s possible during the process of burning, direct
experiment having shown that when this precaution is not observed
another change occurs, whereby the potash, which at low temperatures
becomes soluble, passes again into an insoluble state. A part of the
beneficial effect is no doubt also due to the change produced in the
physical characters of the clay by burning, which makes it lighter and
more friable, and by mixture with the unburnt clay ameliorates the
whole. This improvement in the physical characters of the clay also
requires that it shall be burnt with as low a heat as possible; for if
it rises too high, the clay coheres into hard masses which cannot again
be reduced to powder, and the success of the operation of burning may
always be judged of by the readiness with which it falls into a uniform
friable powder.

The improvement of peat by burning has been practised to some extent in
Scotland, though less frequently of late years than formerly; but it is
still the principal method of reclaiming peat soils in many countries,
and particularly in Finland, where large breadths of land have been
brought into profitable cultivation by means of it. The _modus operandi_
of burning peat is very simple; it acts by diminishing the superabundant
quantity of humus or other organic matters, which, in the previous
section we have seen to be so injurious to the fertility of the soil. It
_may_ act also in the same way as it does on clay, by making part of the
inorganic constituents more really soluble, although it is not probable
that its effect in this way can be very marked. Its chief action is
certainly by destroying the organic matters, and by thus improving the
physical character of the peat, and causing it to absorb and retain a
smaller quantity of water than it naturally does. For this reason it is
that it proves successful only on thin peat bogs, for if they be deep,
the inorganic matters soon sink into the lower part, and the surface
relapses into its old state of infertility. It is probably for this
reason that the practice has been so much abandoned in Scotland, more
especially as other and more economical modes of treating peat soils
have come into use.

_Warping._--This name has been given to a method of improving soils by
causing the water of rivers to deposit the mud it carries in suspension
upon them, and which has been largely practised in the low lying lands
of Lincoln and Yorkshire, where it was introduced about