jointed by the enlargement of the bulk of mortar. I was not, at first, aware of this effect, and having caused a coat of rough-cast, composed of Obernai lime and sand, to be applied to one of the Government buildings, I was surprised, after five or six months, at seeing, on many points of the surface, swellings of the size, nearly, of a half dollar; these portions finally fell off, and it was then seen that they had been caused by particles of blue lime, about the size of a pea, that were beneath. These particles had not had time to slake thoroughly; and, as imperfectly slaked lime has a strong avidity for water, they absorbed water from the air, and increased in bulk. The force must have been great to break the very hard mortar which covered these particles. This rough-cast has now been on six years; it is very hard, and has perfectly resisted all inclemencies of the seasons. The property which hydraulic lime, too much calcined, has, of swelling considerably after being made into mortar, might be availed of, under certain circumstances. If it were required, for example, to fill cracks in old walls where the hand could not be introduced, or to fill spaces washed from under old foundations, mortar made of highly calcined hydraulic lime might be advantageously used, because the mortar by swelling would more perfectly fill the cavities. But especial care should be taken not to use it in new constructions, as it might cause serious accidents. It is stated that two locks having been made, a short time since, with lime that had not been sufficiently slaked, the mortar swelled to such a degree that all the cut stone was displaced, and it was necessary to rebuild the masonry. This accident happened to the works for the improvement of the navigation of

the Vésère.

Table No. II. teaches that the yellow lime and the blue lime gave, alone, greater resistances than when mixed with sand. But I ought to observe that no conclusion should be drawn from this, as the three experiments on hydrates were made with lime different from that used in making the mortars of the table. At the time of making these experiments, I was far from thinking there could be so great a difference between mortars made of pieces of lime which seemed to have sustained the same degree of calcination. The best results with the Obernai lime alone, are 372 lbs. for the yellow, 354 for the gray, and 275 lbs. for the blue. I regret not being prepared for this superiority of the hydrate over mortars, but I was far from expecting it: otherwise, after having made the experiments with the limes alone, I should have made mortars, by adding successively, 1,,, &c. of sand, so as to judge better of the effect of sand; but it was only on breaking the mortars at the end of a year, that I could know the effect, and I have often, on such occasions, obtained results that surprised me, and been conducted to new experiments, requiring another year to furnish results.

3

It is seen that after a certain quantity of sand has been added, the mortars lose much of their tenacity: and that for every degree of calcination, trass considerably augments the resistance. It is remarkable that mortars made of lime, sand, and trass, are in general better than those made with lime and trass, without sand: I have, however, found some exceptions. Mr. Vicat thinks that puzzalonas only slightly energetic, suit best with hydraulic limes; but my experiments do not support this opinion; as the trass I used was very energetic, and it will be seen that I always had very good results on mixing it with very hydraulic lime. I obtained, in the above table, better results with Obernai lime, trass, and sand, than with

Obernai lime and trass without sand, and it will be seen, by experiments which follow, that I found the like results with fat lime. In the 7th number of the Memorial de l'Officier du génie, I have advanced that, in important constructions, it is not prudent to employ hydraulic lime without a little trass. Mr. Vicat is of a different opinion: for we see a note in the Annales des Mines, Vol. X., page 501, wherein he says that the success which has attended the use of hydraulic lime in different works which he cites, ought to suffice to undeceive those who partake my fears as to the insufficiency of hydraulic lime without trass, in important constructions. I have, nevertheless, had an opportunity of seeing the concretes used at the canal Saint Martin, at Paris, and at the basin of the Palais Royal, made of the hydraulic lime manufactured by Mr. Saint-Léger, and they are much less hard than those made at Strasburg. I do not doubt, however, that the concrete employed at the Saint-Martin canal will well fulfil its object. Being obliged to cover all the bottom of this canal, which has an extent of about 4400 yards, with concrete, in order to keep the water from filtering into the cellars of the inhabitants, it would have much augmented the expense, to have added to this mortar, cements analogous to trass. Concrete composed of Obernai lime and sand, without trass, was often used at Strasburg, for the foundations of revetments, of piers of bridges, &c.; but by "important constructions" I meant Locks and Dams, which have a great pressure of water to sustain, perhaps for a long time. There is no doubt that if such a work were executed with gray lime like that of the preceding table, there would be nothing to fear: but this would require the rejection of a great part of the kiln, and would make the mortar very dear: the extremes, only, can be rejected, so that the lime used is a mixture of the three qualities. If a mean be taken of the three kinds of mortar found under Nos. 3, 10, and 17, we shall have 276 lbs. for the result, while the mean term of the three Nos. 6, 13, and 20, is 427 lbs. I have often, however, had results much weaker than those of the above table, as will be seen in the sequel. I have even had occasion to remark that hydraulic limes coming from the same quarry, and appearing to be calcined nearly to the same degree, have, nevertheless, given very different results: in these cases, the lime, which, mixed with sand had given me a weak resistance, had always afforded a very good mortar, when I used sand and trass with the lime. I am convinced that these last mortars have given, on an average, a resistance more than double the first; I persist, therefore, in thinking that in important constructions, such as Locks and Dikes, which have constantly a strong pressure of water to support, it is prudent to put a little trass, or some analogous substance, into the mortar, even when the hydraulic lime used is as good as the very good hydraulic lime of Strasburg. It is for the Engineer who has the work to execute, to examine the quality of the lime he is to employ, in order to fix the quantity of these matters to be added in the mortar. In my opinion a Government should not regard a light additional expense, when the object is to obtain constructions that will last a long time without needing repairs: unfortunately the contrary system, which I think a very bad one, most commonly prevails.

I ought to premise that in the experiments which I shall report, I shall abstain from conclusions too general. With the limes of Alsace I have often noticed contradictory results, and even, as before mentioned, with limes from the same quarry. We have not, as yet, collected enough facts to establish a general theory: that which may be true as to the limes of

one country, may not be true as to those of another. I give the results which I have obtained. It is very desirable that Engineers who have some leisure, should make experiments; as it is only when a great many results shall have been collected that we shall be able to deduce any general principles: those which I shall present must be considered as belonging to only the limes which I shall use.

I now proceed to report other essays upon other limes of the environs of Strasburg.

Observations on the Experiments of Table No. III.

The Altkirch lime comes from a lime-stone which is grayish: all the others from stones that are blue like those of Obernai and Metz. All were slaked to powder with one-fifth their volume of water, and left in that state for twenty-four hours, before making them into mortars. Several being too remote to be brought into use at the works, I contented myself with some experiments on their hydraulic properties. It is seen that the Altkirch lime is good. The piece of lime brought me had, probably, been out of the kiln a number of days, which should considerably influence the result, as will be soon seen. It is possible, therefore, that this lime is better than the experiments indicate. Two different kinds were brought from Villé. I was told that it was as good as the Obernai lime. The first kind is brown, and the second grayish: we see that the resistances are feeble. The limes of Rosheim, Hochfeld, Verdt, Oberbronn, and Bouxviller gave very good results whether alone or in mortar. It will be seen, also, that with the limes slaked to powder and measured in that state, the mortars consisting of lime and sand, generally supported less weight than the lime alone. It is possible that there was too much sand in them. I began with the principle, admitted by several Engineers, that it is better to err by excess of sand than by excess of lime; but it appears this is not always true, for the experiments of tables Nos. II. and III. show that when the mortars had a little too much sand, their resistance was much diminished. It is possible also that some hydraulic limes may become harder when alone, than when mixed with any proportion of sand whatever.

The Oberbronn lime was measured in paste; it appears that with two and a half parts of sand, it gave a better result than with two parts only, and that this result was superior to that obtained with the lime alone.

As the Bouxviller lime might be used at Strasburg I treated it with more detail. We see that this lime loses much by being too much burned, and that it cannot support as much sand as that of Oberbronn; and, also, that with all these limes, the mortars were much improved by adding trass to the lime and sand. It is an error, therefore, to suppose that energetic puzzalonas are not adapted to good hydraulic limes. Tables Nos. II. and III. show that different limes take very different proportions of sand. Thus, as before observed, various experiments induce me to believe that pieces of lime from the same quarry may give very different results, although calcined to the same degree. It also appears that different degrees of burning require different proportions of sand; it becomes, therefore, difficult to fix accurately the proportion of sand which should enter the mortar.

The above experiments were a first essay only: I proposed making others, with a mixture of lime calcined to different degrees, and to repeat the experiments on several burnings, so as to be able to take a mean. At Strasburg we adopted one part of lime to two and a half parts of sand. The mortar was good, but it, possibly, was not the best that could be made. Supposing the best proportions, as to hardness, were one part of quick lime to two parts of sand, it would still remain to determine whether these proportions should be adopted; because, if in putting a little more sand the result should be but slightly inferior, it might be done unhesitatingly, if the economy were sensible. I was about to make these experiments on the different limes of the environs of Strasburg, when I was obliged to quit the place, in consequence of the new functions to which I was called. Towards the end of the table, are the experiments made with Metz lime.