CHEMISTRY OF HORTICULTURE. — LIME. 
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precipitate is then washed, dried, and exposed to a white heat in an open vessel, with free 
access of air. The lime thus obtained, should be slaked, and converted to a hydrate , and 
this again heated as before, when the whole of the carbonic acid will be expelled. 
Herein, I have followed the process of Professor Brande : and though few gardeners can 
be expected to act upon it practically, it is pleasant and profitable to become acquainted 
with the rationale of well-established facts. 
Hot lime, and the hydrate prepared by slaking the shells with water before they 
become air-slaked by exposure, are of great importance in the farm : and its uses were 
particularly dwelt upon by the late Sir John Sinclair in “ The Code of Agriculture.” 
Many writers have attempted to elucidate its mode of action ; and a very valuable article 
was a very few years since contributed to Science, by Professor Johnston of the late 
Chemico- Agricultural Laboratory of Edinburgh. The reader should be made thoroughly 
acquainted, in the first place, with the processes of slaking. If upon lime hot from the 
kiln, water be sprinkled in small quantities, great heat and steam are excited, the shells 
snap, crack, and crumble to a soft and bulky powder, which then is a true hydrate, 
containing one equivalent of water, and this water can again be expelled by heat. In 
such a hydrate there is no carbonic acid — it is still a caustic, alkaline lime. But lime is 
so greedy of water that, if it be exposed for a time to the air, it will equally, but more 
slowly, become slaked by attracting and uniting with the watery vapour of the atmosphere. 
The powdery mass so produced is also a hydrate, but not purely and solely such, for lime 
not only attracts and fixes water, but exerts its chemical affinity for the carbonic acid, which 
also is always present in the air. Air-slaked lime, therefore, is more or less a carbonated 
hydrate, approaching somewhat to the nature of common chalk or whitening, and if a 
small quantity of it be dropped into a glass containing dilute muriatic acid, it will effervesce, 
and produce a beading of air-bubbles. In the garden, fresh made water hydrate, in fine 
powder, acts most effectually in the destruction of slugs and other vermin, and it may at 
all times be employed, with great safety, to vegetable crops, because its affinity for water 
is satisfied by the equivalent which it already contains, and therefore it does not injure 
the vegetable tissues, as hot and unslaked lime would do, by its eager attraction of 
moisture. 
The destruction of inert, useless vegetable and animal matter, is one of the objects 
which the advocates of hot lime hope to effect. Sir Humphry Davy thus expressed his 
view of the subject : “ When lime, whether freshly burnt or slaked, is mixed with any 
moist fibrous vegetable matter, there is a strong action between the lime and the vegetable 
matter, and they form a kind of compost together, of which a part is usually soluble in water. 
By this kind of operation, lime renders matter, which was before comparatively inert, 
nutritive, and as charcoal and oxygen abound in all vegetables, it becomes at the same 
time converted into carbonate of lime. 
“ Quick-lime, in being applied to land, tends to bring any hard vegetable substance that 
it contains, into a state of more rapid decomposition and solution, so as to render it a proper 
food for plants. Chalk and marl will only improve the texture of the soil, or its relation 
to absorption ; it acts merely as one of its earthy ingredients. 
“ Quick-lime, when it becomes mild, operates in the same manner as chalk ; but in the 
act of becoming mild, it prepares soluble out of insoluble matter.” — 7th Agricultural Lecture. 
No one, until Mr. Rowlandson, of Liverpool, wrote his Essay on Lime, had, I believe, 
given a hint of that most essential property of lime, by which it becomes the specific 
reclaimer of inert peat bogs. Davy, without detecting the cause of this agency, had 
observed that lime was injurious when mixed with any common dung, and tended to 
render the extractive matter insoluble. Mr. Rowlandson asserted, and I have proved the 
correctness of his views, that if lime or lime-water be mixed with brown humus, black peat, 
old soil glutted with humic matter, it attracts and fixes the humic acid, and forms an 
insoluble humate of lime. Any gardener can try the experiment, and thus satisfy himself 
of two important facts. First, that humus, when superabounding, is a poison. Second, 
that lime is its specific antagonist. Thus it is shown, that the Scotch, and all reclaimers 
of peat bogs, act upon sound philosophical principle when they apply lime in comparatively 
