38 



Mr. W. Crookes on Repulsion 



[Jan. 17, 



object lias been to produce the molecular pressure by a very intense 

 heat which would not have to pass through glass, and to allow it to act 

 on vanes which are turned in the most favourable position for rotating 

 under the influence of the molecular pressure. The direction is called 

 positive when it is the same as would be produced by a wind blowing 

 from the platinum wire, and negative when it takes the contrary direc- 

 tion. In the apparatus most experimented with a rotating disk of 

 mica is supported on a separate pivot, immediately over the mica 

 vanes. The normal or positive movement of this disk is in the 

 opposite direction to that of the vanes ; thus, if the positive movement 

 of the vanes is in the direction of the hands of a watch, the positive 

 movement of the disk is in the opposite direction. 



Fig. 2. 



With the apparatus full of air at the normal pressure the direction 

 of rotation, both of the vanes and disk, is positive when the platinum 

 wire is ignited. This is probably due to the ascending current of hot 

 air. 



At a pressure of 80 millims. the disk does not rotate. The vanes 

 rotate positively, but slowly. 



At 19 millims. no movement whatever takes place. The disk and 

 vanes are as still when the wire is heated as when it is cold. 



At 14 millims. the disk remains stationary. The vanes move 

 slowly in the negative direction. 



At 1 millim. the disk rotates in the positive direction slowly, whilst 

 the vanes rotate negatively rather fast; both disk and vanes now 

 rotate in the same direction. 



