exposed to the air for one year only, show great tenacity. Tables Nos. XL, XLI, and XLII, contain, also, very good mortars made of fat lime, puz zolana, trass, and several kinds of cement. The use of cement goes back to high antiquity, for Roman and Egyptian structures often contain it. Those who wish to solve this question, without recourse to the quality of the lime, or to cements, object that there are ancient remains which appear to have been made of fat lime, since they are seen in countries where no puzzolana or hydraulic limes are to be found, and they have not the aspect of mortars made with cements. I will observe, touching this point, that if we examine the two tables in page 111, which contain the analyses of several lime stones, we shall see that many limes which are ranked with fat limes contain, nevertheless, small quantities of clay. Although they may not contain enough clay to harden under water, they ought to afford much better mortars in the air than those limes which contain none. Again, hydraulic limestones are often found disseminated amongst the strata of fat lime stones. And, lastly, the important observation of Mr. Girard, on the hydraulic proportions of arenes explains easily how very good mortars may have been made of fat lime. I will observe, in addition, that the Romans, in all the countries they occupied, executed a great many works, of which only those made of good mortars survive to the present day. Saint Augustine complains of the manner in which mortars were made in his day: and the same complaints are found in Pliny; who says, chapter XXII, "that which causes the ruin of the greater part of the edifices of this city (Rome) is, that the workmen employ, from fraud, in the construction of the walls, lime which has lost its quality."* We see, therefore, that all the Roman mortars were not good.

For my part, I am convinced, that if those ancient constructions that have reached our times, be examined with attention, it will be ascertained that they were made either with hydraulic lime and sand, or with fat lime and sand mixed with cement, or with arenes: in a word, that these mortars had all the elements of good hydraulic mortars.

We are in the habit of composing our mortars of fat lime and sand; the preceding experiments show that we are wrong: our mortars have, consequently, little durability. We shall not obtain durable masonry in the air, until we make use, therein, of hydraulic mortars. In countries where good natural hydraulic lime is to be had, no other kind should be used for any purpose whatever. For ordinary masonry, the mortar should, in that case, be made of lime and sand only. In countries where there are no natural hydraulic limes, but where there are arenes, the mortar should be made of fat lime and these arenes: in both these cases the mortars would be cheap. In countries where neither arenes nor hydraulic limes are to be procured, it will be necessary to incur a little additional expense, and make use of fat lime, sand, and hydraulic cement. To combine economy and solidity at the same time as much as possible, the proportions, in cases where there are to be one part of fat lime and two of sand and cement, the mixture may be made as follows, viz: one part of fat lime measured in paste, one and a half of sand, and a half of hydraulic cement; (according to similar proportions made with trass, as shown in table No. XXVII,) we should have, by this means a very good mortar. The proportions of hydraulic cement, stated above, should be used in all common masonry: in works demanding more care, the mortar should be composed of lime, sand, and cement, in equal

*We might infer, from this passage, that Pliny complained because the lime was not used soon after its calcination. AUTHOR.

parts. I have said, that the proportions indicated for common masonry, should augment the expense but little: but were the augmentation more considerable, it is certainly much more economical to incur at once, all the expense necessary to produce a permanent work, than to build at a cost rather less in the first instance, and to be obliged to reconstruct the work entirely, after no great lapse of time. A government should construct for posterity: and I do not doubt that this end would be attained by making all masonry with hydraulic mortar, in the manner I have pointed out.

Officers of engineers often have to construct arched rooms, passages, &c., that are to be always covered with earth. The mortar of these constructions is generally composed of fat lime and sand. To resist filtration and humidity, it is customary to place a cap, or masonry roof, made of hydraulic mortar, over these arches: but experience shows that these caps do not fulfill their object, and that they often afford a passage to the water, especially when made of bad cements. I cited an example at page 84, and I might cite many others. Even when the caps are made of good mortar, it may happen that there is humidity in the casemates, and that the arches leak: for we have seen at page 134 that mortars made of fat lime do not dry completely, even in a century, when the walls are thick. And this result is the more certain when the masonry is covered with earth as in casemates. The walls against which the earth lies, almost always allow the water to transude, although the cap may be impermeable. Besides, from the considerable load that these arches often sustain, the masonry that has been made of mortar slow to harden, will settle, causing cracks and leaks in the caps. The only means of obviating these disadvantages, is to construct all the masonry of the casemates with hydraulic mortar. We shall thus secure the great advantage of a prompt induration: the subsidence will consequently be less sensible, and cracks less apt to occur; the walls will no longer allow moisture to transude, and even should there be some cracks in the caps, the water will, with difficulty, find its way through arches made with hydraulic

mortar.

The manipulation of mortars designed for exposure in the air, should be the same as that described in the first section for mortars designed for

water.

CHAPTER XV.

On factitious Stones and Concretes, exposed to the Air.

Mr. Fleuret, formerly Professor of Architecture in the Royal Military School, of whom I have before had occasion to speak, published in 1807, a work on the art of composing factitious stones. The following passages are to be found in page 12: "The art of building with factitious stones is very old; it was in use with the Babylonians, and the early Egyptians, amongst the Greeks and Romans, and it is still pursued in Barbary, and amongst the nations of Malabar.

"According to Pliny, the columns which adorn the peristyle of the Egyptian labyrinth, were of artificial stones, and this vast edifice has existed three thousand six hundred years. The pyramid of Nynus is formed of a single block. The enormous stones composing the great and strong walls raised in the empire of Morocco, as is reported by the Abbe de Marsi, from writers whom he quotes; the square stone that formed the tomb of Porsenna, spoken of by Varro and Pliny, which was thirty feet wide by five feet high, were composed in the same manner as the pyramid of Nynus, and lead us to believe that these monuments owe their existence

and their preservation to a process as easy as it is simple, and which unites the advantage of solidity with economy.

"All factitious stones of a volume thus considerable, were made by encasement, and the process of massivation; that is to say, that in the great walls built thereof, these stones were formed the one upon the other, by beating the materials, with rammers, into spaces formed by planks, as I shall explain."

Mr. Rondelet states, in his preface, that the columns of the choir of the church of Véselay, in Bourgogne, were ascertained to be factitious stones by Marshal Vauban, and the pillars of the church of Saint Amand, in Flanders, were made in the same manner.

I have had no opportunity to ascertain whether blocks of stone of extraordinary dimensions, were factitious; this question could not be well settled, except by examination of the masses themselves. It is possible that several of the large blocks mentioned were factitious: but it is not easy to believe, in a wall formed of large pieces placed one upon another, that these large pieces are factitious, for it seems to me that it would be much more difficult to construct a wall of that sort, than to make the whole wall of one piece, whereby the moving and transportation of masses of an enormous weight would be avoided. I do not at all doubt that artificial stones of large dimensions may be made, for all depends on making a good mortar: nevertheless it seems proper not to admit such facts as are cited above without a thorough examination.

The Italians, at Alexandria, make very good factitious stones with the Casal lime, and employ them in angles; they are four feet eight inches long, by two feet eight inches wide, and two feet eight inches high; they are buried under ground for two or three years, and there acquire great hardness. These stones are made in the following manner. For one cubic yard they take 0.24 of Casal lime measured in paste, and 0.90 of sand. These are well mixed together, adding the water necessary to form a paste, and there is then added 0.20 of pebbles, (cailloux;) these factitious stones are therefore real concretes. Their goodness depends on the quality of the mortar applied to making them. Mr. Fleuret proposes to make the mortar for these factitious stones in the following manner.

He slakes the lime by immersion, according to the process of Mr. Lafaye, to which he attaches great efficacy. When slaked to dry powder, it is to be deposited in a dry situation, enclosed in casks covered with straw mats loaded with stones. He recommends that, each time the cask is resorted to for lime, a part only of the mat should be raised, and that it be replaced as soon as possible, in order to guard gainst the contact of the air, which he says is very hurtful. He makes a perfect mixture of sand and clay dust, in the proportion of two measures of sand to one of dust; or, which is better, he mixes sand and dust in equal parts. He then takes two measures of the mixture of sand and dust, and one measure of lime after being tempered with water, and makes them into a heap. He then works them dry, moistening them little by little as they mingle. Lastly, they are taken to a trough, where they are beaten by rammers of wood armed with iron, which are suspended from the ends of poles acting with a spring, like the spring pole of a turner's lathe. Mr. Fleuret says the mortar is improved by wetting it in the trough with a little lime slaked thin, and used in the proportion of one-sixth of the mortar. He censures the practice of reworking the mortar with fresh portions of water, which he says weakens it much. The author adds that hard stone pulverized, may be substituted for sand,

and that scoriæ, iron scales, and sea coal ashes, furnished by forges, are still better than pulverized stones and clay dust. These are, in brief, the means pointed out by M. Fleuret, for making the mortar he designs for factitious stones. The mortars being made after this process, they are placed in moulds where they are beaten and strongly compressed. Mr. Fleuret has made much use of factitious stone in making water conduits, pump tubes, troughs, &c.

Mr. Fleuret established a manufactory of factitious stones at Pont-aMousson. We might suppose that, having at his command the good hydraulic lime of Metz, he would obtain good results; but, as I have said, in the first section, when we attempted to apply this process to making factitious conduits at Phalsburg, and caps of arches at Landau and Strasburg-using the lime of the country, we did not succeed. What I have before stated shows that it is less the manipulation than the choice of materials which affords good mortar, and consequently good factitious stones. But Mr. Fleuret has not given us the means of ascertaining the quality of the materials: and it follows that in pursuing the path he has pointed out, we are exposed to bad results, as happened in the above cases.

According to the experiments I have made, the best process for making mortars to be used in forming factitious stones is, if we use bydraulic lime, to slake it to powder with about one quarter of its volume of water, and to cover it with the materials that are to be mixed with it. I have several times remarked on the importance of making the mortar soon after the slaking of the lime. We ought not therefore to slake more lime at one time, than will suffice for eight or ten days. If the lime be eminently hydraulic, sand only need be added; and, of sand, that which is fine should be much preferred. If only a mixed sand can be had, it should be passed through a fine sieve; if it be too coarse, it should be pulverized; and if it be earthy, it should be washed by the process I have given.

If the lime be only moderately hydraulic, it will be necessary to make the mortars by adding to the lime equal parts of clean fine sand and cement. If we happen to be in a country where the only lime is fat lime, we have seen, by table No. XXXIII, that it is quite immaterial whether this lime be used as soon as it is burned, or after having been slaked for some time, to paste or powder. Care should be taken to use only hydraulic lime made of clay, which contained but little lime. The mortar will be made by mixing the lime with the quantities of clean fine sand and hydraulic cement which shall have been found by trial to be the proportion best suited to the lime. In general, the proportion of one part of fat lime measured in paste, to two, or two and a half, of the other ingredients, will afford a very good mortar. There will be no disadvantage in wetting the mortar sufficiently to cause it to work easily, nor in reworking the mortar with a little additional water, should it become too dry. An excess of trituration is useless; it is sufficient if all the materials be well mixed. All my experiments show that iron does not improve mortar. The scoriæ, and the ashes of forges should not therefore be used without our being satisfied before hand, by the means I have pointed out, that they are hydraulic.

When the hydraulic mortar of which the stones are to be formed, has been made as above, it should be placed in moulds, if the stones are to be of small dimensions, and should be loaded with weights, or submitted to heavy pressure, until it has acquired sufficient consistence to be withdrawn from the moulds without breaking. The stones are then to be deposited in a moist place for about one year.

When factitious stones are made for water conduits, or other objects that are to be buried under ground, there is no objection to using red cements, such as are generally made from brick earths; but this colour would be disagreeable to the eye; and in case the objects are to be exposed to view, cements derived from clays that do not take this colour from burning should be used; such, for example, as are used in making tobacco pipes, stone ware and crockery. With the same view, cement derived from slate may be used.

There is no doubt in my mind that with good mortar, factitious stones may be made which will afford, at the end of a year, a resistance approaching that of ordinary bricks, and that the strength will go on increasing with time. We find in the Annales de Chimie, Vol. XXXVII, that "Mr. Monge, in visiting the ruins of Cesaria, remarked, in a temple consecrated to Augustus, that the pillars had wasted away to a great depth, but that the mortar projected. He tried in vain to break off a piece. The mortar was of very fine and equal grain; it appeared to be composed of fine sand and very little lime, very well mixed." It has been remarked that in the ancient Roman constructions which still exist in the Northern districts, the mortar has perfectly resisted the inclemencies of the seasons. I made at Strasburg large cubes of hydraulic mortar, which I withdrew from the water after about one year, and left exposed to the air during several summers and winters without their sustaining any injury. In the 7th No. of the Memorial de l'Officier du Genie, it may be seen that I proposed in 1819, to cover with factitious stone the floor of a sluice de chasse, which was 100 feet wide and was composed of five passages. The foundation was of concrete, and it was covered with a layer eight inches thick of good hydraulic mortar, which became united to the concrete, formed a factitious stone, showing, the next year, great solidity. It has now been made ten years, and it has sustained no injury. This means may be used with great advantage in countries where there is no good stone for cutting, or where such stone is dear. In countries where free stone is not to be obtained, it is very advantageous to be able to compose factitious stones to form the angles, copings, casements of doors and windows, cornices, gutters, water conduits, &c.; this mode of fabrication might even be applied advantageously to objects of the largest dimensions. It often happens that for bridges across the ditches of fortifications, pieces are needed of about three feet in thickness: where stones are not to be had of suitable quality, these pieces may be easily made on the spot in a single piece of factitious stone: under similar circumstances, piers of sluices, columns, and obelisks may, also, be made; but in the cases of such large masses, it will not be easy to bury them in the first instance, and afterwards place them in the proper situations. It will be best, therefore, to construct them on the spot. It does not seem to me to be indispensable to make plank moulds to give them their proper shape: forms such as are used to direct the construction of walls, would suffice. It would only be necessary to take care, when the mortar had been made of ordinary consistency, that it should stiffen a little before being used, so that on being spread with the trowel it would retain the form given it, and at the same time be so ductile as to spread easily and unite itself well with the previous layer. As this mortar dries very quickly, there is no danger that the work of the morning will give way under the pressure of that of the afternoon. After having finished the day's work, care should be taken to cover the top of the wall with wet straw matting, so as to maintain a favourable humidity; in the morning the surface of the mortar should be rammed with a small flat