After considering a variety of circumstances in regard to the solubility of lime in water, and its crystallization, it is remarked, that when a large quantity of sand is mixed in the mortar, that sand will of course bear a great proportion to the whole mass ; so that the water that may be mixed with the mortar will be much greater in proportion to the quan tity of lime contained in this mortar, than if the whole had consisted of pure calcareous matter. And that, as the sand absorbs none of that water, the water, now pure, is at liberty to act once more upon those few particles of caustic lime that may still remain in the mortar, which will be dissolved and converted into crystals in their turn. In this way, it may happen, in some circumstances, that a very large proportion of the lime may become crystallized; so that the mortar will consist almost entirely of sand enveloped in crystalline matter, and become in due time as hard as stone itself ; whereas mortar, consisting of pure lime, without sand, can hardly ever be much harder than chalk. It is not, however, to be supposed, that in any case this dried mortar will assume that transparent crystalline form, or the compact firmness of some sorts of calcareous matters, such as marble and limestone. In mortar, in spite of the utmost care that can ever be taken, a very considerable quantity of the lime must remain undis solved; which undissolved lime, although it may be so much separated by the sand and crystalline limestone as not much to affect the hard less of the mortar, yet it must still retain its white chalk-liLe appearance. As marble and limestone are, however, always formed by those particles of lime that have been wholly dissolved in water, and from which they have been gradually separated by a more slow and inure per feet mode of crystallization, they have nothing of that opaque ealx-like appearance, but assume other colours, and appear more firm, unifbrin, and compact ; the sand and other matters that may be enveloped in them being entirely surrounded with a pure crystalline matter.
Rut to obtain the most perfect kind of mortar, it is not, however, enough that a large proportion of sand should be employed, and that the sand should be intimately mixed with the lime ; it is also of the utmost importance that a large proportion of water be added ; for, without this, it is impos sible that a large proportion of the lime can be crystallized : and the mortar, in that ease, would consist only of a mixture of chalky matter and sand, which could hardly be made to unite at all, and would be little more coherent than sand by itself, and less so than pure chalk. In that case, pure lime alone must afford rather a firmer cement than lime with sand. It is also of very great importance that the water be retained as long in the mortar as possible: for if it be suddenly eva porated, it will not only be prevented from acting a second time upon the lime, after a part of what was first dissolved has been crystallized, but even the few crystals that would be formed when the water was suddenly evaporating, would be of themselves much more imperfect than they otherwise most certainly would have been. In proof of which, instances
of the crystallization of common salt, lump-sugar, and sugar candy, are adduced ; after which it is noticed, that every one knows what a difference there is between the firmness of the different substances; and that as great must be the dill'erence between the firmness of that cement which has been slowly dried, and,that which has been hastily hardened by the power ful action of a warm air.
It is contended, that it is owing to this circumstance that the lime, which remains all winter in a mortar-tub filled with water, is always found to be much firmer and more coherent than the mortar that was taken from the same tub and used in any work of masonry, although in this case the materials were exactly the same. From the same cause, any work cemented with lime under water, it' it has been allowed to remain undisturbed and uninjured until it has once become hard, is always much firmer than that which is above the surface of the water.
In order to render the force of the above reasoning more strong and convincing, lime-cement or mortar is compared to a mass of matter consisting of a congeries of stones closely compacted together, and united by a strong cementing mat ter, that had, while in a fluid state, pervaded all the inter stices between the stones, and afterwards become a solid indissoluble substance. If the cementing matter be exceed ingly hard and coherent, and if the stones bedded among it be also very hard and firm, the whole mass will become like a solid without fissures, that can hardly be broken to pieces by the power of man. But, although the cement should be equally firm, if the stone, of which it consists, be of a soft and friable nature, suppose chalk or sand-stone, the whole mass will never be capable of attaining such a degree of firmness as in the former ease; for, when any force is applied to break it in pieces, although the cement should keep its hold, the solid matter cemented by it would give way, and the whole would be easily broken to pieces. Whereas, in mortar, the sand that is added to it represents the stones of a solid matter, in the composition, the particles of which are united together by the lime which had been formerly dissolved, and now crystallized, which becomes an exceedingly solid and indissoluble concretion ; and as the particles of sand are of themselves exceedingly hard, and the cement by which they are united equally so, it is plain that the whole concretion must be extremely firm, so as to require very great force to disunite any particle of it from the whole mass. But if; instead of employing sand, the only solid body that is entangled among the cementing matter should be chalk, (as in all eases where the mortar consists of pure lime alone,) or any other slightly coherent substance, let the cementing particles of that composition be ever so perfect, it is impossible'that the whole can ever attain a great degree of firmness, as these chalky matters will be easily broken asunder.