MR JOHN AITKEN ON DEW. 



39 



to the idea that the greater wetness of grass on dewy nights is owing to its 

 greater radiating power. The radiating powers of the two surfaces seem to be 

 practically the same; and if neither grass nor soil received heat from the ground, 

 the soil would cool lowest, because the grass in its natural condition would get 

 more heat from the passing air, on account of its surface being irregular and in 

 small pieces, as we know that small surfaces receive from this cause much more 

 heat, per unit of area, than larger ones, this being particularly the case when 

 there is wind. From which we see that the smallness of the blades in grass is 

 really an advantage, and prevents their surfaces being cooled by radiation so 

 much as they would be if they were larger. 



The number of substances tested for their radiating powers at night is not 

 so great as was hoped for, on account of the rare occurrence of evenings on 

 which work of this kind can be done in this climate; for not only must the sky 

 be free from passing clouds, in order that the amount of radiation may be as 

 constant as possible, but the air must also be very dry, in order that the dew- 

 point may be lower than the temperature of the cold radiating surfaces. 

 If vapour gets condensed on the radiating surfaces, the radiation from the film 

 of ice, or water, will interfere with the results. In making the experiments, 

 a large sheet of glass was generally exposed alongside of the radiation boxes, to 

 show if vapour was being deposited on the radiating surfaces. But even with 

 this precaution we cannot be sure we are testing the radiating powers of some 

 substances experimented on, because some kinds of matter have an affinity for 

 water, and condense vapour on their surfaces from unsaturated air. As an 

 example of uncertain results, I may mention a comparison made between salt and 

 sugar. These two substances have been found by other observers to radiate 

 very unequally at 100° C. — sugar radiating twice as much as salt. When tested 

 at night, they were found to radiate equally well; but as both substances have 

 an affinity for water, their surfaces would have a film of moisture over them, 

 which would increase the radiating power of the salt, and thus make the test 

 of no value. 



Among the few substances that have been found to radiate less heat at 

 night than a black surface is sulphur. On the night of the 7th December, when 

 the air was very dry and the glass plate kept undewed, the following read- 

 ings were taken : — 



Temperature of air. 



Temperature of black surface. 



Sulphur. 



27° 

 26° 



21° 

 19° 



23° 

 21°-25 



The sulphur was sifted over the one thermometer box and the other left bare. 



