in 1822, a work wherein he narrates the experiments he made, following the process used by Mr. Vicat, and adding several of his own. Mr. Bergère, chef de bataillon du génie, gave an analysis of this work, in the Annales des Mines of 1824, Vol. IX.

In 1825, Mr. Hassenfratz published a memoir on mortars. This work, which is voluminous, contains many practical details on the calcination of lime-stone in different countries, and exhibits the actual state of knowledge, in the art of making mortars at the period of publication.

In terminating this reference to works on hydraulic mortars, which have appeared up to this time, I must introduce a fact, entirely new, announced by Mr. Girard de Caudemberg, engineer of roads and bridges, in a notice published by him in 1827. He states that the proprietors of mills on the river Isle, in the department of Gironde, discovered by accident, a kind of fossil sand to which they gave the name of aréne, which has the singular property, without any preparation, of forming, with fat lime, a mortar that hardens under water, and has great durability. I shall have occasion to return to this important fact, and to report what Mr. Girard says, as well as to state the principal experiments which have been made with this substance, in other places where it has been found.

I was employed from 1816 to 1825 at Strasburg, at which place they had made no use of hydraulic lime. I ascertained, however, that such lime was to be found in the neighbourhood. Almost all the hydraulic works connected with the fortifications of the place, having been badly constructed, and dating as far back as Vauban's time, were to be rebuilt. Twenty-five years' experience had taught me the great superiority of hydraulic mortars in the air as well as in the water-where, indeed, they are indispensable. I tried, therefore, the hydraulic limes, afforded by the environs of Strasburg, and found them excellent: they were, consquently, used in all the works both in air and water. All the revetments built from port de Pierre to port Royal, having a development of about 1650 yards, were rebuilt or repaired with hydraulic mortar. It was the same with the hydraulic works; they were rebuilt or repaired with the hydraulic lime of the neighbourhood.

An engineer who should use fat lime, even for constructions in the air, when there are hydraulic limes at hand, would be very censurable, because the expense is about the same, and, as regards the strength and durability of masonry, there is a vast difference in favour of the hydraulic mortar. But in countries where no hydraulic lime is to be had, or only that of mediocre quality, what should be done? Shall the engineer adopt the process of Mr. Vicat, which consists in making an artificial hydraulic lime? I answer, emphatically, that I think not; in this case, occurring very often, it is, in my opinion, preferable to make hydraulic mortar by a more direct process which I shall point out.

There are two modes of obtaining hydraulic mortar; the first consists in mixing natural, or artificial, hydraulic lime with sand; the second consists in mixing ordinary fat lime with certain substances such as puzzalona, trass, certain coal-ashes, and brick dust, or tile dust. I feel bound to correct here, an assertion touching these mortars, not perfectly accurate, of Mr. Gauthey, Inspector of roads and bridges. In his excellent Treatise on the construction of bridges, this Engineer says (Vol. II., page 278) that "fat lime is very proper for constructions out of water; but will not answer in the composition of betons to be placed in water, because the mortars in which it is used, even when mixed with puzzalona, and placed in water as soon as made, do not harden, but remain pulverulent." This is far from exact:

because mortar composed of fat lime and puzzalona hardens very soon in water, and acquires, in a short time, very great strength. This fact was known to the ancients, for Vitruvius speaks of it, as will be seen further on.

I should not refer to the error into which Mr. Gauthey has fallen in this instance, if he did not enjoy a reputation so justly elevated. His highly esteemed work being in the hands of every Engineer, it was to be feared that this remark of his would prevent constructors from making hydraulic mortars by the direct union of common lime and substances analogous to puzzalona. My experiments will show that, in countries where hydraulic lime is not to be had, instead of following the process of Mr. Vicat, it is preferable to make hydraulic mortar by a direct mixture of fat lime with substances of a similar nature to puzzalona. These experiments show also that fat lime is far from being always proper for construction out of water; although Mr. Gauthey, in the beginning of the sentence, states it to be.

CHAPTER II.

On slaking Lime; manner of making Mortar; observations on Hydrate of Lime.

There are three modes of slaking lime. The first consists in throwing on the lime, as it comes from the kiln, enough water to reduce it to thin paste. This process is the one generally employed with fat lime. Too much water is added, almost always-that is to say, as much as is required to make it a thin cream. In this state it is run into vats; after some time it thickens, and it is then covered with a layer of sand or earth to preserve it from contact of the air, which would soon convert the upper portion into a carbonate. It is a common opinion that the longer the lime has been kept in this state, the better it is. My experiments will show that this is not true, at least not always true: since some fat lime that I had experimented with, which had been lying in this condition, gave, in the air, when the mortar was composed of lime and sand only, very bad results.

The thickening of the lime in the vats is due to the escape of water by filtration, by evaporation, and also to a third cause: for this thickening which is quite prompt, occurs equally when the vats are constructed in moist ground, and when the season is rainy. This third cause appears to me to be this: that the lime, having a strong affinity for water, solidifies the first portions very promptly, but requires a considerable time to saturate itself completely. These portions of the lime which have been too much or too little burned are, besides, slow to slake. I made the following experiment to satisfy myself on this point. I took a portion of lime that had been lying wet in a vat for four years, it was quite thick, I added a little water to bring it to the consistence of sirup, and placed it in a stoneware vessel. I took an equal portion of fat lime, slaked fresh from the kiln, reducing this also to the consistence of sirup, and placing it in a similar vessel. After a short time, this last had become very thick, while the former retained its consistence of sirup; I then added water to restore the consistence first given. The thickening again occurred, but more slowly than at first. It was necessary to add water several times before the second lime would maintain the sirupy state. It results from this experiment that fat lime, slaked into a clear paste as it comes from the kiln, retains the power of absorbing water for a considerable time.

The second method of slaking consists in plunging quick-lime into water for a few seconds. It is withdrawn before the commencement of ebullition; slakes with the water it has absorbed, and falls to powder. It is preserved in a dry place. The operation is performed with baskets into which the lime, broken to the size of an egg, is put. Mr. de Lafaye, in 1777, proposed this mode of slaking lime, as a secret recovered from the Romans; it made much noise at the time, but experience has not realized the great results anticipated.

The third process consists in leaving the quick-lime exposed to the air. Its strong affinity for water causes it to attract the greater part of that which is in the surrounding air. Lime, thus exposed, slakes slowly without giving out much heat, and falls at last to powder. This mode of slaking is called air-slaking, or spontaneous slaking. It is employed, more or less, in several countries. It is spoken of in several works on constructions, and is generally condemned. Mr. Vicat, however, appears to give it the preference, for, at page 20 of his memoir, he says: "Such are the three modes of slaking lime: the first is generally used; the second has hardly been tried, except as an experiment at certain works; the third is proscribed, and represented, in all the treatises on construction, as depriving the lime of all energy, to such a degree that those portions which have fallen to powder in the air, are considered as lost. We shall not now speak of the processes of Rondelet, Fleuret, and others, because they do not differ much from those described. We shall see, further on, that, as regards spontaneous slaking, these proscriptions of authors who, believing every thing, repeat without examination the errors of those who preceded them, are founded on false observations and are deserving only of mistrust." Mr. Vicat has announced that a mortar made of sand, and fat lime which was airslaked, resisted perfectly at the end of ten years, the test indicated by Mr. Berard for frost-proof stones; he says on this subject "a hint, this, to those who have written and spoken so much against air-slaking, and in opposition to the opinion which I have had to maintain singly, unable to invoke to my aid any experiments but my own." The results I have obtained are far from confirming what Vicat says, as will appear by the experiments reported hereafter.

Mr. Vicat gives, at page 20, experiments made by him to ascertain the amount of swelling of fat limes and of hydraulic limes, on slaking by the three modes. He found the first mode to be that which gave the greatest volume of paste, with both kinds of lime. On comparing the bulks obtained by the second and third modes, it was found that with fat lime, air-slaking gave greater bulk than slaking by immersion, and that it was the reverse with hyraulic lime.

At Strasburg, an attempt was made to apply, on a large scale, the mode of slaking by immersion, pointed out by Mr. Lafaye; but the process was found to be attended with inconvenience and embarrassment. It is necessary to procure a stock of baskets-to break up the larger pieces-to secure workmen who will be faithful in holding the lime under water only the given number of seconds, which is not easy; a portion of the lime is lost, falling in powder to the bottom of the vessel of water; when the lime is reduced to powder, it is requisite to measure it before making the mortar, and should there be a wind, much will be lost. These objections caused the process to be renounced in favour of that about to be described, and which amounts to the same thing.

It is founded on the following observation: if quick-lime be plunged into

water it absorbs, in a certain number of seconds, a quantity sufficient to reduce it well to powder. We shall have then a like result by throwing the same quantity of water on the lime, and avoid the inconveniences attending the plunging into water. Since 1817, this process has been employed at Strasburg, where considerable masses of lime were operated on. A small building was erected near the works, into which the hydraulic lime, not allowed to arrive too fast from the kiln, was put, to be protected from the weather; the building was boarded on the sides and top, and, in case of rain, covered with a tarpaulin. By the side of this lime-house, a larger shed was constructed, the top only being boarded; a plank floor, on which the mortar was mixed, was laid under this shed. There was a measure, without a bottom, which contained about 10 cubic feet, each dimension of the box being about 2.20 feet, this was placed on the floor and filled with lime; which being done, the same measure was used for the sand, which was placed around the lime, without covering it: with large tin watering pots of known capacity, water, equal in bulk to about one-quarter the bulk of the lime, was thrown on: the workmen knew they were to empty the watering pots but a given number of times; and the lime being all in sight they saw that they should throw the greater quantities on those parts of the heap where lay the largest lumps of lime. As soon as the slaking became energetic, the lime was left to itself until the vapours had ceased; it was then turned a little with a shovel, or a rod was thrust in, and if any lumps were found still entire, either for the want of water, or because they were too much burned, a little water was poured on these lumps. A regular form was then given to the heap, and the surface being slightly pressed with the back of the shovel, the lime was covered with the sand that had been placed around it. This process was completed towards evening-as many heaps being prepared as it was presumed would be required during the whole of the ensuing day. By thus leaving the lime, over night, in heaps, the slaking is complete; portions which have too much water impart it to those which have too little, and the water becomes thus uniformly diffused through the heap. In the morning the sand and lime of each heap were mixed together, and passed twice under the rab (rabot) before adding any water: in this way, if there were any stones, or pieces of lime imperfectly slaked, they were easily found and rejected. Water was then added in sufficient quantity to bring the whole to the state of very soft paste; because in this dilute state the mortar is, with less labour, mixed more perfectly. Experiments which follow will show that it is an error to insist that mortar should be mixed with "the sweat of the labourers:" it is enough if the sand be well mixed with the lime; and this mixture is better effected, and in a much more economical manner, when the mortar is in a state rather thin, than when it is thick; another reason for making it rather thin is, that it often becomes stiffer than it ought to be, before it is used, in consequence of the lime preserving, as before stated, for a considerable time, the property of solidifying water. When the lime has been properly burned, the operation just described gives a homogeneous mortar not at all granular, and not exhibiting a multitude of little white specks, which are particles of lime that have been badly slaked. At Strasburg the precaution was always taken of making up only one or two heaps of mortar at a time; so that it should not have too much time to dry before being used, and that the masons might find it in the state of paste, in the heaps in which it was deposited after being well worked. In making the mortar only as it is needed, there is, besides, the advantage of avoiding the labour of remixing, in the frequent case of the

works being interrupted by rain: it is best therefore to make the heaps of slaked lime into mortar, no faster than as they are wanted. Lime may be preserved in this way for eight or ten days without losing quality. If at the end of the day, all the heaps of lime have been consumed, new heaps should be formed for the consumption of the morrow: if a portion only have been consumed, this portion should be made good. This manner of slaking lime, and making mortar, gave very good results at Strasburg, and at other places in the vicinity where it was employed. It is seen to be a method analogous to that by immersion, recommended by Mr. Lafaye; but, by throwing upon the lime, just the quantity of water necessary to reduce it well to powder, instead of immersing it, much inconvenience and embarrassment are avoided, especially when operating with large quantities of lime.

An opportunity was presented of convincing ourselves of the goodness of mortar made by this method, it being necessary, in order to make a postern, to pierce through a revetment wall that had been built two years: the mortar had already acquired such hardness that the tools had much difficulty in cutting through the masonry.

In extensive works, it will be very advantageous, as regards economy, to make mortar with a machine. Several have been contrived with this object, but that which has succeeded best is a two-horse machine proposed and executed by Mr. Saint-Léger, formerly Captain of Engineers. The following is a brief description.

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A circular trench, having the two sides sloping, is built of masonry; the section of the trench is a trapezoid 2 feet wide at bottom, 3 feet 4 inches wide at top, and 1 foot 4 inches deep; the inner circle of the trench is 9 feet 4 inches in diameter; at the centre there is a mass of masonry, in which is fixed vertical axis, of wood, 6 feet 8 inches long, and 8 inches square, and which is bedded in the masonry about 5 feet; the top of this axis is formed into a cylinder 5 inches in diameter, and 6 inches high; around which is fitted a collar of cast-iron, carrying laterally two horizontal trunions 32 inches in diameter, and 4 inches long; a piece of wood, 26 feet 8 inches long, is notched at its middle upon the collar of the vertical axis. (Instead of one piece of wood, two might be taken, each 13 feet 4 inches long, by strongly securing, with iron, their junction with the vertical axis.) This piece is placed horizontally, and is about 18 inches square in the middle, lessening towards the ends, so as to serve as an axletree to two vertical wheels with broad felloes-6 feet diameter of wheel, and 6 inches breadth of felloe. These two wheels rest in the circular trench in such a way that the one touches the exterior and the other the interior slope of the trench. A horse is attached to each extremity of the horizontal bar, and their united efforts cause the wheels to revolve in the trench; behind each wheel, attached to the horizontal bar, by means of a hinge, is a scraper of wood armed with iron, these follow the movement of the wheels, scraping the two sides of the trench so as to throw the mortar under the wheels. These scrapers of which the lower end is within two inches of the bottom of the trench, are attached by hinges in order that they may rise over any obstacle. Mortar is made in this machine in the following manner. A cubic metre (35.34 cubic feet) of lime in the state of paste is thrown into the trench, and the horses are started; a little water is added if necessary, and when the paste has become quite liquid and homogenous, the proper quantity of sand is thrown in by the shovel, without arresting the movement; in about 20 or 25 minutes the mortar is made. With this machine 12 batches of 3