420 Sir Jarr.es Dewar [Jan. 28, 



obtained by grinding down hard carbonised coconut shell. "When 

 spread in a layer about 2 to 3 mm. thick, they present a broken 

 cellular surface well adapted for entangling incident radiation. As 

 already described (Joe. fit.), 0*7 to 0*8 gramme was employed spread 

 over a surface about 2j cm. in diameter. This, when enclosed in a 

 cell under a very thin rubber membrane or rock salt window, and 

 immersed in liquid oxygen, absorbed about 500 cc. of clean air. 

 When it was exposed to radiation from a black surface at normal 

 earth temperature (restricted to an axial pencil of 2 h cm. diameter 

 surrounded by a penumbra of diminishing intensity enclosed within 

 a cone having an angle of approximately 25"), the displacement of 

 the liquid index of the manometer was of the order of 3 cm. in half 

 a minute, which corresponds to an evolution per half minute of 

 0*054 cc. of gas, and a utilisation of 0*0071 gramme calorie per 

 half minute.* With the diaphragms stopped down to approximately 

 one-hundredth of this aperture, the sun (at 3 p.m. April 8, 1921) 

 gave a displacement of 4-4 cm., equivalent to 0*0792 cc, or 0*0104 

 calorie. t When the moon's image was reflected in from a silvered or 

 platinised mirror, a distinct response was produced, in addition to that 

 from the night sky. 



The displacements resulting from exposure to the zenith varied 

 of course considerably with the conditions, low values being recorded 

 with clear night skies. Clear skies by day also frequently gave very 

 low values, especially just before sunset, when the displacements were 

 often as small as those obtained during clear nights. 



The displacements of the manometer on exposure to a black 

 dome at the temperature of the surface air and to the zenith per- 

 mitted the determination of the proportion between E, the emission 

 from the black body at earth temperature, and Z, that from the zenith, 

 the radiations being sifted by the liquid oxygen and rubber membrane 

 or rock salt, and absorbed into the liquid oxygen *' sink" at 90° Abs. 

 The theoretical total calories are easily deduced in the first case when 



* This corresponds with the theoretical emission from the black body area 

 exposed : viz. central uniform ring of 2£ cm. diameter surrounded by 

 penumbral annulus 1| cm. wide, diminishing to zero intensity at the edge ; 

 or the total equivalent to a uniform ring 4 cm. in diameter or 12- 6 sq. cm. in 

 area. Taking black body emission from 15° C, absorbed at 90° Abs., as - 577 

 calorie per minute per sq. cm., then total from 12*6 sq. cm. area in half a 

 minute = 3*64 calories. This total is radiated to the cell 20 cm. distant from 

 the open neck of the flask at 15° C, and therefore is spread over a hemisphere 

 of 20 cm. radius, or 2513 sq. cm. area ; of this surface the cell of diameter 

 2^ cm. occupied only 4*9 sq. cm. The proportion available to the cell is 

 4 "9/2513 of the total of 3*64 calories emitted per half minute, or - 0071 

 calorie, 



t In the same way the sun effect at approximately 1/100 aperture (reflected 

 in from silver heliostat) corresponded to a heat absorption of 0-0104 calorie 

 per balf minute, or 1*04 calorie at full aperture of 4*9 sq. cm., corresponding 

 to 0*42 calorie per minute per sq. cm. utilised in evaporation from the partly 

 isolated charcoal. 



