Absorption of Heat by Leaves. 



341 



be determined. The essential parts consisted of two concen- 

 tric boxes made of sheet tin (O I, fig. 1), drawn with their 

 sides nearest the observer torn away in order to show the 



Fiff. 1. 



internal parts. The two boxes were connected by means of 

 the tin tube (T) soldered into a perforation in the middle of a 

 side of each. 



The inner box which contained the thermopile was mounted 

 upon wooden legs, and running out from its top and also 

 through the top of the larger box was the tube (t). The wires 

 which connected the thermopile with the galvanometer (G), 

 ran through this tube (t), as did also a very delicate thermome- 

 ter giving the temperature of the air surrounding the thermo- 

 pile. The sliding metallic screen (S) could be dropped down 

 over the opening of the tube T, thus shielding the thermopile 

 from all radiant heat. The capacity of the larger box was 

 about 850 cubic inches, that of the inner one being 150. The 

 outer box was jacketed with a two-inch layer of cotton wool. 



By filling the space between the two concentric boxes with 

 cracked ice, the temperature of the thermopile, and of the air 

 in the inner box which surrounded it, could be lowered, so that 

 one might determine the radiation from leaves at the tempera- 

 ture of the air of the laboratory. This apparatus possessed also 

 the very great advantage that there were no air currents im- 

 pinging upon the thermopile, and as a consequence its readings 

 were remarkably concordant. 



. Many determinations both of radiation and absorption of 

 heat were made by the aid of this apparatus, but in the excel- 

 lent constant-temperature room of the Jefferson Physical 

 Laboratory it was found unnecessary to inclose the thermopile, 

 and therefore it was mounted upon an ordinary Melloni's bench. 



