NICHOLS AND HULL. — PRESSURE DUE TO RADIATION. 587 



columns of the table give the zero, direct and reversed reading of the gal- 

 vanometer G v The fourth column gives the temperature of the bath in 

 which the disc was immersed, and the fifth, that of the constant tempera- 

 ture calorimeter. The sixth column gives the deflections of G v The 

 seventh column the means of the alternate deflections. The eighth, 

 the mean of the two columns preceding it. The last column gives the 

 difference in temperature between the two calorimeters in degrees C. 

 For the total temperature range in the table, 39.11°, the deflection of G^ 

 was 393.8 scale divisions for a sensitiveness of G x = 996. A range of 

 one degree would thus give a deflection of 10.03 divisions for a sensitive- 

 ness of C?i = 1000. The mean of two separate calibrations was 9.96 

 scale divisions for one degree temperature difference. 



Before beginning a series of intensity measurements the disc was 

 suspended in an air-chamber containing phosphoric anhydride and sur- 

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

 temperature of about zero degrees and was then quickly transferred to 

 the chamber C (Fig. 6), and the beam was directed upon it. When its 

 temperature had risen to within five or six degrees of that of the chamber 

 G, galvanometer readings were made at intervals of five seconds until the 

 disc was heated to a temperature several degrees above its surroundings. 

 The temperature of the chamber C was determined by removing the 

 disc and cooling it to a point near the room temperature, then replacing 

 it and observing its rate of temperature change for several minutes. 



The notebook record of one series of observations showing the heat- 

 ing of the disc by the light beam is given in full in Table VI. It will be 

 seen from the table that the temperature of the disc passed that of the 

 chamber thirty seconds after the beginning of the series. The readings 

 of G x at equal time intervals on either side of the zero are on horizontal 

 lines. The last column of the table contains the rate at which the 

 galvanometer deflection was changing when the disc and its surround- 

 ings were at the same temperature. 



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 off from the disc 

 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 disc thermo-junctions 

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

 of the disc, which might prevent the mean temperature of the two 

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



