84 THE FAIRY-LAND OF SCIENCE 
thin, arc 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 
surface of the leaves. 
We can easily make artificial dew for ourselves. 1 
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 
coiners with pieces of stick so that it forms an 
awning. 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 sec plenty of drops on the cobwebs themselves 
sparkling like diamonds ; but underneath on the 
leaves there will be none, for even the delicate cobweb 
