1876.] 



Mr. W. Crookes on the Radiometer. 



305 



When the residual gas is air, the viscosity (measured by the logarithmic 

 decrement of the arc of oscillation) is practically constant up to an 

 exhaustion of 250 millionths of an atmosphere, or 0*19 millim. of mercury, 

 having only diminished from 0*126 at the normal pressure of the atmo- 

 sphere to 0*112. It now begins to fall off : at 200 millionths it is 0*110, 

 at 100 millionths it is 0*096, at 50 millionths it is 0*078, at 20 millionths 

 it is 0*052, at 10 millionths it is 0*035, and at 0*1 of a millionth of an 



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'7 135 



atmosphere the log. dec. has fallen to about 0*01. Simultaneously with this 

 decrease in the viscosity, the force of repulsion exerted on a black surface 

 by a standard light varies. It increases very slowly till the exhaustion has 

 risen to about 70 millionths of an atmosphere ; at about 40 millionths the 

 force is at its maximum ; and it then sinks very rapidly, till at 0*1 millionth 

 of an atmosphere it is less than one tenth of its maximum. On continuing 

 the curves of the log. dec. and the force of radiation, and assuming that 

 the torsion-fibre of glass has no viscosity, it is most probable that 



z2 



