It is remarked, in addition, that a variety of conjectures have been made about the nature of the lime•cement em ployed by the ancients. It has been thought that they pos sessed an art of making mortar which has been long since entirely lost ; as the cement in the walls which have been built by them, appears to be, in many cases, much firmer than that which had been made in modern times. Yet, when the mortar of these old buildings is analyzed, it is found to consist of the same materials, and nearly in the same propor tions, in which they are now made use of. And it is thought probable, that their only secret consisted in mixing the mate rials more perfectly than the rapidity or avarice of modern builders will permit ; in employing their mortar in a much more fluid state than is done now ; and in allowing it to dry more slowly, which the immoderate thickness of many of their walls would naturally produce, without any precon certed design on their part. Tradition has .even handed down to the present times the memory of the most essential of these particulars ; as the lower class of people, in every part of the nation, at this moment, invariably suppose and believe that these old walls were composed of a mortar so very thin, as to admit of its being poured, like a fluid, between the stones, after they were laid in the wall : and the appearance of these old walls, when taken down, seems to favour this popular tradition. Nor is it,doubted but that this may have been the ease. The stones in the outer part of the wall, it is thought, were probably bedded in mortar, nearly as is practised at present ; and the heart, after being packed well with irregular stones, might have the interstices between them entirely filled up with fluid mortar, which would insinuate itself into every cranny, and, in time, adhere as firmly as the stones themselves, or even more so, if the stones were of a sandy friable nature. And that, as these walls were usually of very great thickness, it might often happen, that the water in this mortar, by acting successively upon difThrent particles of caustic lime, would at length be entirely absorbed by successive crystallizations, so as to become perfectly dry, without any evaporation at all ; in which case, a very large proportion of the original lime must have been regularly crystallized in a slow and tolerably per fect manner, so as to attain a firmness little inferior to lime stone, or marble itself.
It is supposed that, upon these principles, it is easy to account for the superior hardness of some old cements, when compared with that of modern times, in which a practice very different is usually fl)llowed, without having recourse to any wonderful arcana whatever.
There arc likewise a few other circumstances that may influence the quality of common lime-mortar. If limestone be sufficiently calcined, it is deprived oiall its moisture, and of all its carbonic acid gas, or fixed air. But experience shows, that limestone Will fall to powder on the effusion of water upon it, when it is much less perfectly calcined, and while it still retains almost the whole of its fixed air. And that, as masons have hardly any other rule fir Slidging whether limestone be sufficiently calcined, except this single circumstance of its filling to a powder when water is poured upon it, it may thus easily be perceived, that the same lime may be more or less fitted for making good mortar, according to a circumstance that, in a great measure, eludes the obser vation of operative masons; for if it should happen that all the pieces of lime drawn from a kiln at one time were just sufficiently calcined to make it fall to a powder with water, and no more, that powder would be altogether unfit for making mortar of any kind. This is a calve that can seldom
happen; but as there are a great many intermediate degrees between that state and perfect calcination, it must often happen that the stone will approach nearer to one of these extremes at one time than at another; so that the mortar may be much more perfect at one time than at another, owing to a variation in this particular.
All those who have written on the subject of lime as a cement, have endeavoured to ascertain what is the due pro portion of sand for making the most perfect cement. But a little attention to the matter will show, that all rules which could be prescribed as to this particular, must be so vague and uncertain as to be of little utility to the practical mason ; as, besides the variation which may arise from a inure or less perfect degree of calcination as above, it is a certain fact, that some kinds of limestone are much more pure, and con tain a much smaller proportion of sand, than others du; some being found almost perfectly pure, while others contain eleven-twelfths of sand, and all the intermediate proportions of it. Therefore, it would be absurd to say that pure lime would require as small a proportion of sand, when made into mortar, as that which originally contained in itself a much larger proportion ot' sand than any writer has ever ventured to propose for being put into mortar.
Besides, there are variations caused by the different nature of the calcination in the several sorts of limestone ; from which it may, upon the whole, be concluded, that about one tenth of pure limestone is not enough calcined to admit of being made into mortar ; and that of the most impure sorts of limestone, not above one-fourth part of the lime contained in it is so much calcined as to be in a caustic state.
The variation that is produced by these means, in regard to the proportion of sand that will be required to the lime in the one or the other case, is found to be so extremely great as hardly to be conceived. It is, however, stated, that the best mortar that has been seen made was formed of lime which had been found to contain eleven parts of sand to one of lime ; to this there was added between twice and thrice its whole bulk of sand by measure, which may be allowed to have been at least three times its quantity by weight. There fore, supposing that every particle of that lime had been so perfectly calcined as to be in a caustic state, there could not be less than forty-seven parts of sand to one of lime. As much may, however, be allowed for the uncaustie part of the lime as is desired, and the calculation made accordingly. But it is hardly possible to suppose that above one-hundredth part of this mass, independent of the water, consisted of pure caustic calcareous earth.