THE PRESSURE DUE TO RADIATION. 129 



This method was suggested bj^ Prof. (1. E. Hale and gives very fine 

 and uniform dead-blaek coatings not inferior to good smoke deposits. 



For the energy measurements the Ijell jar and the torsion l)ahince 

 were removed from the platform P (fig. 1) and a double-walled cop- 

 per vessel AB (fig. 5), which served as a water jacket surrounding a 

 small air chamber C, was mounted in the same place. A tul^e 2 cm. in 

 diameter was soldered into the front face of the jacket to admit the light 

 beam into the chamber C. This opening was covered by a piece of plate 

 glass similar to the plates forming the larger windows in the bell jar. 



The needle system in G^, a four-coil du Bois-Rubens galvanometer, 

 was suspended in a strong magnetic field so that its period was about 

 four seconds. The system was heavily damped by a mica air fan of 

 large surface. The'disk junctions and galvanometer responded quickly 

 to the radiation, as was shown by the reversal of motion of the magnet 

 system 1.2 seconds after the light was cut ofl' from the disk, wh(>n the 

 latter was a few degrees above the temperature of the room. 



The disk was calibrated for temperature in terms of the deflection 

 for a definite sensitiveness of the galvanometer G^. * * * 



The mean of two separate calibrations taken several days apart was 

 9.96 scale divisions for one degree temperature difference. 



Before beginning a series of intensitv measurements the disk was 

 suspended in an air chamber containing phosphoric anhydride and sur- 

 rounded by a jacket of ice and salt. The disk was thus lowered to a 

 temperature of about zero degrees and was then quickly transferred 

 to the chamber C (fig. 5), and the beam was directed upon it. When 

 its temperature had risen to within 5 or 6 degrees of that of the cham- 

 ber C, galvanometer readings were made at intervals of five seconds 

 until the disk was heated to a temperature several degrees above its 

 surroundings. The temperature of the chamber C was determined b}^ 

 removing the disk and cooling it to a point near the room tempera- 

 ture, then replacing it and observing its rate of temperature change 

 for several minutes. 



Energy series were made " through air," "through red glass," and 

 "through water cell," as in the pressure measurements. During the 

 experiment the black coatings were frequently cleaned ofl' from the 

 disk and new ones deposited. The final result therefore does not 

 correspond to an individual but to an average coating. 



To correct for any inequality between the two disk thermoj unctions 

 or any lack of symmetry in their positions, referred to the central 

 plane of the disk, which might prevent the mean temperature of the two 

 junctions from representing the mean temperature of the mass, series of 

 observations were made on each face of the disks. The black coating 

 was always cleaned off from the face of the disk away from the light," 



« In the original paper here follow detailed results of 82 energy measurements 

 through air, water, and red glass, some on one face of the disk and some on the 

 other. 



