Lime (chemical compound)

“ Combined with carbonic acid (26) it forms limestone (140) , chalk, and marble; all of which are capable of being converted into lime by burning. Lime may also be obtained from oyster and other sea shells. When pure, it is of white colour, and moderately hard substance, though it is easily reducible to powder. Its taste is burning and acrid; and, like the alkalies, it changes vegetable blue colours to green. It has likewise the property of corroding and destroying animal substances. Lime, when pure, absorbs water rapidly, becomes hot, and falls into powder. ”

“ Usually this lime is regarded only as mechanically mixed with the bleaching-compound, CaOCl2, but some chemists adopt formulae in which this lime is equally represented.
For the manufacture of bleaching-powder, limestone of high degree of purity (especially free from magnesia and iron) is carefully burned so as to drive out nearly all the carbon dioxide without overheating the lime. The quick-lime is then slaked with the requisite quantity of water; the product is passed through a fine-meshed wire sieve and is spread in layers of 2 or 3 in. ”

“ When slaked lime is mixed with water, the fixed air in the water is attracted by the lime, and saturates a small portion of it, which then becomes again incapable of dissolution, but part of the remaining slaked lime is dissolved and composes lime-water. ”

“ If exposed to the air it attracts carbonic acid and becomes again a carbonate, and useless to seeds.
Where lime is employed upon the soil, it is either as a mere article of vegetable food, or, as a chemical agent, to change the condition of other ingredients of the soil. All good soils contain lime ”

“ When it has absorbed carbonic acid, and become reconverted into carbonate, the original caustic lime has no chemical virtue over chalk, rich shell-sand or marl, or crushed limestone. It has, however, the important mechanical advantage of being in the form of a far finer powder, than any to which we could reduce the limestone by [Pg 197] art—in consequence of which it can be more uniformly diffused through the soil, and placed within the reach of every root, and of almost every particle, of vegetable matter that is undergoing decay. ”

“ If more lime be presented to the liquor, the goods stop coming down. If now fresh dung be added, they will continue to come down, but the quantity of lime in solution does not increase; the excess of lime is precipitated, partly as phosphate and partly as oxalate, in the way stated above. ”

“ Of all known substances, lime, in the form of lime-water, acts most energetically in producing this change; but other solutions of lime, as well as most salts and mineral acids, produce the same effects when used in sufficient quantity. Common salt is among the most convenient, because it can most readily be leached out of the clay precipitate thus thrown down. This when white, resembles boiled starch, but being usually colored by iron might be easily mistaken for the mixed precipitate of ferric hydrate and alumina so commonly obtained by chemists in soil analysis. ”

“ It is a safe statement that most buyers of lime in some form or other will profit chiefly through the correction of soil acidity and promotion of bacterial life. This renders the situation more simple as any carbonate, hydrate or oxide of lime will accomplish these purposes.
Composition. The first consideration is the actual content of calcium and magnesium. A guaranteed analysis is the only safe basis of purchase. ”

“ Gas lime is also frequently used as manure; in gas works, quicklime is used for removing the impurities from the gas. Gas lime, therefore, varies considerably in composition, and consists really of a mixture of slaked lime, or calcium hydrate, and carbonate of lime, together with sulfites and sulfides of lime. These last are injurious to young plant life, and gas lime should be applied long before the crop is planted, or at least exposed to the air some time before its application. The action of air converts the poisonous substances in it into non-injurious products. ”

“ One of the reasons for applying ground limestone to soils which are deficient in lime is to furnish the element calcium in suitable form for the formation of nitrates in the process of nitrification. Ground limestone is calcium carbonate (CaCO3) , a compound containing one atom of calcium (Ca) , one atom of carbon (C) and three atoms of oxygen (O3) . This is the same form of lime which is contained naturally in limestone soils—soils which are noted for their great productiveness—and it is generally the most economical form of lime to use for correcting soil acidity and promoting nitrification. ”

“ The day would come when the surface soil could not produce our staple crops if its loss of lime continued, and a means of replenishing the stock were not at hand. The huge deposits of limestone that have not been disintegrated by processes of weathering are assurance that the soil's need can be met forever. The calcium and magnesium in the stone are in chemical combination with carbonic acid forming carbonates, and there is an additional mixture of other earthy material that was deposited by the water when the stone was being formed, but much limestone possesses an excellent degree of purity. ”

“ Thus lime being deprived of its air by calcination, and having a stronger affinity with it than alkaline salts have, being mixed with a lixivium of these salts, absorbs all the air from them, deprives them of their property of effervescing with acids, and renders [Pg 47] them more acrid, at the same time that the lime becomes mild, and incapable of impregnating water, but recovers its power of fermenting when mixed with an acid. ”

“ The lime causes the minute separate particles of clay to flocculate or group themselves together into larger compound grains between which air and water can percolate more freely. It is this power of creating a more crumbly tilth on stiff clays that makes lime so valuable to the farmer. Lime also assists in the decomposition of the organic matter or humus in the soil and promotes nitrification; hence it is of great value after green manuring or where the land contains much humus from the addition of bulky manures such as farm-yard dung. ”

“ The next earth we have to consider is LIME. This is a substance of too great and general importance to be passed over so slightly as the last.
Lime is strongly alkaline. In nature it is not met with in its simple state, as its affinity for water and carbonic acid is so great, that it is always found combined with these substances, with which it forms the common lime-stone; but it is separated in the kiln from these ingredients, which are volatilised whenever a sufficient degree of heat is applied. ”

“ The chemical composition of the original limestone also has an influence on the character of the burnt lime. Limestone consisting entirely of carbon dioxide and lime is so rare that sufficient is never available for making burnt lime on a large scale. Even the purest limestone found native in large quantities—namely marble—is not pure carbonate of lime, but contains a certain proportion of extraneous substances. At the same time it is too expensive to use for technical purposes.
The ordinary impurities present in limestone are91 ferrous oxide, ferric oxide, magnesia and organic matter. ”

“ Davy, therefore, very properly introduces the subject, by a description of the nature and qualities of these bodies, and by marking the distinctions between quicklime and its carbonate.
The substance commonly known by the name of Limestone is a compound of lime and carbonic acid, associated generally with other earthy bodies, the nature and proportions of which vary in different species. "When a limestone does not copiously effervesce in acids, and is sufficiently hard to scratch glass, it contains siliceous, and probably aluminous earth. ”

“ Old heaps will remain in hydrate form for many years, excepting the outside coat, which excludes the air. Complete air-slaking would not reduce ability to correct soil acidity, the total amount of calcium and magnesium remaining constant, but weight would be added in the slaking, and therefore the value per ton would be reduced. The slowness with which air-slaking proceeds gives reason to expect that any bulk of old lime may contain a considerable percentage of the hydrate, and therefore have greater strength than a true carbonate like limestone. ”

“ To obtain it perfectly pure, the earth must be pounded and washed with water, in order to free it from any saline substance which may be contained in it, then dissolved in distilled vinegar, and precipitated by mild alkalies. Lime-stone exposed to heat loses about half its weight, in fixed air and water, and the remainder, called quick-lime, attracts water very powerfully, and their union is attended with much heat, after which it dissolves into a fine powder called slaked lime. ”

“ If we do not add enough lime, it prevents the formation of the flocky precipitate, and, besides, more carbonate of soda is used. By adding too much lime, we also use more carbonate of soda in order to precipitate the excess of lime. We can therefore add so much lime, that there is only a very small excess of hydrous lime in the water, and that after well stirring, a red litmus paper being placed in the water for twenty seconds, appears only slightly blue. After a short time of practice, an attentive person can always get the exact amount of lime which ought to be added. ”

“ Lime combines with most of the acids, to which the carbonic (as being the weakest) readily yields it; but these combinations we shall have an opportunity of noticing more particularly hereafter. It unites with phosphorus, and with sulphur, in their simple state; in short, of all the earths, lime is that which nature employs most frequently, and most abundantly, in its innumerable combinations. It is the basis of all calcareous earths and stones; we find it likewise in the animal and the vegetable creations.
EMILY.
And in the arts is not lime of very great utility? ”

“ Every day, or even twice a day, the hides are drawn out ("hauled") , and the pit is well plunged up, to distribute the undissolved lime through the liquor. The hides are then drawn in again ("set") , care being taken that they are fully spread out. How much lime is required is doubtful, but owing to its limited solubility, an excess, if well slaked, is rather wasteful than « 142 » injurious. Great differences exist in the quantity of the lime used, the time given, and the method of working. Lime, as we have seen (p. 140) , is only soluble to the extent of about 1·25 grm. ”

“ A word or two as to lime. When this is spoken of, let it be understood always what is meant; whether pure lime, that is what is called burnt lime, or the same substance in combination with fixed air, or carbonic acid, of which the process of burning deprives it. The effects of these two preparations are exceedingly different on animal bodies; the former causing rapid decomposition and consumption; the latter being, on the contrary, quite inert. Loaf-sugar, though prepared by means of lime, ought never to contain a particle of it, and scarcely ever does. ”

“ The explanation of this state of matters commonly assumed by those unacquainted with chemistry is, that the land has become too full of lime; but a moment's consideration of the very small fraction of the soil which even the largest application of lime forms, will serve to shew that this cannot be the cause. Ten tons of lime per acre amounts to only one per cent of the soil, and as a considerable part of the lime is carried off by drainage in the course of years, it is obvious that even very large and frequently repeated doses are not likely to produce any great accumulation of that substance. ”

“ As food for plants, lime is of considerable importance. All plants contain lime—some of them in large quantities. It is an important constituent of [Pg 161] straw, meadow hay, leaves of fruit trees, peas, beans, and turnips. It constitutes more than one third of the ash of red clover. Many soils contain lime enough for the use of plants, in others it is deficient, and must be supplied artificially before they can produce good crops of those plants of which lime is an important ingredient. The only way in which the exact quantity of lime in the soil can be ascertained is by chemical analysis. ”

“ Another objection to the use of litmus is that the degree of acidity is not accurately indicated, and therefore the farmer is sometimes at a loss to know just how much lime should be applied to make soil conditions favorable for growing crops. ”

“ When mixed with water, lime forms a pasty mass known as slaked lime and in a more dilute form as milk of lime, because when shaken up in water it remains suspended in it for a long time and presents the appearance of a milky liquid. But, besides this, lime is directly soluble in water, not to any considerable extent, but still in such a quantity that lime water is precipitated by carbonic anhydride, and has clearly distinguishable alkaline properties. One part of lime requires at the ordinary temperature about 800 parts of water for solution. ”

“ The ordinary 74 lime is preferable to the chalk lime, as it is usually stronger, though it sometimes contains more impurities. Gas lime is the poorest of all. Lime should be stored in a dark place, otherwise the outside of the heap carbonates quickly, forming chalk (Ca O + CO2 = Ca CO3) which is of no use for the liming process. ”