Absorption of Heat by Leaves. 343 



ture, as nearly as possible, as much higher than the tempera- 

 ture of the room as the melting ice was lower than that tempera- 

 ture. A five minute exposure was again given and the reading 

 of the galvanometer taken. From the results of these two 

 experiments it became possible to compare the radiation of the 

 dewed leaf with the heated one. For example: let t l rep- 

 resent the temperature of the air of the room, supposed con- 

 stant, t that of the melting ice, T that of the heated water 

 in the Leslie cube, d the deflection produced by the dewed 

 leaf, and d 1 that caused by the dry heated leaf. Then assum- 

 ing the deflections of the galvanometer to be proportional to 

 the radiation's we would have 



Instead of x coming out equal to d Q as it would do if the 

 coefficient of radiation of the dewed surface were equal to that 

 of the heated surface, it was always greater than d Q ; thus prov- 

 ing that the coefficient of radiation of the leaf was lowered by 

 the dew which had collected upon it. 



III. Absorption of Heat by Leaves. — In performing these 

 experiments the side of the Leslie cube was lampblacked, and 

 the cube filled with water which was kept gently boiling. The 

 reading of the galvanometer produced by five minutes ex- 

 posure to the hot lampblacked surface of the Leslie cube was 

 taken and compared with that produced when the heat from 

 the Leslie cube was obliged to pass through a leaf, placed over 

 a diaphragm, between the cube and the thermopile. Great care 

 was taken to insure that all the heat which reached the thermo- 

 pile had passed through the leaf. From the results of these 

 two experiments it became a simple matter of ratio to calculate 

 the percentage of heat absorbed by the leaf. Thus : Let D 

 be the deflection produced when the heat passed uninterrupt- 

 edly from the cube to the thermopile, and d that produced 

 when the path of the heat was intercepted by a leaf, placed 

 between the cube and the thermopile. Then the percentage 

 of heat transmitted by the leaf would evidently be given by 



the expression — -^- = per cent transmitted. To find the 



per cent absorbed we have merely to subtract the per cent 

 transmitted from 100. By causing the heat to pass through 

 successively, one, two, and three leaves placed in its path, some 

 facts relative to the selective absorption of leaves were obtained. 

 By determining the heat-absorbing ability of a fresh green 

 leaf, and then dissolving out its chlorophyl in alcohol or ether 

 and testing the same leaf again, it became possible to ascertain 

 how much of the absorption was due to the chlorophyl of the 

 leaf. 



