258 Sir James Dewar [Jan. 16, 



the same temperature as at the lower pressure of 1*06 mm., we 

 should have — 



T^ dp = T 2 dp dp' I = 760 



p dT p' dT° ° dT p 1-06 



= 648*8 mm. of sulphuric acid per degree. 



Since the indication of the sulphuric acid manometer with the 

 charcoal thermoscope was frequently of the order of 3 cm. for 

 \ minute's exposure to a Leslie cube at 15° C, the corresponding 

 rise of temperature would therefore be ' 05° Abs. 



The measured evolution of gas at constant pressure on the 

 horizontal scaled tube under the same circumstances was about 

 4 cc. Now, taking the thermal evolution . of oxygen in charcoal 

 as 3146 calories per gramme molecule,* corresponding to approxi- 

 mately 7 • 2 cc. evolved per calorie absorbed, the thermoscope registered 

 l/20th of a calorie for \ minute's exposure. This corresponds 

 approximately to 56*6 mm. displacement in the scaled tube of 

 3 mm. bore, so that each mm. represented approximately 0*001 

 calorie. For more exact measurements of smaller transmissions a 

 tube of less diameter was used. 



The maximum evolution of gas corresponding to the theoretical 

 total emission from the black body was not usually obtained, but the 

 proportion registered was higher with small disturbances and lower 

 with exposures to more intense radiations. Two typical cases may 

 be quoted. In the first, the cell was exposed in the ordinary way at 

 the lower end of a cylindrical vacuum vessel with black absorptive 

 walls ; while in the second, the walls were made reflecting with a 

 lining of thin polished metal. In the first case the cell was distant 

 18 cm. from the open neck covered by a black cube at 15° 0. Xow, 

 with cell 2 J cm. diameter (4' 9 cm. 2 area) 18 cm. below the neck, of 

 diameter 5 J cm. (neck area = 23*7 cm. 2 ), theoretical calories 

 diffused through vessel from a black cube at 15° C, covering the 

 whole open neck, and absorbed at 90° Abs. (see Fig. 13) : 

 = 1-374 x 10" 12 (288 4 - 90 4 ) 

 = 0*00945 calorie per second per cm. 2 . 



Therefore total flux from exposed area of 23*7 cm. 2 = 0*2245 

 calorie per second, or 13*47 calories per minute. 



These 13*47 calories per minute may be regarded as spread out 

 over a hemisphere of 18 cm. radius, i.e. over an area of 2036 cm. 2 , of 

 which surface the cell occupies 4 * 9 cm. 2 ; and will therefore absorb 

 4*9/2036 of the total radiation, viz. 0*032 calorie per minute. If 



* Derived from a Rankine formula log# = A — B/T, where B was found 



T 2 d v 

 by experiment to be 684, whence the value of — rf =2x 684 x log e l0 = 



3146. (Proc. Boy. Inst., xix. p. 416.) 



