a perfectly neutral position. 



In an atmosphere of hydrogen at ordinary atmospheric tensions there is 

 no indication of attraction or repulsion when the distance between the pith 

 and glass disks is 100 millims. and the time of illumination 15 seconds, 

 a period which experience has led us to adopt. The first unmistakable 

 indications of pressure on the swinging disk at this distance occur when 

 the tension is about 50 millims., at which tension there is a very feeble 

 repulsion. As the tension is reduced the repulsion increases. An ex- 

 tensive series of experiments have been made for the purpose of deter- 

 mining the ratio in which the repulsion increases for given reductions in 

 the tension of the residual gas. The accompanying Table exhibits the 

 results of one set of experiments : — 



T = tension of the residual gas. 



D = distance between the disks in millims. 



P=pressure on the swinging disk, as represented by the space, in 

 degrees of the scale, through which the index passes in 15 

 seconds. 



T. 



D. 



P. 



50 



100 



10 



4 



20 



5) 



i 



10 



5) 



1* 



5 



5) 



2i 



4 



i1 



3 



3 



5? 



4 



2 



5) 



5 



1 



)) 



Si 



intensity of these forces the matters to be considered along with the directions of the 

 motions in the intervening gas, are the proximity and extent of the opposed surfaces, 

 and not the degree of their parallelism. It is probable that all the phenomena of 

 radiometers with crumpled or inclined vanes, which Mr. Orookes has lately investi- 

 gated, admit of explanation by these considerations. 



It is manifest that in the apparatus described in the text the chief part of the re- 

 action on the containing vessel, when the disks are at a considerable distance asunder, 

 consists of tangential forces acting on the inside of the tube. 



