and Repulsion resulting from Radiation, 247 



highest rarefaction, there is not found any difference in the results 

 which can be traced to the residual gas. A hydrogen-vacuum appears 

 the same as a water- or an iodine-vacuum. 



With this apparatus the effect of exposing a torsion-balance to a 

 continuous radiation is described, and the results are shown graphi- 

 cally. The effect of a short (11*3 seconds) exposure to radiation 

 is next described, and the results are given in the form of a Table. 



In another Table is given the results of experiments in which a 

 constant source of radiation was allowed to act upon one end of the 

 torsion-beam at a distance of 140 or 280 millims., various substances 

 being interposed. The sensitiveness of this apparatus to heat-rays 

 appears to be greater than that of an ordinary thermo-multiplier. 

 Thus the obscure heat-rays from copper at 100°, passing through 

 glass, produce a deflection on the scale of 3*25, whilst under the 

 same circumstances no current is detected in the thermo-pile. The 

 following substances are used as screens, and the deflections pro- 

 duced (when the source of radiation is magnesium wire, a standard 

 candle, copper at 400°, and copper at 100°) are tabulated: — 



Rock-salt, 20 millims. thick; rock-crystal, 42 millims. thick; 

 dark smoky talc; plate glass of various thicknesses, both white and 

 green ; a glass cell containing 8 millims. of water ; a plate of alum 

 5 millims. thick; calc-spar, 27 millims. thick; ammonio-sulphate of 

 copper, opaque to rays below P ; ditto, opaque to rays below Gr. 



The author considers that these experiments show that the re- 

 pulsion is not entirely due to the rays usually called heat, i. e. to 

 the extreme and ultra red of the spectrum. Experiments have 

 been tried with the electric and the solar spectrum formed with a 

 quartz train, which prove the action to be also exerted by the lumi- 

 nous and ultra violet rays. Some numerical data have been obtained ; 

 but unfavourable weather has prevented many observations being 

 made with the solar spectrum. 



The barometric position of the neutral point dividing attraction 

 from repulsion is next discussed. The position of this point varies 

 with the density of the substance on which radiation falls, the ratio 

 of its mass to its surface, its radiating and conducting-power for 

 heat, the physical condition of its surface, the kind of gas filling 

 the apparatus, the intensity of radiation, and the temperature of 

 the surrounding atmosphere. The author is inclined to believe that 

 the true action of radiation is repulsion at any pressure, and that 

 the attraction observed when the rarefaction is below the neutral 

 point is caused by some modifying circumstances connected with 

 the surrounding gas, but not being of the nature of air-currents. 



The neutral point for a thin surface of pith being low, and that 

 for a moderately thick piece of platinum being high, it follows that 

 at a rarefaction intermediate between these two points pith will 

 be repelled, and that platinum will be attracted by the same 

 beam of radiation. This is proved experimentally ; and an appa- 

 ratus showing simultaneous attraction and repulsion by the same 

 ray of light is described and illustrated in the paper. 



