Fig. 3. Columns showing what 100 parts of dried soil and subsoil were made of
Summary. The experiments made so far have taught us these facts:—
1. Soil contains water, grit or sand, silt, clay, a part that burns away, and some white chalky specks.
2. The top layer of soil to a depth of about eight inches is different from the soil lying below, which is called the subsoil. It is less sticky, easier to dig, and darker in colour. It contains more of the material that burns away, but less clay than the subsoil.
3. When soil is dried it is not sticky but hard or crumbly; as soon as it is moistened it changes back to what it was before. But when soil is burnt it completely alters and can no longer be changed back again.
[1] See p. xiv for explanation of the figures in square brackets.
CHAPTER II
MORE ABOUT THE CLAY
Apparatus required.
Clay, about 6 lbs.; a little dried, powdered clay; sand, about 6 lbs. Six glass jars or cylinders [2]. Six beakers [1]. Six egg-cups [1]. Six funnels and stands [2]. Six perforated glass or tin disks [2]. Six glass tubes [2]. Two tubulated bottles fitted with corks. Some seeds. Six small jars about 2 in. x 1 in. [2]. Bricks. The apparatus in Fig. 9. Pestle and mortar.
We have seen in the last chapter that clay will float in water and only slowly settles down. Is this because clay is lighter than water? Probably not, because a lump of clay seems very heavy. Further, if we put a small ball of clay into water it at once sinks to the bottom. Only when we rub the clay between our fingers or work it with a stick—in other words, when we break the ball into very tiny pieces—can we get it to float again. We therefore conclude that the clay floated in our jars (p. 6) for so long not because it was lighter than water, but because the pieces were so small.
Clay is exceedingly useful because of its stickiness. Dig up some clay, if there is any in your garden, or procure some from a brick works. You can mould it into any shape you like, and the purer the clay the better it acts. Enormous quantities of clay are used for making bricks. Make some model bricks about an inch long and half an inch in width and depth, also make a small basin of about the same size, then set them aside for a week in a warm, dry place. They still keep their shape; even if a crack has appeared the pieces stick together and do not crumble to a powder.
If you now measure with a ruler any of the bricks that have not cracked, you will find that they have shrunk a little and are no longer quite an inch long. This fact is well known to brickmakers; the moulds in which they make the bricks are larger than the brick is wanted to be. But what would happen if instead of a piece of clay one inch long you had a whole field of clay? Would that shrink also, and, if so, what would the field look like? We can answer this question in two ways; we may make a model of a field and let it dry, and we can pay a visit to a clay meadow after some hot, dry weather in summer. The model can be made by kneading clay up under water and then rolling it out on some cardboard or wood as if it were a piece of pastry. Cut it into a square and draw lines on the cardboard right at the edges of the clay. Then put it into a dry warm place and leave for some days. Fig. 4 is a picture of such a model after a week's drying. The clay has shrunk away from the marks, but it has also shrunk all over and has cracked. If you get an opportunity of walking over a clay field during a dry summer, you will find similar but much larger cracks, some of which may be two or three inches wide, or even more. Sometimes the cracking is so bad that the roots of plants or of trees are torn by it, and even buildings, in some instances, have suffered through their foundations shrinking away. We can now understand why some of our model bricks cracked. The cracks were caused by the shrinkage just as happens with our model field. As soon as the clay becomes wet it swells again. A very pretty experiment can be made to show this. Fill a glass tube or an egg-cup with dry powdered clay, scrape the surface level with a ruler, and then stand in a glass jar full of rain water so that the whole is completely covered. After a short time the clay begins to swell and forces its way out of the egg-cup as shown in Fig. 5, falling over the side and making quite a little shower. In exactly the same way the ground swells after heavy rain and rises a little, then it falls again and cracks when it becomes dry. Darwin records some careful measurements in a book called Earthworms and Vegetable Mould—"a large flat stone laid on the surface of a field sank 3.33 millimetres[1] whilst the weather was dry between May 9th and June 13th, and rose 1.91 millimetres between September 7th and 19th of the same year, much rain having fallen during the latter part of this time. During frosts and thaws the movements were twice as great."
