A DROP OF WATER. 87 
very quickly, because the blades, being very thin, are 
almost all surface. In consequence of this they part 
with their heat more quickly than they can draw it 
up from the ground, and become cold. Now, the air 
lying just above the grass is full of invisible vapour, 
and the cold of the blades, as it touches them, chills 
the water-particles, and they are no longer able to hold 
apart, but are drawn together into drops on the sur- 
face of the leaves. 
We can easily make artificial dew for ourselves. I 
have here a bottle of ice which has been kept outside 
the window. When I bring it into the warm room a 
mist forms rapidly outside the bottle. This mist is 
composed of water-drops, drawn out of the air of the 
room, because the cold glass chilled the air all round 
it, so that it gave up its invisible water to form dew- 
drops. Just in this same way the cold blades of grass 
chill the air lying above them, and steal its vapour. 
But try the experiment, some night when a heavy 
dew is expected, of spreading a thin piece of muslin 
over some part of the grass, supporting it at the four 
corners with pieces of stick so that it forms an awn- 
ing. Though there may be plenty of dew on the 
grass all round, yet under this awning you will find 
scarcely any. The reason of this is that the muslin 
checks the heat-waves as they rise from the grass, 
and so the grass-blades are not chilled enough to draw 
together the water-drops on their surface. If you 
walk out early in the summer mornings and look at 
the fine cobwebs flung across the hedges, you will see 
plenty of drops on the cobwebs themselves sparkling 
like diamonds ; but underneath on the leaves there will 
