96 Transactions of the South African Philosophical Society. 



that radiation, like absorption and unlike reflection, is not a surface 

 phenomenon, but takes place from the whole body of matter in 

 question. In fact it is known that a stratum of any substance, 

 however slight its emissivity for particular radiations, will, if only 

 thick enough, behave exactly like a black body.* 



It is to be noted that the suggestion that terrestrial radiation 

 depends more upon the relative than upon the absolute humidity 

 does not depend solely upon the observations of temperature cited 

 at the commencement of this paper. Some years ago I published 

 some results showing that under absolutely clear skies the air itself 

 seemed to cool more rapidly when the relative humidity was low 

 than when the dew-point was low.f Indeed no variation arising 

 out of the absolute humidity could be detected with certainty. If, 

 then, we can accept it as proved that the solar radiation is absorbed 

 in proportion to the absolute humidity alone, while terrestrial 

 radiation is absorbed in proportion to the relative humidity alone, 

 the protective value of the atmosphere appears in a much more 

 effective aspect. To start with, we shall have terrestrial radiation 

 (say, e.g., nocturnal cooling) proceeding more and more slowly as 

 the temperature falls, even though the dew-point fall as dew is 

 condensed out of the lower air. At high temperatures, under full 

 sunshine, the emission to space may, in some cases, be almost as 

 rapid as the reception of heat from the sun. At lower temperatures, 

 even with the same quantity of moisture, the emission may be 

 extremely slow. An elevated sheet of air containing a given 

 quantity of aqueous vapour at a given temperature may, on account 

 of a low absolute humidity, permit the solar radiation to pass with 

 comparative freedom, while on account of a high relative humidity 

 the return terrestrial radiation might be effectually checked. In 

 this case the sheet of air does bear some analogy to the glass of a 

 greenhouse so dear to the heart of the orthodox meteorologist. 

 Should the temperature of the elevated sheet of air, however, 

 happen to be high, so that its relative humidity is low, then the 

 analogy breaks down, for the terrestrial radiation is no longer 

 checked by it. By way of restoring the analogy, does a hot sheet 

 of glass absorb as much dark heat as a cold sheet ? 



* See P. G. Tait, " Heat," 1895, p. 262 ; J. Tyndall, " Heat a Mode of Motion," 

 1880, p. 312. The former gives a mathematical demonstration. 



J. R. Sutton, "Aqueous Vapour and Temperature," Symons's Met. Mag., 1895, 

 vol. 30, p. 104. The matter is being re-examined. 



