Radiant Heat to Aqueous Vapour. 43 



anomalies were due to differences in the amount of aqueous 

 vapour in the air, which escaped the sense of vision. Leslie 

 himself connects the effect with aqueous vapour by the following 

 remark : — •" The pressure [apparently a misprint for presence] of 

 hygrometric moisture in the air probably affects the indications 

 of the instrument." In fact, the absence or presence of moisture 

 opened or closed an invisible door for radiation from the ( ' sentient 

 bulb " of the instrument into space. The following observation 

 in reference to radiation-experiments with Pouillet's pyrhelio- 

 meter, now receives its explanation. u In making such experi- 

 ments/' says M. Schlagintweit, " deviations in the transparency 

 are often recognized which are totally inappreciable to the tele- 

 scope or the naked eyes, but which afterwards announce them- 

 selves in the presence of thin clouds/'' &c. 



In his beautiful essay on Dew, Wells gives the true explana- 

 tion of the formation of ice in India, by ascribing the effect to 

 radiation. I think, however, his theory needs supplementing. 

 Given the same day-temperature here as at Benares, could we, 

 even in clear weather, obtain a sufficient fall of temperature to 

 produce ice ? I think not. The interception of the calorific rays 

 by our humid air would too much retard the chill. It is apparent, 

 from the descriptions given of the process, that a dry still air 

 was the most favourable for the formation of the ice. The nights 

 when it was formed in greatest abundance were those during 

 which the dew was not copious. The flat pans used in the pro- 

 cess were placed on dry straw, and if the straw became wetted it 

 was necessary to have it removed. Wells accounts for this by 

 saying that the wetted straw is more dense than the dry, and 

 hence more competent to transfer heat from the earth to the 

 basins. This may be to some extent true; but it is also certain 

 that the evaporation from the moist straw, by throwing over the 

 pans an atmosphere of aqueous vapour, would check the radia- 

 tion and thus tend to diminish the cold. 



Melloni, in his excellent paper "On the Nocturnal Radiation 

 of Bodies," gives a theory of the serein, or excessively fine rain 

 which sometimes falls in a clear sky a few moments after sunset. 

 Several authors, he says, attribute this effect to the cold resulting 

 from radiation of the air, during the fine season, immediately on 

 the departure of the sun. "But," writes Melloni, "as no fact 

 is yet known which distinctly proves the emissive power of pure 

 transparent elastic fluids, it appears to me more conformable to 

 the principles of natural philosophy to attribute this species of 

 rain to the radiation and subsequent condensation of a thin veil 

 of vesicular vapour distributed through the higher strata of the 

 atmosphere "*. Now, however, that the power of aqueous vapour 



* Taylor's Scientific Memoirs, vol. v. p. 551. 



