we have the larger but still invisible particles of vapour hanging about among the air-atoms.  Now, these watery particles, although they are very few (only about one twenty-fifth part of the whole atmosphere), do hinder the sun-waves.  For they are very greedy of heat, and though the light-waves pass easily through them, they catch the heat-waves and use them to help themselves to expand.  And so, when there is invisible vapour in the air, the sunbeams come to us deprived of some of their heat-waves, and we can remain in the sunshine without suffering from the heat.

This is how the water-vapour shields us by day, but by night it is still more useful.  During the day our earth and the air near it have been storing up the heat which has been poured down on them, and at night, when the sun goes down, all this heat begins to escape again.  Now, if there were no vapour in the air, this heat would rush back into space so rapidly that the ground would become cold and frozen even on a summer’s night, and all but the most hardy plants would die.  But the vapour which formed a veil against the sun in the day, now forms a still more powerful veil against the escape of the heat by night.  It shuts in the heat-waves, and only allows them to make their way slowly upwards from the earth — thus producing for us the soft, balmy nights of summer and preventing all life being destroyed in the winter.

Perhaps you would scarcely imagine at first that it is this screen of vapour which determines whether or not we shall have dew upon the ground.  Have you ever thought why dew forms, or what power has been at work scattering the sparkling drops upon the grass?  Picture to yourself that it has been a very hot summer’s day, and the ground and the grass have been well warmed, and that the sun goes down in a clear sky without any clouds.  At once the heat-waves which have been stored up in the ground, bound back into the air, and here some are greedily absorbed by the vapour, while others make their way slowly upwards.  The grass, especially, gives out these heat-waves 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 surface 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.