In countries where there is neither coal nor turf, it seems to me that the conical furnace might still be used for calcining clays. It will suffice to take dry wood, and in preference round sticks, which should be cut into lengths of four inches: these little pieces should be mixed with the clay balls, and the operation of burning should be like that with coal. doubt that a good result would be obtained.

I do not

Clays which are greasy to the touch are easily formed into balls, or shaped like bricks, without any preparation; but it is possible, it may be necessary to temper such clay as is used in brick making, this being generally more meagre.

When very greasy clays, containing no lime, are used, it will sometimes be difficult to reduce the cement to powder without machinery. In such cases, clays should be preferred which are not too greasy and which contain about five per cent, of lime: these clays are, moreover, more common than others. It is not to give energy to the cements that we should prefer earths containing a little lime: we have seen that opposite effects are sometimes produced. But the presence of this substance has two considerable advantages; the first is that the clay requires less burning to yield good puzzalona, whence results an economy as to the fuel: the second is that the clay containing lime is more easily reduced to powder. Thus it might be well to mix a little lime with clays which contain none, or to mix clays which contain no lime with clays that contain too much: but as these operations require a good deal of work, it will be necessary to calculate the relative expense of the several modes, namely, burning the clays hard, and pulverizing them, though with difficulty-mixing a little lime with the clay or mixing clays together. The relative expense will depend on the price of fuel, and the means at coramand for breaking down the clay. A pestle mill, or mill with large stones, like those used in pulverizing plaster of Paris, would, it appears to me, be most convenient.

The name puzzalona expresses that this substance is obtained from the village of Pouzzol in Italy-that of trass, has no etymology. In the notice which I published in 1825, I proposed to give the name hydraulic cements to substances substituted for trass and puzzalona. This denomination appears to me to be convenient, and I shall generally employ it. I have entered into many details as to the fabrication of these productions because they are of great importance in constructions. I proceed to report sundry experiments that I made with hydraulic mortars.

CHAPTER VII.

Various Experiments on Mortars placed under water.

Mr.

Much importance has been attached to the manner of slaking lime. Lafaye published in 1777, a memoir in which he gives, as a secret recovered from the Romans, the mode of slaking lime by plunging it into water for a few seconds, and then withdrawing it to slake and fall to powder in the air. This powdered lime is preserved in a covered place Other Engineers have asserted that there is great advantage in stifling lime as it slakes; that is to say, covering it with sand before it begins to slake, in order to retain the vapours liberated during the process. Mr. Fleuret attributes great efficacy to this vapour, for he says "This vapour awakens and excites the appetite of the workmen, whence I conclude that it contains principles proper to the regeneration of lime and consequently to the

hardening of mortars. But it is proved that nothing escapes but the vapour of water accompanied by some particles of lime. I made, on this point, the experiments contained in the following table.

Observations on the Experiments of Table No. XXV.

To make the three experiments above, I took a piece of Obernai lime which I divided into three portions. The first portion was slaked by throwing on one fifth of its volume of water, leaving the lime at rest in the air for twelve hours before making it into the mortar No. 1. The hardening was slow because the experiment was made in November.

No. 2 was slaked in the same manner with this difference, that I covered the lime with the sand as soon as I had thrown on the water. This, also, was left to itself for twelve hours before making it into mortar. These two experiments gave, we see, the same results.

The

The third experiment differs from the first in this, that to slake the lime, I plunged it into water for fifty seconds-atterwards treating it in the same manner as No. 1. The result was less by 22 lbs. It is singular that I got the same results, as will be seen further on, by making similar mortars and leaving them in the air, instead of placing them under water. result, it seems to me, is owing to the lime, immersed for fifty seconds, absorbing too much water, which is hurtful, as the experiments in the following table will show. I purposed repeating the trial, by varying the time during which the lime should be immersed, but I had not an opportunity.

We have seen, in tables Nos. IV, V and VII, that the Obernai and Metz lime soon lose a great part of their properties, when they have been slaked to dry powder and left exposed for some time to the air. It is true that a piece of Obernai lime gave me, in this way, the most favourable result of table No. VI, but only in the first month after slaking: for, afterward, this lime rapidly lost its quality. We ought then, by no means to apply the process of Mr. Lafaye to hydraulic limes, unless assured of a different result from mine, which, I think, need not be looked for.

As to fat limes, table No XIV shows that when they were slaked to powder, and left in this state, in the air, for a couple of months, I obtained, by mixing this lime with trass, results sensibly better than when I used lime just slaked. I think, therefore, that it is best to slake fat limes to powder and to leave them for a month or two in this state in the air, before using them in mixture with hydraulic cements; but the process of Mr. Lafaye

may be much simplified, by throwing on the lime a quarter, or a third, of its volume of water: it was in this way that I operated; avoiding, thereby, the embarrassment of panniers or baskets. We have seen, above, that lime slaked to powder has the property of absorbing oxygen; and it is to that property I attribute the benefit derived from leaving lime exposed to the air after being slaked to powder. Table No. XV offers a similar result; but as the proportions were not the same, I do not know which of the two modes is preferable-slaking fat lime to powder, or reducing it at once to paste.* We see that in both cases, there is an advantage in not making the mortars immediately. If time had been allowed me I should have repeated these experiments; making the proportions the same. It sometimes happens that hydraulic lime has been wet, either in the lime house from carelessness, or out of doors, when slaked and covered with sand. I accordingly made the following experiments with lime slaked with more than the usual quantity of water-making mortars therewith at different periods.

1

3 15 242 16 209 20 165 22 132 24 12126 9730 77 32 66

Composition of the mortar.

Au.

Observations on the experiments of Table No. XXVI.

To make the above experiments I took Obernai lime fresh from the kiln, and slaked it by throwing on a bulk of water equal to the bulk of lime: thus slaked, it was in the state of powder, but on pressing it between the fingers it was felt to be a little moist. I made at once, a mortar, taking one part of this lime in powder, and two parts of sand; and I put it in water, after having left it for twelve hours in the air: I made other mortars in the same way, every twelve hours, and at the end of a year, I broke them all. The table shows that after twelve hours the mortar had already lost much of its force, since the second mortar supported 33 lbs. less than the first. That which was made after twenty-four hours lost 77 lbs., and at the end of thirty-six hours, this lime gave a mortar that had lost almost half its force. The table shows that the strength of these mortars went on diminishing in a very rapid manner, to the last, which being made after eighty-four hours, supported only 66 lbs., in lieu of the 242 lbs. which the same lime sustained at first. We see also, that the rate of hardening decreased in a very rapid manner. These experiments show how dangerous

* Comparing the series of Nos. 1 and 3 of table No. XV, we see that at the end of six months, lime slaked to powder supported 44 lbs. more than that slaked to paste. but in the following mortars, the advantage was only 11 lbs. which is but trifling.

it would be to slake hydraulic lime with too much water, and how important it is to keep it from the rain. In the experiments I have made, I almost always slaked hydraulic lime with one-fifth of its volume of water, and fat lime with one-fourth; because, as we have seen in the first table, the latter absorbs more water than hydraulic lime, before being reduced to paste. But in slaking on a large scale, the workmen always waste a portion of the water on the ground: there will therefore be no disadvantage in slaking hydraulic lime with one-fourth, and common lime with one-third of its bulk of water.

The experiments in the above table explain why mortar No. 3 of table No. XXV gave a result inferior to the others: the reason is, probably, because during the fifty seconds that the lime was plunged in water it absorbed too great a quantity; and as it remained twelve hours exposed to the air, it lost part of its strength. I will now give some experiments that I made in in order to ascertain the influence of different quantities of trass on the qualities of mortars.

Observations on the experiments of Table No. XXVII.

We see that in the above experiments I first used a very small quantity of trass, and that I augmented it little by little. No. 2, containing very little trass, gave a result superior to many mortars made of natural or artificial hydraulic limes: such a mortar would not be dear, and might be employed with advantage in gross masonry.

I made no experiments to determine the quantity of sand or factitious puzzalona that would suit the hydraulic and fat lime, of the neighborhood of Strasburg, because I think these experiments should not be made on a small scale. We have seen that the same quarry furnishes limestone giving different results. A multitude of essays, on several pieces of lime calcined to different degrees, would have been necessary, which would have demanded much time. Instead of following this course, I proposed, (after getting some facts as to proportions from the above experiments) to take mortar already made at the works, the lime being there slaked in large quantities at a time: repeating this several times, I should have determined the quantity of sand and hydraulic cement which it would be proper to mix with the lime used at the works: but I quitted Strasburg before it was

in my power to make these experiments. There was used at that place, as I have before stated, one part of quick lime (measured as quick lime) to two and a half of sand: the mortar was good; as there was occasion to ascertain in several partial demolitions.

Mr. Raucourt has advanced the opinion that sands require different quantities of lime according to their degree of coarseness; and to know the quantity of lime, it will suffice to measure the void spaces between the particles of sand, these void spaces being the measure of the lime. Accordingly, he filled a vessel, successively, with several kinds of sand and determined the quantity of water which could be poured on each without overflowing the vessel, whence he obtained for each the quantity of lime that should be added. But experiments should be made to ascertain whether the best mortar is obtained when these voids in the sand are just filled with lime; this Mr. Raucourt has not ascertained.

Before the publication of Mr. Rau court's work, Captain Henry Soleirol of the Engineers, had engaged in researches of the same sort. Capt Soleirol, also, thinks that no more lime should be added than will fill these void spaces in the sand. But his experiments presented anomalies difficult to explain; and they are not sufficient to cause us to admit the principle, in anticipation. This theory is ingenious, but it wants, as yet, the support of facts. The subject is important and deserves to occupy the leisure of Engineers.

The next table presents several experiments made with fat lime in reference to the manipulation of mortars.