344 A. G. Mayer — Radiation and 



Results of the Experiments. 



In conducting the experiments upon radiation it was deemed 

 wise to select leaves of widely different genera of plants; 

 accordingly the leaves of a few forest trees, bushes, weeds liv- 

 ing in both sunny and shady places, aquatic plants, cultivated 

 plants and grasses were chosen. The results of a series of ex- 

 periments upon the following leaves demonstrated that the 

 coefficient of radiation of dark heat from both their upper and 

 lower surfaces was exactly the same as that of lampblack. 



The leaves tested were elm, oak, maple, horse chestnut, bass- 

 wood (Tilia Americana), silver poplar, beech, lilac, mullein 

 ( Yerbascum thapsus), plantain (P. major), lilly pads (Nujyhar 

 advena), cultivated grape, blackberry and clover. A single 

 interesting exception, however, was discovered. The upper 

 surface of burdock leaves (Arctium lappa) radiates exactly as 

 do all other leaves, but the under surface radiates only 81 per 

 cent of this amount. 



These leaves are very broad and thick, and as they lie for 

 the most part spread out horizontally very near the ground, 

 the under surface is largely shaded from the sun's rays, and 

 therefore receives but little direct heat. Moreover it is prob- 

 able that being so near to the ground less dew would form upon 

 the under surface than upon the upper. Both of these causes 

 would combine, as we shall see later, to make it very advanta- 

 geous to such leaves to possess a poorly radiating lower surface. 



Of course, as leaves radiate exactly as lampblacked surfaces, 

 they also possess correspondingly good absorbing surfaces. 

 Numerous experiments were made by the method already 

 described to ascertain the effect of dew upon the radiation of 

 leaves. If we call the radiation from a dry leaf 100, that from 

 a leaf covered by a thin film of dew is about 78, and if the 

 dew stand out in beads over the surface the radiation is reduced 

 to 66. 



A polished tin surface which only radiated 11 per cent as 

 much heat as a lampblacked surface, radiated 96*8 per cent of 

 the lampblacked one when both were covered with beads of 

 dew. The coefficient of radiation of the lampblacked surface 

 was lowered, and that of the polished one raised until the radia- 

 tion was nearly the same from both. So potent is a film of 

 dew in altering the nature of a radiating surface. 



Upon the importance of this remarkable fact we need hardly 

 dwell. The surface of leaves being one of the best known 

 radiators of heat, is therefore an equally good absorber of that 

 heat. If then we imagine a forest in the tropics where the 

 days and nights are of almost equal length the whole year 

 round, very much of that heat which had been absorbed by the 

 leaves during the day, would be lost by their great radiation 



