A DROP OF WATER. 85 
has been strong enough to shut in the heat-waves and 
keep the leaves warm. 
Again, if you walk off the grass on to the gravel 
path, you find no dew there. Why is this ? Because 
the stones of the gravel can draw up heat from the 
earth below as fast as they give it out, and so they 
are never cold enough to chill the air which touches 
them. On a cloudy night also you will often find 
little or no dew even on the grass. The reason of 
this is that the clouds give back heat to the earth, 
and so the grass does not become chilled enough to 
draw the water-drops together on its surface. But 
after a hot, dry day, when the plants are thirsty and 
there is little hope of rain to refresh them, then they 
are able in the evening to draw the little drops from 
the air and drink them in before the rising sun comes 
again to carry them away. 
But our rain-drop undergoes other changes more 
strange than these. Till now we have been imagining 
it to travel only where the temperature is moderate 
enough for it to remain in a liquid state as water. 
But suppose that when it is drawn up into the air it 
meets with such a cold blast as to bring it to the 
freezing point. If it falls into this blast when it is 
already a drop, then it will freeze into a hailstone, 
and often on a hot summer's day we may have a 
severe hailstorm, because the rain-drops have crossed 
a bitterly cold wind as they were falling, and have 
been frozen into round drops of ice. 
But if the water-vapour reaches the freezing air 
while it is still an invisible gas, and before it has 
