500 
On the Food of Plants. 
other places on the continent of Europe and in the East, where 
granite is exposed at the surface, the progress of this decomposi- 
tion can be distinctly traced from the sound, unaltered felspar 
with its brilliant cleavage, to the dull earthy mass, so soft as to be 
cut with the utmost ease, although still showing its peculiar 
structure and the form of the crystals contained in it. It is from 
this source that all the fine white clay used in the manufacture of 
porcelain is obtained, by simply crushing the decomposed granite 
to powder and washing it over in a stream of water, whereby 
the coarser and heavier portions, the quartz and the mica, arc 
separated. 
Subjoined is the composition of one of the most celebrated 
clays of this description, which may be taken as an excellent type 
of the substance itself in its present form, namely, that employed 
in the Sevres porcelain works : — 
Silica , . . .48-8 
Alumina . . . 3*7 -3 
Potash . . . 2-5 
Water .... 11-4 
100-0 
The red and yellow clays of the secondary strata, which are 
alone of importance in connexion with agriculture, differ from 
that above described in containing a verv much larger proportion 
of silica, not as sand, but in an impalpable state, in intimate union 
with the other constituents, and a large quantity of oxide of iron. 
The latter sometimes equals and even surpasses in amount the 
alumina, and has probably been derived from the decomposition 
of hornblende or augite — minerals rich in oxide of iron, and 
abundantly contained in basaltic rocks. 
It must not be supposed that anything like a pure clay, even of 
the last kind, is ever the subject of tillage : the heaviest and 
stiffest clay soil contains probably, in most cases at least, half its 
weight of sandy matter, chiefly siliceous, easily separable by the 
process of washing; it is very surprising, indeed, how small a 
proportion of alumina suffices to confer great plasticity on such a 
substance. The most remarkable circumstance connected with 
such soils is the quantity of potash they appear to contain in an 
insoluble state, as one of the silicates of that base. In a soil of 
the kind mentioned, a stiff deep-red clay from one of the midland 
counties, the finely divided matter got by washing contained 
above 3^ per cent, of potash in this condition, a thing quite un- 
looked for. We have reason to think that this potash plays a 
very important part in the nutrition of plants, and that the in- 
soluble state is by no means without its use. 
2. Calcareous Soils. — These are derived from the disintegration 
