1877.] 



On Crookes's Force. 



553 



487. 20533 88090 34907 59753 59342 91581 10882 95687 88501 02669 

 40451 74537 98767 96714 57905 54414 76386 03696 09856 26283 

 36755 64681 72484 59958 93223 81930 18480 49281 31416 83778 

 23408 62423. 



69499. 42 16402 64958 74250 00737 87046 36777 99375 12912 73311 

 43614 89064 75972 82950 13260 58633 29520 29987 05564 38657 

 66605 24773 47376 76509 15591 83535 79093 38910 22857 11876 

 34134 30929. 



60 66853 69514 29876 69949 02151 63927 52953 46570 

 07023 75019 62711 34797 37766 20182 42900 32259 88844 52386 

 92150 48569 40061 68976 19403 38122 61378 99960 27151 79868 

 94597 53571. 



February 22, 1877. 



Dr. J. DALTON HOOKER, C.B., President, in the Chair. 



The Presents received were laid on the table, and thanks ordered for 

 them. 



The following Papers were read : — 



I. ^'On Crookes^s Force/^ By Gr. Johnstone Stoney, M.A., 

 F.R.S., and Richard J. Moss, F.C.S. Received January 12, 

 1877. 



In two papers by one of the authors of the present communication, 

 which appeared in the Philosophical Magazine for March and April 1876, 

 it has been shown that the motion of the blackened disks of a Crookes's 

 radiometer can be explained by the known dynamical properties of the 

 trace of gas which is present, and the term " Crookes's force " is pro- 

 posed to designate the reaction which comes into play between the 

 blackened disks and the walls of the exhausted chamber when a difference 

 of temperature exists between them. Shortly after the first of these 

 papers appeared we commenced an experimental investigation of the 

 subject with the view of learning, if possible, the laws to which the 

 force conforms. The investigation is still in progress, and, being ex- 

 ceedingly tedious, it will require a great expenditure of time before it is 

 completed ; we propose, however, in this preliminary paper to describe 

 the apparatus and methods of observation employed, and to give some of 

 the results already obtained. 



If the pressure which is exerted on the blackened pith surfaces reacts 

 on the sides of the glass envelope, it follows that a transparent disk 

 delicately suspended close to a stationary disk of blackened pith ought 

 to move away from the pith, and therefore towards the light, when the 



