obtained at Harpenden:--

Amount Percentage
Colour before of loss on Colour of
burning smouldering burning residue

Leaf mould dark brown much 78.3 light grey

Mould from
dead tree black much 60.6 light grey

Soil from
wood dark brown less 43.4[1] white

Soil from
garden almost black less 10.1 red

Soil from
field brownish still less 5.4 red

Subsoil red none 2.0 red

The mould nearly all burns away and its dark colour entirely goes, so
also does the dark colour of the soil.

Our supposition explains why, in the case of soils, the less the
blackness, the less the loss on burning. If the {36} brown or black
combustible part is really mould formed by the decay of plant roots,
etc., then we should expect that as the percentage of mould in the soil
increased, so its blackness would increase and its loss on burning
would become greater. This actually happens.

This, then, is our idea. We suppose that the plants that have lived in
past years have decayed to form a black material like leaf mould which
stops in the soil, giving it a darkish colour. The more mould there
is, the darker the colour of the soil. We know that along with this
decay there is a great deal of shrinkage. As the black material is
formed from the plant, it only extends as far into the soil as the
plant roots go, so that there is a sharp change in colour about 6
inches below the surface (see also p. 2). Like the plant the black
material all burns away when the soil is heated sufficiently.

Thus we can explain all the facts we have observed, and in what seems a
very likely way. This does not show that our supposition is correct,
but only that it is useful. When you come to study science subjects
you will find such suppositions, or hypotheses as they are called, are
frequently used so long as they are found to be helpful. In our
present case we could only get absolute proof that the black
combustible part of the soil really arose from the decay of plants by
watching the process of soil formation. We shall turn later to this
subject.

The black material is known as humus. Farmers and gardeners like a
black soil containing a good deal of