ide 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 {58} 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.

[Illustration: 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 {59} 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 {60} 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.

[Illustration: Fig. 27. Bottle containi