These busy little creatures are called micro-organisms because of their small size. But they are not all useful. Some can turn milk bad as we have already seen, and therefore all jugs and dishes must be kept clean lest any of them should be present. Others can cause disease. It has happened that a child who has cut its finger and has got some soil into the cut, and not washed it out at once, has been made very ill. You may sometimes notice sheep limping about in the fields, especially in damp fields; an organism gets into the foot and causes trouble.
SUMMARY. The soil is full of living things, some large like earth worms, others very small. Earthworms are very useful: they make burrows in the soil, thus allowing air and water to get in: they drag in leaves and they keep on covering the surface with soil from below. Besides these and the other large creatures, there are micro-organisms so small that they cannot be seen without a very good microscope: they live and breathe and require air, water and food. Some are very useful and change dead parts of plants or animals into valuable plant food. Almost anything that can be consumed by fire can be consumed by them. Others are harmful.
CHAPTER VIII
THE SOIL AND THE PLANT
Apparatus required.
Dry powdered soil, sand, clay, leaf mould, seeds. Six funnels, disks, stands and glass jars [3]. Six glass tubes about 1/2 in. diameter and 18 in. long [2]. Muslin, string, three beakers. Six lamp chimneys standing in tin lids [3]. Pot experiments (p. xiii), growing plant. Two test tubes fitted with split corks (Fig. 35).
If you have ever tried to grow a plant in a pot you must have discovered that it needs much attention if it is to be kept alive. It wants water or it withers; it must be kept warm enough or it is killed by cold; it has to be fed or it gets yellow and starved; also it needs fresh air and light. These five things are necessary for the plant: