The Science of the Soil 99 



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 ot 

 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 com- 

 posed 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 atmos- 

 pheric 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 3'30 Ib. to 4'84 Ib. 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 25'3 Ib. of common salt, and 17 '41 sulphuric acid per acre, have also been 

 found in the annual rainfall at Rothamsted. 



Frost. This is a powerful agent in producing a powdery soil. When 

 water in the soil or in the crevices of hard rocks becomes frozen, it swells 

 up and occupies more space. In cultivated soils the particles are easily 

 pushed asunder, and are often raised up a good deal. In the case of rocks 

 the force exerted by the swelling ice is so tremendous that it is irresistible. 

 The rocks are therefore forced apart, splitting along the line of least resist- 

 ance, and when a thaw sets in great pieces are broken off. Fresh surfaces 

 are thus exposed to the weather, and the process of disintegration goes 

 steadily on. 



Heat. This has the effect of warming the soil, and water in it, causing 

 both to expand and one of them (water) to evaporate. As water is driven 

 out of the soil in this way air enters, and thus makes the soil warmer than 

 it was before. As the temperature of the air varies greatly between mid- 

 day and midnight, sometimes as much as 60 F., one can readily imagine 

 a kind of opening and closing or expanding and contracting movement 

 going on continually on the crust of the earth, much in the same way 

 that the tides rise and fall, although not so conspicuous. This variation of 



