234 Royal Society. 



3. The appearance of the induction-spark between the platinum 

 wires. 



1 measures the viscosity ; 2 enables me to calculate the force 

 of radiation of the candle ; and 3 enables me to form an idea of 

 the progress of the vacuum according as the interior of the tube 

 becomes uniformly luminous, striated, luminous at the poles only, 

 or black and non-conducting. 



The apparatus is also arranged so that I can try similar experi- 

 ments with any vapour or gas. 



The following are some of the most important results which 

 this apparatus has as yet yielded. 



Up to an exhaustion at which the gauge and barometer are 

 sensibly level, there is not much variation in the viscosity of the 

 internal gas (dry atmospheric air). Upon now continuing to 

 exhaust, the force of radiation commences to be apparent, the 

 viscosity remaining about the same. The viscosity next commences 

 to diminish, the force of radiation increasing. After long-con- 

 tinued exhaustion the force of radiation approaches a maximum ; 

 but the viscosity measured by the logarithmic decrement begins 

 to fall off, the decrease being rather sudden after it has once com- 

 menced. 



Lastly, some time after the logarithmic decrement has com- 

 menced to fall off, and when it is about one fourth of what it 

 was at the commencement, the force of radiation diminishes. At 

 the highest exhaustion I have yet been able to work at, the loga- 

 rithmic decrement is about one twentieth of its original amount, 

 and the force of repulsion has sunk to a little less than one half 

 of the maximum. The attenuation has now become so excessive 

 that we are no longer at liberty to treat the number of gaseous 

 molecules present in the apparatus as practically infinite-; and, 

 according to Professor Clerk Maxwell's theory, the mean length of 

 path of the molecules between their collisions is no longer very 

 small compared with the dimensions of the apparatus. 



The degree of exhaustion at which an induction-current will 

 not pass is far below the extreme exhaustions at which the loga- 

 rithmic decrement falls rapidly. 



The force of radiation does not act suddenly, but takes an ap- 

 preciable time to attain its maximum — thus proving, as Prof. 

 •Stokes has pointed out, that the force is not due to radiation di- 

 rectly but indirectly. 



In a radiometer exhausted to a very high degree of sensitive- 

 ness, the viscosity of the residual gas is almost as great as if it 

 were at the atmospheric pressure. 



With other gases than air the phenomena are different in 

 degree, although similar in kind — aqueous vapour, for instance, 

 retarding the force of repulsion to a great extent, and carbonic 

 acid acting in a similar though less degree. 



The evidence afforded by the experiments of which this is a 

 brief abstract is to my mind so strong as almost to amount to con- 

 viction that the repulsion resulting from radiation is due to an 

 action of thermometric heat between the surface of the moving 



