NICHOLS AND HULL. — PRESSURE DUE TO RADIATION. 591 



to a temperature difference of 1° C. Consequently the rise in tempera- 

 ture of the silver disc per second when the light passed : — 



(a) through air = 0.966 -^ 9.96 = (0°.0970 ± 0° .00034) C. ; 



(b) through red glass = 0.942 + 9.96 = (0°.0946 ± 0°.00036) C. ; 



(c) through water cell = 0.880 -f- 9.96 = (0°.0884 ± 0°. 00064) C. 

 The mass of the silver disc was 4.80 grams, its specific heat* at 18° 



C.= 0.0556 ; the mechanical equivalent of heat at 18° C. = 4.272 X 10 7 

 ergs.f Consequently the energy of the standard radiation is 



(a) through air, 0.0970 X 4.80 X 0.0556 X 4.272 X 10 7 



or E a = (1.108 ± 0.004) x 10 6 ergs per second. 



(b) through red glass, E g = (1.078 ± 0.004) x 10 6 " " " 



(c) through water cell, E w = (1.008 ± 0.007) X 10 6 " " « 



Reflecting Power of the Surfaces Used. 



According to Maxwell, the pressure in dynes per square centimeter 



for normal incidence is equal to the energy in ergs in unit volume of the 



medium. The energy in unit volume is made up of both the direct and 



reflected beams. If E is the intensity of the incident beam and p the 



p/i I \ 

 reflection coefficient, the pressure p = — , where V is the velocity 



of light. The methods for measuring p and E have already been de- 

 scribed. The determination of p for both sides of the vanes C and D 

 was made as follows. The supports of the torsion balance were replaced 

 by the divided circular plate A (Fig. 7), of a force table which could be 

 rotated about a central, vertical axis. The rod about which the plate 

 turned passed up through the plate and at its top the mirror holder b b 

 was fastened. The vanes were freshly silvered and mounted on a plate- 

 glass carrier a a, which was held by a clamp against the back face of b b. 

 The beam was directed on the vanes by the lens L s (Figs. 3 and 7) 

 exactly as it had been in the pressure observations. After reflection 

 from the vane the beam fell on a concave mirror M which projected an 

 image of the vane upon a simple sheet bolometer B, forming the unknown 

 resistance of a postoffice-box bridge. The current was supplied from 

 storage cells and the galvanometer was the same used in the energy 

 determinations but fitted with low resistance coils. The bolometer was 

 covered by the bell-jar used earlier. The mirror M, the bell-jar and 

 bolometer were attached to the plate of the force table. The full line 



* U. Behn, Ann Phys., IV. 266 (1900). 



t Mean of Rowland's and Griffith's values, Phil. Trans., V. 184, 496 (1893). 



