QUICKLIME, a general name for all calcareous substances when deprived of their fixed air; such as chalk, limestone, oyster-shells, &c. calcined.
Quicklime has the following properties. 1. It is entirely soluble in water, with which it unites so rapidly as to occasion considerable heat. When exposed to air, it imbibes moisture from thence. When united with as much water as is sufficient to make it a fluid paste, it is called slaked lime. Water saturated with quicklime is called lime water. According to Brandt, lime-water contains about one part of quicklime to 700 or 800 parts of water. Slaked lime, or lime-water, being exposed to the atmosphere, attracts from thence particles of fixable air which float in it, by which means the quicklime is rendered mild, insoluble in water, and therefore appears on the surface of the lime-water, or of the slaked lime where this combination happens, in the state of mild or combined calcareous earth, convertible by a second calcination into quicklime, and is called cream of lime.
If the earth dissolved in lime-water be precipitated from thence by any substance containing fixable air, as by mild alkalies or magnesia, it will unite with this air, become mild, and resume its former weight and properties which it possessed before calcination. But if it be precipitated from the water by means of some substance which does not contain fixable air, but which is more strongly disposed than the earth to unite with the water, for instance, spirit of wine, the earth thus precipitated will be in the state of quicklime, that is, caustic, and soluble in water.
2. Quicklime unites with acids without effervescence, which is nothing else than an extrication of the fixable air, of which quicklime has been already deprived. It nevertheless saturates as much acid as it would have done if it had not been calcined.
3. Quicklime is more powerfully disposed to unite with fixable air than fixed or volatile alkalies, or magnesia. Hence, when treated with these substances, it takes from them their fixable air, and is itself rendered mild, and restored to its original weight and properties. Thus two drams of chalk, having been by calcination reduced to one dram and eight grains of quicklime, were thrown into a filtrated solution of an ounce of mild fixed alkali in two ounces of water, and digested during some time; by which the calcareous
earth became mild, and weighed one dram and 58 Quicklime grains. By means of magnesia, the calcareous earth may be precipitated from lime-water; and this earth is found to be mild, and to have deprived the magnesia of its fixable air. By depriving alkalies of their fixable air, quicklime renders them more caustic and solvent, for the same reason that itself is by this privation of air rendered more caustic and powerfully solvent. This increase of causticity and dissolving power is consistent with a general rule, namely, that the more simple or less compounded any body is, that is, the less its general tendency to union is satisfied, the more disposed it is to unite with or dissolve other substances.
4. Quicklime has a disposition to unite with sulphur, with which it forms a liepar of sulphur, similar to that made by sulphur united with an alkali, and, like this, soluble in water. It is also disposed to unite with oils and with animal and vegetable matters, with respect to which it discovers a caustic and corrosive property.
4. Quicklime mixed with sand forms a mass which hardens, and is used as a cement or mortar.
All these properties of quicklime have been the objects of consideration to the chemists and philosophers; who have, as usual, been divided in their opinions on the subject. The evident resemblance of the action of quicklime to fire, has given occasion for one party to derive all the active properties of this substance from fire; while, on the other hand, its want of heat, and incapacity of setting bodies on fire, unless by an accession of water, were objections altogether insurmountable. On the other hand, those who denied the materiality of fire, and affirmed that it consists only in a motion mechanically produced among the particles of bodies, were altogether at a loss to show a reason why this motion, or any thing resembling it, should continue perhaps for months after the exciting cause is taken away. To remove this difficulty, some have had recourse to the action of a latent acid communicated to the quicklime by the fire; and which one chemist (Mr Meyer) has distinguished by the name of acidum pingue.
But on this hypothesis it may be remarked in the first place, that the action of acids is as difficult to be explained as that of fire; and, in the second place, that as all substances, by calcination into quicklime, lose considerably of their weight, it seems very improbable that they should acquire an acid or any other substance which could increase their weight. Besides, from the experiments of Dr Black, it appears, that the diminution of weight in calcareous substances, is owing to their parting with a quantity of fixed air, the weight of which is much more considerable than that of any moisture or fatty matter they contain. The loss of this fixed air is now also universally allowed to be the reason of the causticity of the quicklime, as its superior attraction for fixed air is looked upon to be the reason why it renders fixed and volatile alkalies caustic like itself. The only question therefore can be, By what means are the calcareous earths deprived of their fixed air? To this question the answer is evident, namely, that the action of the fire expels the fixed air; and if this is the case, it is evident, that to this action of fire, continued, the caustic properties of the lime are owing.
Quicklime. We come now to the discussion of the question, Whether quicklime is to be considered as a pure earth, or a combination of it with something else?—Most of the chemists, since the discovery of fixed air, have been inclined to think, that quicklime is a pure earth uncombined with any thing else, and that it approaches more nearly to the state of elementary earth than any other. But this opinion seems not to have a solid foundation; for there are other earths, such as the basis of alum, which, as far as they can be examined by us, are equally pure with quicklime, and yet discover not the smallest causticity, even after the most violent calcination. Besides, from the property which quicklime has of depriving alkaline salts of their fixed air, we may learn, that there exists in it, when kept by itself, a certain principle which prevents it from absorbing again the fixed air, with which it was once so closely united, except in certain circumstances. It is well known, that fixed alkalies, as well as those which are volatile, will absorb fixed air from the common atmosphere: and hence, tho' they are prepared in the most caustic state, they will in a very short time become mild, by an exposure to the atmosphere; nay, it requires no small degree of care to prevent the atmosphere from having as much access to them as is necessary to change them from a caustic to a mild state. Now, as these substances thus attract the fixed air from the atmosphere, it thence appears that the atmosphere parts very readily with the fixed air which it contains. The quicklime, however, though it has a greater attraction for fixed air than the alkalies, yet does not become near so soon mild from exposure to the air as the alkalies which have less attraction than itself. Hence the necessary inference must be, that quicklime, after being once calcined, instead of attracting, repels fixed air, unless it is placed in certain circumstances, wherein the repelling power is destroyed, and the attractive power again manifests itself. Now it is manifest, that the power which originally repelled the fixed air was the action of fire; and consequently, while the quicklime refuses to attract fixed air, we must conclude that it is the same action which prevents the union. Quicklime, therefore, is not a pure earth, but a combination of a pure earth with fire; just as chalk, or limestone uncalcined, is not a pure earth, but a combination of a pure earth with fixed air. In all chemical trials, then, where quicklime is used, the double elective attraction will manifest itself as much as in a combination of different salts, metals, and acids. Thus, when water is poured on quicklime, the attraction between that element and earth is stronger than the attraction between earth and fire. The consequence is, that the water expels the fire, just as vitriolic acid poured upon sea-salt expels the marine acid. The fire then, having nothing with which it can form a chemical combination, becomes sensible to the touch, first making the lime very hot, and then gradually dissipating in the atmosphere. However, as the water combines with the earth but in very small quantity, it can only expel the fire from that quantity with which it does combine; and consequently the lime still retains its caustic quality, though in a degree somewhat milder than what it was originally. We must also consider, that water itself has a considerable attraction for fire as well as for earth; and the conse-
quence of this must be, that part of the lime will be dissolved in the water, if more of that element is added than what the earth can absorb without losing the form of a dry powder. Hence the origin of lime-water, which is only a small quantity of lime in its caustic state dissolved in a large quantity of water. This dissolution is owing to the double attraction of fire to earth and water; for as long as the water can admit the calcined earth to that intimate union with itself which is called a chemical combination, the earth must still retain all the causticity which the fire gives it, and dissolve in the water. When the earth is in too large quantity to be thus combined with the water, the latter is only absorbed into the pores of the earth, where by its bulk it splits the stone or calcined matter all to pieces, and reduces it to an impalpable powder, expelling a proportional quantity of fire from those pores which it now occupies. The water, however, is capable of radically dissolving but a very small portion of calcined earth: and therefore, the same quantity of quicklime will serve for preparing lime-water a great number of times over; but at last a large quantity is left, which seems to be quite inert, and has lost the properties of quicklime. Those who have tried the experiment of lixiviating lime with fresh quantities of water till it ceases to be soluble, have fixed the proportion of soluble matter in the lime at about one-third of the whole; but from Dr Black's experiments it appears that quicklime may all be dissolved in water at once, provided the water is in sufficient quantity. Its inactivity, therefore, after repeated affusions of water, must be owing to some change produced by the water; but whether this is owing to an absorption of all the fire it contained by the great quantity of water, or to a supply of fixed air given by the water, has not yet been determined by any experiment.
If, instead of pouring cold water upon quicklime, we pour that which is already heated, the absorption is much less complete; because the water, having already a superfluous quantity of heat, is resisted by that which is contained in the quicklime in a latent state; and hence it is a general observation, that hot water is less proper for slaking lime than cold. But if we pour on any acid upon quicklime which contains a great quantity of fire in a latent state, and has likewise a violent attraction for the earth, a much greater degree of heat is produced than with simple water. With the vitriolic acid, indeed, this is not so well perceived, if the common calcareous earths are made use of; because their insolubility in this acid diminishes its effect: but if, instead of these earths, we take magnesia newly calcined, the heat is so great, that the aqueous vapour, not having time to evaporate slowly, is driven off with a considerable explosion. If the common calcareous earths, well calcined, are dissolved in the nitrous acid, a most violent degree of heat is produced; more indeed than in any other case where a liquid is concerned: for the nitrous acid itself contains a great deal of latent heat; the quicklime does the same; and by the intimate union of the earth with the acid, all this latent heat, at least a great part of it, both in the quicklime and spirit of nitre, is displaced, and attacks the aqueous fluid as being nearest to it; from whence it is dissipated in the air, or
Quicklime, absorbed by the neighbouring substances. The same Quicksilver thing happens, only in a less degree, when the marine acid is employed.
When quicklime is mixed with a solution of mild alkali, a double decomposition, and two new compositions, take place. The quicklime may be considered as a combination of earth and fire, while the alkali in the present case acts as a combination of salt and air. These two substances, therefore, are no sooner put into such circumstances as enable them to act on each other, than the quicklime attracts the air from the alkali, and gives its own fire in exchange, which the alkali takes up, and thus is rendered caustic, while the quicklime becomes mild. Nevertheless, though the alkali here seems to have the greater attraction for fire, and the quicklime for air; yet it appears, that the alkali is by no means capable of keeping the fire which it has imbibed for any length of time; for no sooner is it exposed to the action of the air, than it parts with the fire which it had imbibed, regains its air, and becomes mild. This, however, in all probability is owing to its extreme solubility in water while in a caustic state; for quicklime itself when dissolved in water very easily regains its fixed air, nay, even more than it contains in a natural state. See the article SALT.
On the whole, then, the properties of quicklime may be explained in a very easy manner on Dr Black's principle of latent heat. That heat consists in a latent state in quicklime, as well as in vapour, we have incontestable proofs; because, in all cases where quicklime changes its nature and becomes more mild, a degree of heat is produced, and which is always proportionable to the change made on the quicklime. In the making of quicklime, therefore, the air is expelled, and a proportional quantity of fire enters; in dissolving it in an acid, flaking, &c. an acid, air, or water, expels part of the heat, which then becomes sensible. By long exposure to the air, the heat gradually evaporates; the fixed air resumes its place; and the quicklime, being thus increased in bulk, embraces those bodies very closely which lie nearest to it, inasmuch, that, when mixed with sand and stones, it will harden with them almost into the solidity of a rock. See CEMENT and MORTAR. When mixed with animal or vegetable substances, it destroys or decomposes them both by the action of its internal heat, and by its attraction for a certain acid contained in the animal substances, and an oily matter in the vegetables; and hence its property of burning cloth, though its attraction for the oily matter just mentioned makes it an excellent whitener when properly applied. See BLEACHING.