er present in the soil supplying carbonic acid,
it may facilitate their development; but it is not essential to
their growth. If we furnish the soil with ammonia, and the
phosphates, which are indispensable to the cerealia, with the
alkaline silicates, we have all the conditions necessary to ensure
an abundant harvest. The atmosphere is an inexhaustible store of
carbonic acid.

A no less favourable influence than that of lime is exercised upon
the soil of peaty land by the mere act of burning it: this greatly
enhances its fertility. We have not long been acquainted with the
remarkable change which the properties of clay undergo by burning.
The observation was first made in the process of analysing the clay
silicates. Many of these, in their natural state, are not acted on
by acids, but they become perfectly soluble if heated to redness
before the application of the acid. This property belongs to
potters' clay, pipe-clay, loam, and many different modifications of
clay in soils. In their natural state they may be boiled in
concentrated sulphuric acid, without sensible change; but if feebly
burned, as is done with the pipe-clay in many alum manufactories,
they dissolve in the acid with the greatest facility, the contained
silica being separated like jelly in a soluble state. Potters' clay
belongs to the most sterile kinds of soil, and yet it contains
within itself all the constituent elements essential to a most
luxurious growth of plants; but their mere presence is insufficient
to secure this end. The soil must be accessible to the atmosphere,
to its oxygen, to its carbonic acid; these must penetrate it, in
order to secure the conditions necessary to a happy and vigorous
development of the roots. The elements present must be brought into
that peculiar state of combination which will enable them to enter
into plants. Plastic clay is wanting in these properties; but they
are imparted to it by a feeble calcination.

At Hardwicke Court, near Gloucester, I have seen a garden (Mr.
Baker's) consisting of a stiff clay, which was perfectly sterile,
become by mere burning extremely fertile. The operation was extended
to a depth of three feet. This was an expensive process, certainly;
but it was effectual.

The great difference in the properties of burnt and unburnt clay is
illustrated by what is seen in brick houses, built in moist
situations. In the town of Flanders, for instance, where most
buildings are of brick, effloresen