342 A. G. Mayer — Radiation and 



Of course, as the thermopile was then at the temperature of 

 the air of the room it became necessary to raise the tempera- 

 ture of the leaves which radiated to it. It was found, how- 

 ever, by numerous experiments, that the temperature of the 

 leaves might be raised to 45° C. without altering in the least 

 the coefficient of radiation from their surfaces. 



Methods of Experimenting. 



I. Radiation of Leaves: — Two leaves of the same species 

 of plant and as nearly alike as possible were procured, and 

 each was glued, flat, upon one of the polished tin sides 

 of the Leslie cube (L, fig. 1). One of the leaves was then 

 painted over with lampblack in alcohol, which caused it to 

 present a dull, dead black appearance. The Leslie cube was 

 then filled with water which was heated to about 40° C, at 

 which temperature it was maintained constantly by means of a 

 low luminous flame of a Bunsen burner. A side of the cube 

 bearing a leaf was turned so as to face the thermopile, and 

 after the temperature of the water had become steady, the 

 falling screen (F, fig. 1) was lowered, the slider S raised, and 

 the deflection of the galvanometer after five minutes exposure 

 to the radiant heat carefully observed. An exactly similar ex- 

 periment was then gone through with, upon the lampblacked 

 leaf, in order that the radiation from the green leaf might be 

 accurately compared with that from a similar surface of lamp- 

 black. Great care was taken in these experiments that the 

 thermopile cooled completely, so as to cause the galvanometer 

 to return to its zero reading before another experiment was 

 performed. As the deflections of the galvanometer were small 

 the radiation was assumed to be proportional to the deflection 

 produced. 



II. The Effect of Dew upon the Radiation of Leaves. — As 

 a copious film of dew commonly forms upon the leaves of 

 plants during summer nights, it occurred to me to ascertain 

 whether their radiating ability was thereby altered to any per- 

 ceptible degree. 



The experiments were conducted as follows : — A leaf was 

 glued to the polished side of the Leslie cube as in experiments 

 upon radiation. The cube was then filled with finely cracked 

 ice. This soon caused a film of dew to form upon the surface 

 of the cooled leaf. The temperature of the air of the room 

 and of the melting ice in the cube was then determined. The 

 leaf was placed facing the thermopile, the shielding screens 

 removed and the deflection of the galvanometer after five 

 minutes exposure observed. The Leslie cube was then filled 

 with water which was heated up and maintained at a tempera- 



