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Mr. W. Crookes on the Radiometer. 



[Nov. 16, 



Sniythe, hayiDg been nominated by tlie President, were elected by ballot 

 Auditors o£ the Treasurer's Accounts on the part of tbe Society. 



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

 them. 



The following Papers were read : — 



I. ^' Experimental Contributions to the Theory of the Radio- 

 meter."" — Preliminary Notice. By William Crookes, F.R.S. 

 &c. Received November 15, 1876. 



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 more 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 recent experiments before men of science, I might 

 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 repulsion 

 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 experimenting at still higher exhaustions, and 

 viith different gases in the apparatus ; and by means of a McLeod gauge 

 attached to the mercury pump I have been able to measure the atmo- 

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

 measured the force of repulsion, but also the viscosity of the residual 

 gas ; and from the results I have plotted the observations in curves 

 which accompany this paper, and which show how the "\dscosity of the 

 residual gas is related to the force of repulsion exerted by radiation. 

 These curv es must not, however, be considered as representing more than 

 the broad facts, for I have not included in them my final observations, 

 which in all 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 yxrVo atmosphere : each millimetre, 



therefore, stands for the millionth of an atmosphere. 



My results have principall}^ been obtained at the top of the scale ; and 

 it is the last quarter of a metre which supplies the diagrams accompanying 

 this paper. 



