[ 467 ] 

 LXIII. Proceedings of Learned Societies. 



ROYAL SOCIETY. 

 [Continued from p. 392.] 

 Nov. 10, 1876.— Dr. J. Dalton Hooker, C.B., President, in the Chair. 

 HPHE following paper was read : — ■ 



" Experimental ( onf ributions to the Theory of the Radiometer." 

 — Preliminary Notice. By William Crookes, F.R.S. &c. 



Instead of bringing another preliminary notice before the Society, 

 I should have preferred reserving the announcement of my new 

 results on the Repulsion resulting from Radiation until they were 

 fit to be offered in a inore complete form ; but the radiometer is 

 now so much occupying the attention of scientific men, and results 

 of experiments with this and allied instruments are appearing so 

 frequently in the scientific journals at home and abroad, that were 

 I not to adopt this method of bringing the results of my more re- 

 cent experiments before men of science, I jnight find myself 

 anticipated in some or all of the conclusions at which I have 

 arrived. 



On June 15th last I mentioned to the Society that the re- 

 pulsion resulting from radiation increases up to a certain point 

 as I exhaust the air from the torsion-apparatus. After long- 

 continued exhaustion the force of radiation approaches a maximum, 

 and then begins to fall off. I have since succeeded in experi- 

 menting at still higher exhaustions, and with different gases in 

 the apparatus ; and by means of a McLeod gauge attached to the 

 mercury pump I have been able to measure the atmospheric 

 pressure at any desired stage of exhaustion. I have not only 

 measured the force of repulsion, but also the viscosnry of the 

 residual gas ; and from the results I have plotted the observa- 

 tions in curves, which accompany this paper, and which show how 

 the viscosity of the residual gas is related to the force of repulsion 

 exerted by radiation. These curves must not, however, be con- 

 sidered as representing more than the broad facts ; for I have not 

 included in them my final observations, which in a]l probability 

 will introduce modifications in them. 



In plotting these curves I have supposed my scale to be 1000 

 metres long, and to represent one atmosphere. Halfway up the 

 scale therefore, or 500 metres, represents half an atmosphere ; 999 

 metres up the scale represents an exhaustion of jjnru °^ an atmo- 

 sphere : each millimetre, therefore, stands for the millionth of an 

 atmosphere. 



My results have principally been obtained at the top of the scale ; 

 and it is the last quarter of a metre which supplies the diagrams ac- 

 companying this paper. 



When the residual gas is air, the viscosity (measured by the loga- 

 rithmic decrement of the arc of oscillation) is practically con- 

 stant up to an exhaustion of 250 millionths of an atmosphere, or 

 0*19 millim. of mercury, having only diminished from 0*126 at the 



2 H 2 



