On these considerations it is conceived, that it is impossible to prescribe any determinate proportion of sand to lime, as that must vary according to the nature of the lime, and other incidental circumstances, which would form an infinity of exceptions to any general rule. But it would seem that it might be safely inferred, that the moderns, in general, rather err in giving too little sand than in giving too much. It deserves, however, to be noticed, that the sand, when natu rally in the limestone, is more intimately blended with the lime than can possibly be ever effected by any mechanical operation ; so that it would be in vain to hope to make good mortar artificially from pure lime, with such a sinall propor tion of caustic calcareous matter as may sometimes be effected when the lime naturally contains a very large proportion of sand. But there seems to be no doubt, that if a much larger proportion of sand were employed, and if that were inure carefully blended and expeditiously worked than is common, the mortar would be much more perfect than is usual in modern times, as has been proved by actual trials.
Another circumstance that tends greatly to vary the qua lity of cement, and to make a greater or smaller proportion of sand necessary, is the mode of preparing lime before it is beaten up into mortar. When for plaster, it is of great importance to have every particle of the limestone slaked before it is worked up; for, as smoothness of the surfAce is the most material point, if any particles of lime should be beaten up in it, and employed in work before sufficiently fhllen, the water still continuing to act on them after it was worked up, would infallibly slake such particles, which, forcibly expanding themselves, would produce those excres cences on the surface of the plaster commonly termed blisters. Consequently, in order to obtain a perfect kind of plaster, that will remain smooth on the surface, and free of blisters, there is an absolute necessity to allow the lime to lie for a considerable time macerating or souring in water, before it is worked up. And the same sort of process is necessary for the lime, when intended for use as mortar, though not so absolutely. Great care is, however, required in the management in this respect ; the principal things being the getting of well-burnt lime, and the allowing it to macerate or sour with the water for only a very short time before it is used ; but that which is the best burnt will require the maceration of some days in the water before it is sufficiently slaked in the whole mass for this purpose.
It has been almost universally admitted, that the hardest limestone affords a lime that will consolidate into the firmest cement ; hence it is generally concluded, that lime made of chalk produces a much weaker cement than what is made of marble or limestone. It would seem, however, that if ever this be the case, it is only incidentally, and not neces sarily so. As, from the nature of calcareous matter, every kind of lime is equally fit for becoming a firm cement, if it be first reduced to a proper degree of causticity, and has after wards a due proportion of sand properly mixed with it, before it be employed in work. Different sorts of lime, without doubt, differ much from each other in the proportion of sand they naturally contain, and, of' course, require very different proportions of sand to be added to them before they can be made equally perfect as a cement ; which is an economical consideration of no small moment, in some cases, as it may make one sort of lime a great deal cheaper than another on some occasions, and, of course, deserves the attention of builders in general.
The excellencies and defects of other substances, that may be occasionally mixed with lime, in making cement, may be just noticed. Those commonly used as an addition to mortar, besides sand of various denomMations, are powdered sand stone, and And, for forming plaster, where closeness rather than hardness is required, they use lime that has been slaked and kept long in a dry place till it has become nearly effete, powdered chalk, or whiting, and gypsum, in various proportions ; besides hair and other mate rials of that nature. But some others have been more lately
advised, such as earthy slightly burnt and pounded, powdered and sifted old mortar rubbish, and others of a similar kind. All of which substances are found objection able, in some respect or other, for this use, sand being the only perfectly suitable material that can be easily met with ; on which account, it has been always justly preferred. Pure firm crystallized sand is the best ; though all pure sands are not equally proper for this purpose.
It is stated by Sir I lumphrey Davy, in his work on Agri cultural Chemistry, that there arc two modes in which lime acts as a cement ; in its combination with water, and in its combination with carbonic acid. When quick-lime is rapidly made into a paste with water, it soon loses its softness, and the water and the lime form together a solid coherent mass, which consists of seventeen parts of water, to fifty-five parts of lime. When this hydrate of lime, while it is con solidating, is mixed with red oxyde of iron, alumina, or silica, the mixture becomes harder and more coherent than when lime alone is used ; and it appears, that this is owing to a certain degree of chemical attraction between hydrate of lime and these bodies : and they render it less liable to decompose by the action of the carbonic acid in the air, and less soluble in water. It is thought that the basis of all cements that are used for works which are to be covered with water must be formed from hydrate of lime ; and that the lime made from impure lime-stones answers this purpose very well. Puzzo lana, it is said, is composed of silica, alumina, and oxyde of iron ; and it is used mixed with lime, to form cements intended to be employed under water. It is stated that Mr. Smeaton, in the construction of the Eddystone lighthouse, used a cement composed of equal parts, by weight, of slaked lime and puzzolana. Puzzolana, it is said, is a decomposed lava. Tarras, which was formerly imported in considerable quantities from Holland, is found to be a mere decomposed basalt : two parts of slaked lime and one part of tarras form the principal part of the mortar used in the great dykes of holland. It is supposed that substances which will answer all the ends of puzzolana and tarras, are abundant in the British islands. An excellent red tarras may be procured in any quantities from the Giant's Causeway, in the north of Ireland : and decomposing basalt is abundant in many parts of Scotland, and in the north of' England where coal is found.
It is observed that Parker's cement, and cements of the same hind, are mixtures of calcined ferruginous, siliceous, and alominous matter, with hydrate of lime.
It is noticed, that the cements which act by combining with carbonic acid, or the common mortars, are made by mixing together slaked lime and sand. These mortars at first solidify as hydrates, and are slowly converted into carbo nate of lime by the action of the carbonic acid of the air. It was found by Mr. Tennant, that a mortar of this kind, in three years and a quarter, had regained sixty-three per cent. of the quantity of carbonic acid gas, which constitutes the definite proportion iu carbonate of lime. The hardness of the mortar in very old buildings is also thought to• depend upon the perfect conversion of all its parts into carbonate of lime. The purest lime-stones are the best adapted, it is said, for making this kind of mortar. The magnesian lime-stones make excellent water-cements, but act with too little energy upon carbonic acid gas, to make good common mortar. The 1Zomans, on Pliny-'s authority, made their best mortar a year before it was used ; so that it was partially combined with carbonic acid gas before it was employed, it is supposed. See more on this subject under the articles CEMENT, LIME, MORTAR, and PLASTERING.