How does rain purify the air?
By washing away many things that would, if left, decay and make the air impure and unhealthy.
When the air is cooled below thirty-two degrees, what becomes of the moisture in it?
The moisture in it is frozen.
What is this frozen moisture in the air called?
It is called snow.
What are the particles of snow called?
They are called snow-flakes.
Are these snow-flakes all alike?
No; they vary very much in size and shape.