322 
THE TROPICAL 
AGRICULTURIST. [October r, i88r.' 
Composition of Corals, Shell-sands, and Marls. — 
1. Corals, as they are gathered fresh from tbe sea on 
the Irii-h (Bantry Bay) and other coasts, contain, be- 
sides calcic carbonate, a small percentage of calcic 
phosphate, and sometimes not less than 14 per cent of 
animal matter. This animal matter adds considerably 
to the fertilising value of coral-sand, when laid upon 
the land in a recent state, or when made into 
compost. 
2. Shell-sand consists of the fragments of broken 
•shells of various sizes, mixed with a variable propor- 
tion of sea-sand. It contains less animal matter than 
the recent corals, and its value is diminished by the 
admixture of sand, which varies from 20 to 70 per cent 
of the whole weight. On the shores of many of the 
Western Islands, shell-sand is found in large quanti- 
ties, and is extensively and beneficially applied, 
especially to the hillside pastures and to peaty 
soils. 
3. Marls consist of calcic carbonate— generally the 
fragments of shells— mixed with sand, clay, or peat, in 
various proportions. Thpy contain from 5 to as much 
as 80 or 90 per cent of calcic carbonate, and are con- 
sidered more or less rich and valuable for agricultural 
purposes as the proportion of lime increases. They are 
formed, lor the most part, from accumulations of shells 
at the bottom of fresh -water lakes which have gradu- 
ally been filled up by clay or sand, or by the growth of 
peat. 
The Burning and Slaking of Lime. — 1. Bwning. — 
Limestones, when of a pure variety, consist almost en- 
tirely of calcic carbonate, which, as we have seen, con- 
tains 56 per cent of lime, or lljcwt. to the ton. 
When this limestone is put into a kiln, with as much 
coal as, when set on fire, will raise it to a sufficiently 
high temperature the carbonic acid is driven off iu the 
form of gas, leaving the pure lime behind, 
In this state it is known as burnt lime, limeshells, 
caustic lime, and quicklime, and possesses properties 
very different from those of the unburnt limestone. It 
lias a hot, alkaline flavour, absorbs water with great 
Tapidity, falls to powder, or slakes, and finally dissolves 
in 732 times its weight of cold water. This solution is 
known by the name of lime-water. 
2. Slaking. —Its tendency to combine chemically with 
water is shown in the process of slaking. Almost every 
one is familiar with the fact that, when water is poured 
apon quicklime, it heats, emits steam, swells, cracks, 
and at last falls to a fine, usually white, powder, which 
' is two or three times as bulky as the lime in its unslaked 
state. When thus fully slaked and cool, the fine powder 
consists of — 
Lims 76 per cent (nearly). 
Water 24 
100 
Or 20 cwt. of pure burnt lime absorb and retain in the 
solid state G£ cwt. of water, forming 26^ cwt. of slaked 
lime, called hydrate of calcium by chemists. 
When qu cklime is left exposed to the air, even in 
dry weather, it gradually absorbs moisture from the 
atmosphere, and falls to powder without the artificial 
addition of water. In this case, however, it does not 
become ;ensibly hot as it does when it is slaked rapidly 
by immersion, or by pouring water upon it. Some 
chemists state that this powder contains both hydrate 
and carbonate of calcium. 
Effects of exposing Lime to the Air. — When lime 
from the kiln is slaked by means of water, it still re- 
tains its quick or caustic quality. But if, after it has 
fallen to powder, it be left uncovered in tbe open air, 
it gradually absorbs carbonic acid from the atmosphere 
gi»es off its water, and becomes reconverted into dry 
calcic carbonate. 
When Lime in allowed to slake spontaneously in the 
air, it lirat absorbs water, and slakes, and falls to 
powder, and then absorbs carbonic acid and is changed 
into carbonate. 
But as soon as a portion of the lime slakes, it begins 
to absorb carbonic acid, probably long before the whole 
is slaked Thus the two processes go ou together; so 
that, in lime left to slake spontaneously, as it often is 
on our fields and headlands, the powder into which 
it falls consists in part of caustic hydrate, and in part 
of mild carbonate of calcium. Its composition is nearly 
as follows : — 
Per cent. 
Calcic carbonate 57.4 
Calcic hydrate ^.gjj ,. 
100 
When it reaches this stage or composition, the re- 
mainder of the hydrate absorbs carbonic acid much 
more slowly, so that, when spread upon or mixed with 
the soil, it takes a mudi longer time to convert it into 
carbouate. At last, however, after a longer or shorter 
period of tune, the whole of the lime becomes saturated 
with carbonic acid, and is brought back to the same 
stiite of mild iiu-caustic carbonate in which it existed ia 
the native chalk or limestone before it was put into the 
kiln. 
Advantages of Burning Lime.— If the lime return 
to the same chemical state "f carbonate in which it ex- 
isted in the state of chalk or limestone, — what is the 
benefit of burning it ? 
The benefits are partly mechanical and partly chemic- 
al. 
a. We have seen that, on slaking, the burnt lime 
falls to an exceedingly fine bulky powder. When it 
afterwards becomes converted into carbonate, it still 
retains this exceedingly minute state of division ; and 
thus, whether as caustic hydrate or as a mild carbonate, 
can be spread over a large surface, and be intimately 
mixed with the soil. No available mechanical means 
could be economically employed to reduce our lime- 
stones," or even our softer chalks, to a powder of equal 
fineness. 
b. By burning, the lime is brought into a caustic 
state, which it retains, as we have seen, for a longer or 
shorter period, till it again »bsorbs carbonic acid from 
the air or from the soil. In this caustic stats, its ac- 
tion upon the soil and upon organic matter is more- 
energetic than in the state of mild lime ; and thus it 
is fitted to produce effects which mere powdered lime- 
stone or chalk could not bring about at all, or to produce 
them more effectually and in a shorter period of time. 
c. Limestones often contain sulphur in combination 
with iron (iron pyrites). The coal or peat, with which 
it is burnt, also contains sulphur. During the burning 
a portion of this sulphur (oxidised) unites with the lime 
to form gypsum, by this means adding to the propor- 
tion of this substance, which naturally exists in the 
limestone. 
d. Earthy and silicious matters are sometimes pre- 
sent in considerable quantity in our limestone rocks. 
When burnt in the kiln, the silica of this earthy matter 
unites with lime to form calcic silicate. This silicate 
being diffused through the burnt and slaked lime, and 
afterwards spread, in a minute state of division, over 
the soil, is in a condition in which it may yield silica 
to the growing plant, supposing silica to be essential. 
Thus tbe benefits of burning are, as we have seen, 
partly mechanical and partly chemical. They are me- 
chanical inasmuch as, by slaking, the burnt lime can be 
reduced to a much finer and more bulky powder than 
the limestone could be by any mechanical means ; and 
they are chemical, inasmuch as, by burning, the lime is 
brought into a more active and caustic state, and is, at 
the same time, mixed with variable proportions of 
sulphate and of silicate of lime — which may render it 
more useful to the growing crops." 
