THE USES OF LIME IN AGRICULTURE AND HORTICULTURE. 237 
in contact with the air it combines with oxygen and becomes calcium 
oxide (CaO). This is the caustic or burnt lime of commerce ; it is still 
very reactive, or, as the chemist expresses it, a " very active base," and 
when exposed to the air absorbs water, or " slakes," as it is termed, and 
becomes CaH 3 0 3 — that is, calcium hydrate or slaked lime. This 
again is still a reactive compound, and under the influence of the 
atmosphere will absorb carbonic acid gas (C0 3 ), forming, as is shown on 
the chart, CaC0 3 : that is, calcium carbonate, limestone, or chalk. This 
description of the chemical processes leading from the metal calcium 
to the chalk or limestone with which we are all familiar is not intended 
to be taken as chemically exact, as the processes are not as simple as 
they are therein shown, but it is sufficiently accurate for our purpose. 
Thus it is only as calcium carbonate (CaC0 3 ) that lime exists in 
nature, forming whole mountain-chains of limestone, chalk, marble, 
&c, and comprising altogether approximately one-sixth of the earth's 
crust. We are therefore not dealing with a rare or unobtainable sub- 
stance, but one that occurs in large and well distributed quantities 
in the British Isles. Many of these deposits, such as the latest (geo- 
logically) and purest soft chalks, are made up of the microscopic 
remains of minute animals ; the harder limestones, such as the sili- 
ceous and argillaceous deposits, are of older formation. 
Lime is the chief constituent of coral, and of the shells of birds' eggs, 
and of molluscs, and is also found in bones. 
Commercial Preparation. — Let us now turn to the commercial 
preparation of lime. As it occurs only in the form of calcium carbo- 
nate, or limestone, it is obvious that the other forms in which we are 
accustomed to use it must be obtained from this. We must mention 
in passing that lime is now freely used in its natural form, being merely 
ground to a fine powder and put on the market as " ground carbonate 
of lime." 
The preparation of the other forms of lime consists in the first 
place of burning the natural stone in kilns by means of coal fuel ; as 
a result of this process carbonic acid gas is driven off and the caustic 
or burnt lime (CaO) is formed. This is commonly known as lump, 
agricultural, or " through " lime, and is generally put on the market 
in three grades, viz. : — 
(1) Hand-picked lime, which is composed of a selection of lumps 
of the pure burnt lime, and consequently attains a high percentage of 
purity. 
(2) Nutty small lime, which is the screenings from the hand-picked ; 
and 
(3) Lime ashes, the residue from the burning, which is cleared 
from the bottom of the kilns. 
It is obvious that (1) is the purest, that obtained from the best 
deposits sometimes containing as much as 98 per cent. CaO ; (2) is not 
quite so valuable, as it will contain a certain proportion of the natural 
impurities of the original stone ; and (3) will often contain about 30 per 
cent, of ashes. Burnt lime is also ground and sent out as ground 
