Whenever rain falls it brings down a small quantity of carbonic acid gas from the atmosphere with it. It falls on the earth and washes away fine particles from the hill and mountain sides into the plains and valleys. The mountain stream often becomes a torrent, and tears away great boulders, churning one against another, until they become rounded and worn away. The streams become rivers and eventually flow into the sea, and on their course they bring down masses of sand and silt, and deposit it in the lowlands. Many soils have been made in this way, and are said to be alluvial, because they have been washed on to a soil perhaps of a totally different nature.

Running water not only performs this work, but also gradually dis solves particles of rocks into a fine powder and wears away the face of them. This is called denudation. Water also fills the chinks and crevices in the rocks and carries out the same work slowly but surely. Being composed of the gases oxygen and hydrogen, and having a little carbonic acid in it, certain combinations with minerals and metals take place. What applies to rain and river water applies also to dew. If a piece of steel or iron is left in the open air for a night it soons turns rusty. This shows that the oxygen in the dew or rain has eaten into or combined with the steel or iron and produced rust. This eating away of metals by atmospheric gases is constantly going on, and in a few months a bright knife will be almost worn away by their action.

Rain is not merely a combination of the gases oxygen and hydrogen; it also contains small quantities of nitrogen and ammonia, chlorine, and sulphuric acid. From the Rothamsted experiments it has been proved that from 330 lb. to 4.84 lb. of nitrogen and ammonia is distributed over an acre of ground during the year; and it sometimes happens that a small annual rainfall will produce a larger supply of these gases. Chlorine equal to 253 lb. of common salt, and 17.41 sulphuric acid per acre, have also been found in the annual rainfall at Rothamsted.