All these things are very useful and so a gardener should never want to kill worms. The great naturalist, Darwin, spent a long time in studying earthworms at his home in Kent and wrote a very interesting book about them, called Earthworms and Vegetable Mould. He shows that each year worms bring up about 1/50th of an inch of soil, so that if you laid a penny on the soil now and no one took it, in 50 years it might be covered with an inch of soil. Pavements that were on the surface when the Romans occupied Britain are now covered with a thick layer of soil.
Fig. 25. Fresh soil turns milk bad, but baked soil does not
But besides these there are some living things too small to see, that have only been found by careful experiments, but you can easily repeat some of these experiments yourselves. Divide a little rich garden soil into two parts and bake one in the kitchen oven on a patty tin. Pour a little milk into each of two small flasks, stop up with cotton wool (see Fig. 25) and boil for a few minutes very carefully so that the milk does not boil over, then allow to cool. Next carefully take out the stopper from one of the flasks and drop in a little of the baked soil, label the flask "baked soil" and put back the stopper. Into the other flask drop a little of the untouched soil and label it; leave both flasks in a warm place till the next day. Carefully open the stoppers and smell the milk: the baked soil has done nothing and the milk smells perfectly sweet; the unbaked soil, on the other hand, has made the milk bad and it smells like cheese. If you have a good microscope you can go further: look at a drop of the liquid from each flask and you find in each case the round fat globules of the milk, but the bad milk contains in addition some tiny creatures, looking like very short pins, darting in and out among the fat globules. These living things must have come from the unbaked soil or they would have been present in both flasks: they must also have been killed by baking in the oven.
Fig. 26. Soils contain tiny things that grow on gelatine
Another experiment is easy but takes a little longer to show. Mix two sheets of leaf gelatine with a quarter of a pint of boiling water, pour into each of three saucers, and cover over with plates. Then stir up some baked soil in a cup half full of cold boiled water, and quickly put a teaspoonful of the liquid into a second cup, also half full of cold boiled water. Stir quickly and put a spoonful on to the jelly, tilting it about so that it covers the whole surface and label the saucer "baked soil." Do the same with the "unbaked soil," labelling the saucer; leave the third jelly alone and label it "untouched." Cover all three with plates and leave in a warm place. After a day or so little specks begin to appear on the jelly containing the unbaked soil, but not on the others (Fig. 26); they grow larger, and before long they change the jelly to a liquid. The other jellies show very few specks and are little altered. These creatures making the specks came from the soil because so few are found on the jelly alone; they were killed in the baking and so do not occur on the baked soil jelly.