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CHEMISTRY OF HORTICULTUEE. — LIME. 
CHEMISTEY OF HORTICULTURE.— LIME. 
By John Towers, Usq. 
(Continued from Page 261.) 
Ltme — its combinations. — The subject of the last article naturally leads to the considera- 
tion of that peculiar substance which is found to be so important to the gardener and 
agriculturist. Lime — properly so called, that is, in the state of causticity which it 
assumes when burned in the kiln — is never found in nature : it is a preparation of art, 
by a process which shall be described in few words, Although pure lime be absent, the 
earth still abounds with native carbonate of lime in various forms and conditions, known 
as chalk, limestone, marble, and shell-marl. 
" Lime in an impure state is prepared for building and agricultural purposes, by 
calcining in a kiln the ordinary limestones which abound in many districts, and very 
frequently from common pit or rock-chalk ; in the former case it is termed stone-lime, 
and — if well and thoroughly burned in a kiln, so constructed as to admit a free passage of 
atmospheric air at the bottom — is found stronger and to make a better mortar than the 
lime made from common chalk. The process of burning the carbonate must be continued 
so long as is sufficient to discharge the whole of the carbonic acid ; which, according to 
the calculation of Professor Clark (quoted in the last chemical article) amounts to seven- 
sixteenths of its weight, leaving nine ounces of common lime from every avoirdupois 
pound of the raw material. In the best contrived limekilns, the process is carried on 
continuously, broken limestone and fuel being constantly thrown in at the top, and the 
burned lime raked out at intervals from beneath." Dr. Fownes adds — " Sometimes, 
when limestone contains flinty matter (silica), and the heat has been very high, the lime 
refuses to slake, and is said to be overburned — in this case a portion of silicate has 
been formed." 
Flinty matter, or siliceous sand, will unite chemically with lime, and Nature has 
produced this union abundantly : hence the reason that so much unslakeable lime is 
frequently met with. 
Slaked lime is a combination of good lime with about one-third of its weight of water ; 
so that 55 parts of quick or hot lime, absorb 17 parts of water by weight, and in this 
slaked state it is called by chemists hydrate of lime, a term which implies the chemical union 
of lime and water, by which the solidification of the latter is induced. The hardening of 
mortars used for building (wherein slaked lime is mixed with a given quantity of siliceous 
sand) is effected by the gradual absorption of carbonic acid, which, combined with the 
attraction or affinity exercised between lime and silica, produce a solid silicate of lime. 
A great length of time is, however, required, before these combinations become complete. 
There are other lime cements which harden like a rock, and resist the action of water. 
One of these is the substance now so much employed as Concrete, for the foundation of 
buildings. The other is called Roman or Parker's cement. Concrete is formed by 
incorporating gravelly pebble-stones with a known proportion of dry, powdered (not slaked) 
quick-lime, on which a quantity of cold water is poured by one labourer, while another 
works and thoroughly blends the materials with a shovel. Chemical action takes place, 
the lime slakes, and a silicate is gradually formed, solid and indestructible, as is the natural 
concrete called " plum-pudding stone," in which doubtless our artificial preparation found 
its origin. These natural cements comprise oxide of iron, flinty matter, and alumina, 
under the agency of decomposing water. Parker's cement, according to Dr. Fownes, is 
made from the nodular masses of calcario-argillaceous iron-stone found in London clay. 
" When ground to powder and mixed with water, solidification speedily ensues from causes 
not thoroughly understood, and the cement once in this condition, is unaffected by wet. 
Lime, chemically pure, can be made by dissolving white marble in dilute muriatic 
acid, (i. e. spirit of salt — now called hydrochloric acid). To the clear solution a little 
caustic ammonia is added drop by drop : it is then filtered, and liquid carbonate of 
ammonia added till it ceases to produce any precipitation of pure carbonate of lime. The 
