NICHOLS AND HULL. — PRESSURE DUE TO RADIATION. 697 

 A comparison of observed and computed pressures follows : — 



Observed values 

 in 10~5 dynes. 



Through air, p = 7.01 ± 0.02 * 



Through red glass, p = 6.94 ± 0.02 

 Through water, p = 6.52 ± 0.03 



An estimate of the approximate magnitude of the gas action, not elimi- 

 nated by the ballistic method of observation, may be reached from the 

 following considerations. 



When radiation falls upon a vane of the torsion balance, part of it is 

 absorbed by the silver surface. From the amounts directly and diffusely 

 reflected, as given in Table X, the amount transmitted by the average 

 surface (experimentally determined but not given in Table X), the effect 

 of the glass rod and the reflection coefficient of the glass surface, it was 

 found that, when the silver side of the vane was toward the radiation 

 source, the absorption coefficient for radiation through air was 6 per cent, 

 and when the glass surface was forward, it was 18 per cent. 



The total force acting on the vane is made up of two parts, that due 

 to radiation pressure and that due to gas action. Let F^ be the force 

 due to the first cause, assuming tliat all the radiation is absorbed, and Fg 

 the effect due to the second, on the same condition. Then the total 

 effect, when the silver side of the vane is forward and the radiation is 

 " through air," is 1.92 F^ + 0.06 Fg. When the glass side is forward 

 the total effect is 1.776 i^^ — 0.18 Fg. Making these expressions equal 

 to the reduced deflections (Table III, columns 11 and 12) on the silver 

 and glass surfaces respectively, we have two equations by means of 

 which the values of F^ and Fg may be obtained. Hence the effect due 

 to gas action on each face of the vane is approximately determinate, as is 



* The pressure and energy measurements for the three different wave groups 

 through air, red glass, and water cell, constitute three independent experiments. 

 In the values for pressure, 7.01, 6.94, and 6.52, equality is not to be looked for. 

 The difference arises from the different reflecting power of the 45° glass plate 

 (Fig. 3) for the different beams and from the fact that the indications of the lamp 

 galvanometer G, connected with the bolometer R, were probably not strictly pro- 

 portional to energy for throws differing as widely as 3.3, 60, and 100, which, roughly, 

 were the relative intensities of the beams through water cell, red glass, and air. 

 The function of the lamp bolometer and galvanometer was purely to keep a check 

 on the small variations of the lamp which rarely fluctuated more than 10 per cent 

 on either side of the mean value. 



