slaking; it would be necessary to reduce it by pestles: in this, it presents similar character to the Boulogne pebbles.

I reduced a portion of this lime to paste with water, and placed it at the bottom of a glass filled with water: the hardening was complete at the end of six days, although the experiment was made towards the close of November. I mixed a portion of this lime with an equal quantity of sand: the hardening took place in eight days; and it seemed to me that this lime would not bear much sand. I was not able to ascertain the tenacity of mortar made of this lime; but the manner in which it hardened, and the degree of induration it had acquired in a short time, led me to think that it is a very good hydraulic lime; and that it will be of great advantage in all hydraulic constructions on the coast of Bretagne. I am astonished that alumine was not found in this lime, so eminently hydraulic..

CHAPTER IV.

Of Artificial Hydraulic Limes.

I stated in the first chapter, that, for a long time, the property possessed by certain limes of hardening in water, was attributed to the presence of oxides of manganese and of iron; several very hydraulic limestones were, however, at last found that contained no oxide of manganese and very little iron. It was observed, at a later period, that almost all hydraulic limestone contained from one to three-tenths of clay. This led to the opinion that when a certain proportion of clay is disseminated in limestone, it combines, by calcination, with the lime, and imparts to it the property of hardening in water. I stated that Mr. Vicat, Engineer of roads and bridges, published, in 1818, an interesting memoir on hydraulic mortars, and that he announced that by reburning fat lime with a certain quality of clay, he obtained very good hydraulic lime. I was bound to state, also, that Dr. John, of Berlin, presented to the Society of Sciences of Holland, a memoir on the subject which was crowned in 1818, and published the following year. He gave the analysis of many common, and hydraulic limes, as well as of many ancient mortars, and he showed that the hydraulic proper ty of lime is due to a portion of clay which combines with it by calcination, and he calls this clay the cement of the lime.

The Minister of War sent the memoir of Mr. Vicat to those places where public works were in progress, and directed the experiments announced by this Engineer, to be repeated. I was then occupied at Strasburg, in rebuilding the sluices and other badly constructed hydraulic works; and was using the Obernai hydraulic lime. I had caused trass to be brought from Andernach, having had occasion to employ it at the great works of Vésel in 1806, 1807 and 1808, and knowing its excellence in constructions under water. In the erection of the sluices at Strasburg, we composed the mortar of the Obernai hydraulic lime, sand and trass; but this last substance being dear, I had begun making some essays towards replacing it with cement, when the memoir of Mr. Vicat was sent to me. I caused the experiments given in that work to be repeated, in the first place by an officer of Engineers, who used a clay of which bricks are made in the environs of Strasburg: he obtained no satisfactory result. A second officer was directed to recommence the experiments, and he was not more successful than the first. I repeated them, then, myself, and I took great pains in making the mixture of clay and lime; they were then calcined and I obtained a result, but it was a very feeble one. I then began anew, using

other argillaceous earths, richer in clay, (plus grasses) and I got much better results. The mortars made with these artificial hydraulic limes, are

Observations on the experiments of Table No. IX.

The hydraulic limes of this table were made by the process of Mr. Vicat: care being taken to temper the clays in water, and to make the mixture when they were reduced to the consistence of a homogeneous sirup. If the clays be not diluted, the union with the lime cannot be so well made. The compound of lime and clay was made by taking one part of lime measured in paste, and the several proportions of clay mentioned in the table. All the limes were reburnt by placing them in Hessian crucibles, in the middle of the lime kiln; there was no exception but No. 14, which, as will be stated below, was more strongly calcined than the other limes. All the mortars were made as soon as the artificial limes were withdrawn from the kilns. The proportion of the first seven mortars was got by taking one part of lime in powder to two parts of sand. Some experiments having shown me that in this mixture there was too much sand, the mortars from No. 8 to No. 23 were made by taking one part of lime in paste to two parts of sand.

No. 1 consists of fat lime reburnt with of brick-earth of the environs of Strasburg: this did not give a good result, as may be seen in the last column.

Nos. 2, 3 and 4 were intended as a trial of the clay of the Village of Holsheim, near Strasburg. Lime was mixed, successively, with 1, 2 and 3 tenths of that clay: it will be seen that the best result was with 2 10; the mixture in which there was of clay gave a mortar which did not bear the scale pan, and broke easily in the fingers. The Holsheim clay is reddish, and greasy to the touch. Further on, I will give its composition.

Nos. 5, 6 and 7 are a repetition of the three preceding experiments, with this difference-that lime made from white marble was employed, in

stead of the fat lime of the country. The greatest resistance that I obtained with this lime was in a mixture with of clay. It is remarkable that with of clay, I obtained a passable result, while in No. 4, with the same proportion of clay, and differing from No. 7 only in the kind of lime, the result was very bad. I do not know to what this can be owing.

No. 8 was made with a greasy and very white clay, brought from the neighborhood of Cologne, to Strasburg, to make pipes. This clay gave a tolerable result.

No. 9 is a repetition of No. 3: there is no difference except in the proportions. No. 3 was made by taking one part of lime when reduced to powder after the second burning, and two parts of sand, while for No. 9 one part of lime, when reduced to paste after the second burning, was taken, to two parts of sand: as there is more lime in a given bulk of paste than in an equal bulk of powder, this mortar contained less sand than No. 3, and it gave a better result. It may be seen by Tables No. II and III that the natural hydraulic limes of the environs of Strasburg, will not bear a large proportion of sand; and I think that, in general, hydraulic lime takes less than is commonly supposed.

No. 10 was made with the yellow ochre of commerce, and No. 11, with a substance known under the name of sanguine: the results were weak, especially No. 11.

No. 12 was made with the Sufflenheim earth, which is used in making refractory bricks that are highly esteemed in the country, and employed at the cannon foundry at Strasburg. No. 13 contained clay which is brought, because of its containing little or no iron, from the neighborhood of Frankfurt, to Strasburg, to make alum. The results of these two trials are not very satisfactory.

No. 14 was made in the same manner, and same proportions as No 8; it differs from it only in the lime being more highly calcined. The result was better. It is possible that each kind of clay requires a particular degree of calcination; this result may be owing also to this-that the lime and the clay contain very little iron, which appears to facilitate vitrifica

tion.

The lime of No. 15 was calcined like that of No. 8: but in lieu of making the mortar of sand, it was made of trass: the result was much superior.

No. 16 was made with of alumine mixed with lime from white marble. There was no result: the mortars had no sort of consistence.

No 17 was composed of lime from white marble, mixed with a factitious clay composed of equal parts of silex pounded very fine, and of alumine: this gave a better result. The bad result of No. 16 induced me, at first, to conclude that alumine alone had not the property of rendering lime hydraulic: but a mixture of silecious sand and alumine not having yielded a better result, although forming a factitious clay, I have thought it might be owing to the following circumstance: the alumine which I used was extracted from a quality of alum that I decomposed; this alum had been made of the Frankfort clay, of which I will give the analysis below. In making the alum, the clay is calcined before being dissolved in sulphuric acid. It is possible that the calcination takes away from the alumine the property of combining with lime in the dry way. We ought not, therefore, to conclude, from No. 16, that alumine has not the property of rendering lime hydraulic, but only, that, when it has been calcined, it has not

that property. What makes me think that calcination deprived the alumine of the property of combining in the dry way with the lime, is, that I have calcined one part of fat lime with of puzzalona, without getting any result. To know if alumine has the property of rendering lime hydraulic, it will be necessary to use this substance before it has been calcined, or acted upon by any acid. Pure alumine exists in some countries: but it is rare: I had none at command.

Nos. 18 and 19 are, respectively, mixtures of marble lime and fat lime, with white sand, and common sand broken very fine. It is remarkable that I obtained a result with fat lime, and no result with the marble lime. I do not know to what to attribute this difference: it may happen perhaps in this way, that the fat lime, of which I shall give the analysis below, contains the red oxide of iron which might facilitate the union of the lime with the white sand, which is entirely silecious.

No. 20, made with fat lime, and with ordinary sand, which is granitic, and coloured by the red oxide of iron, gave a result nearly like No. 9. I regret not having made a fourth experiment with marble lime and ordinary sand, in order to prove, further, that oxide of iron facilitates the combination of silecious and granitic sands with lime, as appears to be inferable from a comparison of Nos. 18 and 19.

No. 21 was designed to ascertain the influence of magnesia: that which I used in this experiment had been calcined to disengage the carbonic acid. We see that no result was obtained. I repeated the experiments, using carbonate of magnesia in order to avoid the use of a substance which had already been calcined: the heat of the lime kiln would drive off the carbonic acid of the magnesia, so that the magnesia would be free to combine with the lime; I, however, got no result.

No. 22 was an attempt to ascertain whether manganese imparted any properties to the lime. I mixed, successively, one part of fat lime with one, two, and three tenths, of oxide of manganese, and calcined the mixtures: I did not observe that the lime had undergone the least change. It is quite astonishing that an opinion should have prevailed for so long a time, that it was to oxide of manganese, hydraulic lime owed its quality of hardening in water: mixing fat lime with this oxide would have shown that lime acquires no new property thereby.

No. 23 indicates that I made various experiments with iron. I took one part of fat lime, which I mixed successively with diverse proportions of filings of iron obtained from a locksmith's shop,and which I reduced to very fine powder. I did the same thing with black oxide of iron, with brown oxide at different degrees of oxidation, and with steel filings: all these mixtures were burned in a lime kiln and I then made mortars by uniting them with sand; the lime had acquired no hydraulic property; but I had occasion to remark that the oxide of iron had, nevertheless, a marked action on the lime. It is known that if we take lime made of white marble, and throw water on it as it comes from the kiln, it slakes at once, giving out much heat and vapour. In making the above experiments with iron, I remarked, that the higher it was oxidized the less heat and vapour were given out by the lime on throwing water upon it, and that it did not slake until after it had been wet for some time: the lime which had been calcined with of brown oxide of iron gave no vapour; I placed in it a thermometer of Réaumur, and at the end of half an hour it had risen but five or six degrees. The Boulogne pebbles contain, as I have observed, a considerable quantity of brown oxide of iron; it is owing to this, no doubt, that if water be

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