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Proceedings of the Royal Society 
nights were experimented on. They were connected by means of an 
india-rubber tube with a head of water of about 1 metre, and the leaf 
surrounded with saturated air. All were found to exude a watery 
liquid after being subjected to pressure for some hours, and a broccoli 
leaf got studded all along its edge with drops, and presented exactly 
the same appearance it did on dewy nights. A stem of grass was 
also found to exude at the tips of one or two blades when pressure 
was applied. 
The question as to whether these drops are really exuded by the 
plant by internal pressure, as in the above experiments, or 
are produced in some other way, is then considered. The tip 
of a blade of grass was put under conditions in which it could not 
extract moisture from the surrounding air, and, as the drop grew as 
rapidly under these conditions as did those on the unprotected 
blades, it is concluded that these drops are really exuded by the 
plant. Grass was found to get “ dewed ” in air not quite saturated. 
On many nights no true dew is formed, and nothing but these 
exuded drops appear on the grass ; and on all nights when vegeta- 
tion is active, these drops appear before the true dew, and if the 
radiation is strong enough and the supply of vapour sufficient, true 
dew makes its appearance, and now the plants get equally wet all 
over, in the same manner as dead matter. The difference between 
true dew on grass, and these exuded drops, can be detected at a 
glance. The drops are always exuded at a point near the tip of 
the blade, and form a drop of some size, while true dew is distri- 
buted all over the blade. The exuded liquid forms a large diamond- 
like drop, while the dew coats the blade with a pearly lustre. 
Towards the end of the paper the radiating powers of different 
surfaces at night are considered. By means of the radiation-thermo- 
meter, described by the author in a previous paper, the radiating 
powers of different surfaces were compared. Black and white cloths 
were found to radiate equally well : soil and grass were also almost 
exactly equal to each other. Lamp-black was equal to whiting. 
Sulphur was about 2/3rds of black paint, and polished tin about 
1 /7th of black paint. Snow in the shade on a bright day was at 
midday 7° colder than the air, while a black surface at the same 
time was only 4° colder. This difference diminished as the sun got 
lower, and at night both radiated almost equally well. In the con- 
cluding pages of the paper some less important subjects are considered. 
