1876.] 



Repulsion resulting from Radiation. 



139 



eye that there is a yiscosity between the suspended body and the vessel ; 

 but once having ascertained that, and admitting that the logarithmic 

 decrement of the arc of oscillation (when no candle is shining on the 

 plate) is a measure of the viscosity, there is no further necessity to 

 complicate the apparatus by having the ground and lubricated stopper. 

 A movement of the whole vessel bodily through a small arc is equally 

 effective for getting this logarithmic decrement ; and the absence of the 

 stopper enables me to have the whole apparatus sealed up in glass, and 

 I can therefore experiment at higher rarefactions than would be possible 

 when a lubricated stopper is present. 



The apparatus, which is too complicated to describe without a drawing, 

 has attached to it : — a, a Sprengel pump ; h, an arrangement for producing 

 a chemical vacuum; c, a lamp with scale, on which to observe the 

 luminous index reflected fyom the mirror ; d, a standard candle at a fixed 

 distance ; and e, a small vacuum-tube, with the internal ends of the 

 platinum wires close together. I can therefore take observations of : — 



1. The logarithmic decrement of the arc of oscillation when under no 

 influence of radiation ; 



2. The successive swings and final deflection when a candle shines on 

 one end of the blackened bar ; 



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 pro- 

 gress of the vacuum according as the interior of the tube becomes uni- 

 formly luminous, striated, luminous at the poles only, or black and non- 

 conducting. 



The apparatus is also arranged so that I can try similar experiments 

 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 ^-iscosity remaining about the 

 same. The viscosity next commences to diminish, the force of radiation 

 increasing. After long-continued exhaustion the force of radiation ap- 

 proaches a maximum ; but the viscosity measured by the logarithmic 

 decrement begins to fall off, the decrease being rather sudden after it has 

 once commenced. 



Lastly, some time after the logarithmic decrement has commenced to 

 fall off, aiid it is about one fourth of what it was at the com- 



mencement, the force of radiation diminishes. At the highest exhaustion 

 I have yet been able to \^ ork at, the logarithmic 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 



L 2 



