Some hailstones are of great size; you will find it interesting to examine them. If you do this, notice the rings of clear and clouded ice that are often to be seen. These are caused when the hail, after forming, settles to a place where it melts a little, then is lifted again by another current, growing larger by the addition of more vapor. This continues until finally the ice ball sinks to the ground. There is thunder and lightning in such storms. Few things in nature are grander than these, and those who will watch the lightning flash will see many beautiful and interesting sights (Fig. 16). Sometimes the flash goes from cloud to cloud, again from the cloud to the ground. No one knows exactly why the lightning comes; but we do know that it is an electric spark, something like that which one can often see pass from the trolley to the wire of an electric car line. The main difference is that the spark in a thunder storm is a powerful lightning bolt that passes over a space of thousands of feet and often does great damage where it strikes. The thunder is a sound which may be compared to the crack heard when a spark passes from the trolley, though of course the noise is very much louder. The crack of the lightning echoes and reverberates among the clouds, often changing to a great rumble; but this rumbling is mainly caused by the echo, the sound from the lightning being a loud crack or crash like that which we sometimes hear when the lightning strikes near by. Some of the vapor of the air, on condensing, gathers on solid objects like grass, or glass; but some, as fog, floats about in the air. Really this, too, is often gathered around solid objects. Floating about in the air are innumerable bits of "dust" which you can see dancing about in the sunlight when a sunbeam enters a dark room. Some of these "dust" particles are actual dust from the road, but much of it is something else, as the pollen of plants, microbes, and the solid bits produced by the burning of wood or coal. Each bit serves as a tiny nucleus on which the vapor condenses; and so the very "dust" in the air aids in the formation of rain by giving something solid around which the liquid can gather. The great amount of dust in the air near the great city of London is believed to be one of the causes for the frequent fogs of that city. That there is dust in the air, and that the rain removes it, is often proved when a dull hazy air is changed to a clear, bright air by a summer shower. Watch to find instances of this. Indeed, after such a hazy day, when the rain drops first begin to fall, if you will let a few drops fall upon a sheet of clean white paper, and then dry it, you will find the paper discolored by the dust that the rain brought with it. So the rain purifies the air by removing from it the solids that are floating in it.