pieces of lime; the proportions are shown in the table. The best result answers to one of lime and two of sand, and it will be remarked that these three mortars are superior to the first three, which I attribute principally to a smaller quantity of sand.

The numbers from 7 to 11, were made of lime from another burning: the sand was augmented each succeeding trial by a quarter of the bulk of lime, while in the preceding experiments it had been augmented by a half. The best result corresponds with one part of lime in paste to two parts of sand; it is possible that a smaller proportion of sand would be still better, for we see that No. 19, which is lime reduced to paste without any sand, gavě a stronger result; but as No. 19 was made of lime of another burning, I placed it at the end of the table that it might not lead to any false conclusions. I did not anticipate that there may be good hydraulic limes which will support only a very small quantity of sand; otherwise, I should have commenced the series with a much smaller proportion.

The numbers from 12 to 16, are mortars made of the hydraulic lime of Bouxviller, a small village situated at the foot of the Vosges, between Haguenau and Savern. This lime was treated in the same way as the Obernai lime that precedes it in the table. We see that the Bouxviller lime will bear more sand than that from Obernai, and that the best result corresponds to the proportion of one part lime measured in paste to two and a half parts of sand. The lime was of the same burning as that used in table No. III. Comparing the figures of the two tables, I was surprised to see that the same lime supported more sand in the air than in the water, which has happened very rarely in the course of my experiments; I am ignorant whether this is owing to some peculiar circumstance, or is a quality pertaining to certain hydraulic limes.

When

The experiments Nos. 17 and 18, were made with lime from Oberbronn, a village at the foot of the Vosges, to the left of the road from Haguenau to Bitche. I obtained a very good result with one part of this lime measured in paste and two parts of sand; but No. 18 shows that by putting two and a half parts of sand in the mortar, it lost nearly half its force. making these two experiments with Oberbronn lime, I put, on one side, a fragment which I slaked with one-fifth of its volume of water, so as to reduce it to dry powder. I left it in this state, in the air, for fifteen days. I then made a mortar like No. 17. This mortar broke with the weight of 132 lbs., in lieu of the 297 lbs. which the mortar supported when made of lime just slaked. I obtained a similar result with the Bouxviller lime. All the limes, therefore, which I have used in my experiments in the air, and which are included in the preceding tables, require to be used a very short time after being burned: they lose, otherwise, a great portion of their energy; as is also the case when they are used in water.

No. 19 was made, as I have said, of Obernai lime, alone, reduced to paste with water. When I saw the hardness of this hydrate, my intention was to make similar trials with other limes, in order to know if the sand added in the composition of mortars, always, or only with some peculiar limes, diminished the resistance of the hydrates: but I was obliged to quit Strasburg before it was in my power to make the essays.

ARTICLE XIII. Various experiments with Mortars exposed to the Air.

In this article I shall give several experiments which I made on mortars

exposed to the air. Several are repetitions of those contained in article

VII, of mortars placed in water.

Observations on the experiments of Table No. XXXVII.

The above experiments were made with the same lime, and in the same proportions, as those of table No. XXVI, of which the mortars were put in water. For the manner of operating, I refer, therefore, to the observations on table No. XXVI.

These air mortars behaved like those placed in water. The mortar made immediately, afforded, in the air, a resistance rather less than the corresponding mortar of table No. XXVI; the others gave results about the same in the two cases. It is therefore important not to slake hydraulic lime with too much water, whether it is to be used in the air or in water; and we perceive that the method proposed by Mr. Lafaye is very inconvenient for hydraulic lime, because it requires the quick lime to be broken into small pieces of nearly equal size, which is a long and expensive operation. Unless this be done, the small pieces will absorb too much water, and the large pieces too little.

123

45678

Mortar made of equal parts of common lime, sand and trass
Same mortar, worked anew, without water, after 12 hours
Same mortar, worked anew with a little additional water, after 12
hours

(Weights sup-1 ported before breaking.

Ibs.

143

154

209

154

176

Mortar of 1 part of Obernai lime and 2 parts of sand
Same mortar, worked anew with a little additional water, after 12
hours

Observations on the experiments of Table No. XXXVIII. These experiments were made with the same lime as, and in a similar manner to, those recorded in table No. XXVIII. The first six mortars correspond to Nos. 1, 3, 4, 6, 8, and 10, of that table, which were put under water. I made similar trials with lime and sand only; but, as I used lime which had been slaked for some time, I obtained only feeble resultsthe greater part of the mortars not supporting the weight of the scale pan. If we compare No. 2 of the above table with No. 1, we see that the mor*This mortar was cracked, which may have lessened its resistance.-AUTHOR.

tar that was reworked without water, after twelve hours, gained but little upon that which had not been reworked; but that No. 3, which had been reworked with a little water, had gained a good deal. After twenty-four hours, the mortar No. 4 had become so dry that it was impossible to rework it without water; I was therefore obliged to moisten it a little, like No. 3. We see that the result was about the same as at the end of twelve hours. After thirty-six hours the hardness had become sensibly weaker, and at the end of forty-eight hours it had become stronger than at the end of thirtysix hours-an anomaly which I cannot explain.

Comparing the results of the above table with the corresponding ones of table No. XXVIII, we find that the resistances of the mortars left in the air are sensibly less than those placed in water. We see also that in both cases, I obtained no advantage of consequence, by reworking the mortars without additional water; but that there was a material augmentation of force, on reworking the mortars with a little water.

Nos. 7 and 8, of the above table, answer to Nos. 13 and 14 of table No. XXVIII, and were made of the same lime. There is in these mortars, also, much resemblance as to the effect of reworking upon mortars in air and in water; that is to say, the mortar which had been reworked with a little water after twelve hours, presented a greater resistance than that which had not. The resistances of these tables are generally rather weak, because of my using lime which had been slaked for about fifteen days.

At all works, it is often necessary to rework the mortars in the course of the day, especially in hot weather, when it dries very quickly and becomes too stiff to be used. During the hours of recess from labour, and when the works are interrupted by rain, the mortar will harden considerably. It would sometimes demand a great deal of labour to restore the mortar to a suitable consistency without water. My practice has always been to add a little water whenever the mortar had become, from any cause, too stiff, and required reworking. The preceding remarks show that no disadvantage attends this mode of operating. There is, no doubt, a limit, both as to the quantity of water to be added, and as to the time elapsed before reworking; and I purposed making other experiments in order to ascertain these limits, but I had not time. The experiments cited are nearly sufficient, and they prove that the opinion on which is founded the saying that "mortar should be tempered with the sweat of the workmen," is a prejudice leading to considerable useless expense. The essential point is, that the lime, and the substances added to it, be intimately mixed; and this is most easily accomplished when the mortar is soft.

Observations on the experiments of Table No. XXXIX.

These experiments are the repetition of those described in table No. XXV, and they were made of the same lime. The results are similar to those given by mortars put under water; only they are a little weaker. The comparison of Nos. 1 and 2 shows that there is no advantage, when slaking lime, to cover it with sand. This is called stifling the lime; and several constructors think that the vapour which rises, on slaking, possesses valuable properties.

Mortar No. 3 of table No. XXXIX was made of the same lime as No. 3 of table No. XXV, and I used, in slaking the lime, the process recommended by Mr. Lafaye, which consists in dipping the quick lime into water. The resistance was less than No. 1, which was slaked by throwing on a little water. This appears to me to be owing to the pieces of lime absorbing too much water from being in small pieces; and as the lime remained twelve hours in this state, it lost a part of its force-for it follows, from tables Nos. XXVI and XXVII, that lime, slaked to a humid powder has already lost a part of its energy, when it has lain twelve hours exposed to the air.

Observations on the experiments of Table No. XL.

The above experiments are of the same kind as those made with mortars put under water, and reported in table No. XVI. These air-mortars gave similar results with those put in water; that is to say, those made of pulverized bricks, or tiles, but lightly calcined, gave much better resistances than those made of more highly burned bricks or tiles. We see that the dust of No. 5, which had been recalcined by being kept red hot for six hours, gave with the same lime, a mortar that had no strength. The dusts used in these trials, like those of table No. XVI, contained a notable quantity of lime. The resistances of the mortars left in the air, were, in this instance also, rather less than those of the mortars put in water.

Observations on the experiments of Table No. XLI.

These experiments correspond with those of table No. XVII, that is to say, having used brick dusts which gave resistances the feebler the more they were burned, as those of tables Nos. XVI and XL-other brickdusts were found, as those of tables Nos. XVII and XLI, which gave results exactly opposite. The mortars of the two tables XVII and XLI, gave in air and in water, results so much the stronger as the cements had been the more burned. In speaking of tables Nos. XVI and XVII, I said, that the differences were owing to the cements of table No. XVI, containing much lime, while those of table No. XVII, contained little. But the cements of tables Nos. XL and XLI, were the same as those of tables Nos. XVI and XVII, and the results in the air were just like those in the water. The comparison of these four tables leads to an important conclusion; namely, that clay-dusts which give results with fat lime, when placed under water, will also give good results, when exposed to the air: and if the results be bad in the first case, they will be equally so in the second. Whence it follows that to know whether clay dust which is to be used in air mortar, will give good results, it should be tried with fat lime in water; following the process pointed out in the first section. We may be certain that cements which have not the property of causing fat lime to harden in water, will give, in the air, no better results than the same lime mixed up with sand alone. On the other hand, the more hydraulic the cements are, the better, for all uses in the air. The trials of which we have spoken, should not be neglected, for they are of great importance, as regards the solidity of masonry exposed to the air.