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GARDENING AS A SCIENCE. 
and without sufficient observation, has been clearly shown by several philosophers, 
among whom the great Liebig stands pre-eminent. He has demonstrated the truth 
of that which the experience of every observant gardener must also have confirmed 
^ — namely, tliat plants^ so far from exhaustmg a soil^ add manuring matter to it. 
To prove this let us go back to first principles, and inquire — what is earth ? 
Loam is a term of familiar occurrence, though it admits of so many variations 
as scarcely to be intelligible. However, we may safely assume that when a large 
proportion of siliceous sand is naturally united with a less proportion of pure clay 
or alumine, and with certain small quantities of chalk and oxide of iron, the earth 
resulting from such a combination is, strictly speaking, a loam. But what is the 
origin of such an earth, and wiience was it derived ? To this inquiry we safely 
answer that — the formation of every particle of laborable earth which now 
covers the surface of the globe is a process of the disintegration of rocks. On 
this great primitive operation of natural chemistry, Davy thus expressed himself 
in his fourth lecture addressed to the late board of agriculture. 
" It is easy to form an idea of the manner in which rocks are converted into 
soils, by referring to the instance of soft granite, or porcelain granite. This sub- 
stance consists of three ingredients, viz., quartz, feldspar, and mica. The quartz is 
almost pure siliceous earth, in a crystalline form. The feldspar and mica are very 
compounded substances ; both contain silica, alumina, and oxide of iron : in the 
feldspar there is usually lime and potassa ; in the mica, lime and magnesia." 
In this short passage — which, thirty years ago, was scarcely listened to by the 
agricultural body — we now find the base, the substance, of all the modern 
acknowledged theory ; and in it we obtain the key to the true philosophy of horti- 
culture. In quartz or rock crystal, we have the origin of sand — a substance which 
breaks up and modifies the texture of loam, yet in itself is perfectly, or all but, 
insoluble ; silica is an oxide of crystal or pure flint. Alumina — pure clay and 
oxide of iron, are also insoluble substances ; the former confers tenacity on sandy 
loams, and renders them plastic ; the latter gives colour in all its shades, from pale 
buff to deep ochreous red. Lime, as it exists in earth, is found combined with 
carbonic acid, and then is neither more nor less than insoluble chalk. As, therefore, 
the earthy ingredients of loams are portions of rocks broken up by natural agents, no 
particle of them in their pure state can contribute by any possibility to the organic 
structure of a vegetable. But the case is widely different when we discover potash 
among the ingredients of the disintegrated rocks ; for in it we discover the base of 
all those combinations with vegetable acids which are formed in the cellular tissue 
of a great variety of plants most valuable to man and animals. Potash also is a 
solvent of silica, and by its agency that otherwise intractable substance is conveyed 
into the structure of the cereal grasses, and of a variety of other plants. "When," 
Davy adds, "a granitic rock of this kind" (soft granite containing quartz, feldspar, 
and mica.) " has been long exposed to the influence of air and water, the lime and 
the potassa contained in its constituent parts are acted upon by water or carbonic 
