558 



On Crookes's Force. 



[Feb. 22, 



In the aboTe experiments it will be observed that the distance between 

 the pith disk and the swinging glass disk was constant, viz. 100 millims. 

 T\'"e ha^'e made several series of experiments with the disks at various 

 distances in atmospheres of various tensions. The means of several of 

 these series are graphically represented in the following diagram. The 

 ordinates represent the distance in millimetres through which the index 

 moved in a period of 15 seconds ; the abscissas represent the distance 

 in millimetres between the disks. 



20 40 80 



Our expectation of being able to ascertain the thickness of the Crookes's 

 layer has not been entirely fulfilled with the apparatus as at present con- 

 structed, owing to the presence of the exceedingly feeble force that 

 is represented by the horizontal portions of the diagram. We have 

 foimd this feeble force nearly constant at each tension when the 

 two disks are at any distances asunder exceeding 20 millims. So far 

 as we have been able to ascertain, it seems to arise partly from the sudden 

 expansion of the gas throughout the extent of the Crookes's layer as 

 soon as the light is tiu-ned on, an expansion which acts on the swinging 

 disk as if a feeble explosion had taken place in front of it ; and partly 

 from a Crookes's force acting between the upperside of the tube and the 

 swinging disk, caused by the inside of the tube becoming sensibly heated 

 by the convection- current that commences as soon as the Crookes's layer 

 is established. We do not think that any appreciable part of it is due 

 to the direct action of the convection-current. 



