130 THE PKESSURE DUE TO RADIATION. 



As the general result of all the energy ineasurement.s it was found 

 that the rise in temperature of the silver disk per second when the 

 light passed:" 



((0 through air=(0°.0970=h0°.00034) C; 



(h) through red glass-=(0°.0946±0°.00036) C; 



(c) through water cell = (0°.0884±0°. 00064) C. 



The mass of the silver disk was 4.80 grams, its specific heat^' at 18° 

 C. =0.0556; the mechanical equivalent of heat at 18° C. =4.272X10' 

 ergs.'' Consequentl}^ the energy of the standard radiation is 



{(i) through air, 0.0970 X 4.80 X 0.0556 X 4.272 X 10' 



or Ea = (1.108 d= 0.004) X 10" ergs per second. 

 (/>) through red glass, Eg = (1.078 ± 0.004) X 10*^ ergs per second. 

 {(■) through water cell, E„, = (1.008 ±0.007) X 10^ ergs per second. 



REFLECTING POWER OF THE SURFACES USED. 



According to ^Maxwell and Bartoli. the pressure in d^mes per square 

 centimeter for normal incidence is Qqual 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 inci- 

 dent ))eam and p the reflection coefficient, the pressure^? = — ^^ ^, 



where V is the velocity of light. The methods for measuring p and 

 E have already been described. The determination of p for ])<)th 

 sides of the vanes C and D was made l)y means of a bolometer,'' 



In all, three series of measurements were made on the silver and 

 two series on the glass-silver faces of each vane. To get average 

 coefficients which would represent the range of condition of the mir- 

 rors during the pressure measurements, the vanes were cleaned and 

 new silver coatings deposited between each two series on the same 

 vane. The reflection coefficients are as follows: 



Connected refiecHon, coefficients in ■percentages. 



Through air 

 Red glass . . . 

 Water 



Air silver. Glass silver. 



92.0 

 93.4 

 89.0 



77.6 

 76.2 

 80.5 



Average coefficients through air, 84.8: red glass, 84.8; water, 84.8. 



The diffuse reflection of black coatings deposited by the method 

 used in blackening the silver disk was measured and computed in the 

 same manner as the diffused reflection from the vanes C and D. Five 

 determinations of this reflection were made under different conditions 



«See footnote to table on page 131. 

 611. Behn, Ann. der Phys., 4, 266, 1900. 



"Mean of Rowland's and Griffith's values. ' Phil. Trans., 5, 184, 496, 1893. 

 t^In the original paper here follow the details of experiments on the reflecting 

 power of the surfaces. 



