cubic metres each (12 × 3 × 35.34 equals 1272:24 cubic feet) may be made in 10 hours labour; the requisite agents being 4 labourers, 2 horses and their driver, and 1 superintending mason. The expense of making a cubic metre of mortar, amounts in Paris to about $0.10; this is a considerable saving over the common mode of making mortar.* It is desirable therefore that frequent use be made of this machine, in places where there are important constructions. The description just given is extracted from the devis-modèle of the corps of Engineers, and was prepared by Lt. Col. Bergère of the Engineers. It is stated above that at Strasburg, lime which was to be made into mortar, was slaked to dry powder, and left in that state for twelve hours at least before giving it the quantity of water necessary to convert it into paste. I made the following experiments with limes of the environs of Strasburg, to ascertain the volume obtained in powder and in paste, when the proper quantities of water are used to produce those states. All the limes in the above table, were used fresh from the kiln. I reduced them to powder in a mortar, sifted them, and used, for quantity, about one quart. Thus, for example, I took a measure of quick lime of white marble, and throwing upon it half a measure of water, I obtained 21 measures of lime slaked to powder, which I measured after it was cold. The quantity of water thrown on is shown in the second column, and the Greasing wheels one day, 0 07 Wear and tear of shovels and barrows, 1 day for 4 labourers, 0 07 0 32 Equal to $0 10 per cub. metre-or $0:0028 per cubic foot. The quality of the mortar is superior to that made by the common process; and it is well to remark, that the time during which the mortar is made is precisely that in which the labourers repose: it is therefore their interest to let the machine go as long as possible, and consequently to render the mortar more perfect, so that the supervi sion will be directed chiefly to the proportions of the mixture. This note is extracted from the devis-modèle du corps du Génie, p. 71.-Tr. quantity of lime obtained in powder is given in the third column. I was obliged to throw upon this lime in powder, one measure and one-tenth of water in addition to reduce it to paste. Adding this last quantity of water to the half measure used in the first instance, the total is 1 measures of water, absorbed by the lime, in being reduced to paste: this is shown in the fourth column. The fifth column shows that I obtained 1 measure of lime in paste. I followed the same process for all the limes of the above table, producing a uniform consistence of paste, by adding the water little by little. Experience had taught me that these times were reduced to dry powder by throwing on one-fifth of their bulk of water; and that as much as one-half their bulk might be thrown on without the powders ceasing to be dry: beyond this term, a moist powder would be obtained. The only lime on which I threw less than half its bulk, was that at the bottom of the table, of the Boulogne pebbles; on this I poured but its bulk of water; as this lime forms a moist powder with its bulk of water, I was obliged to restrict myself to one-third. This table shows that these different limes afforded very different volumes of powder with the same quantity of water: that the quantities of water absorbed to produce the state of paste were very different, and, also, that the volumes of paste differed much. Experiments which follow will show that, of the limes in the table, those are the most hydrau lic which absorbed the least water in passing to the state of paste, and which gave the smallest bulk both of powder and of paste. Those limes, of the table, which are not hydraulic, are those which gave the greatest volumes in powder and in paste. There are in the table two kinds of Obernai lime, one yellow and the other blue; they are of the same limestone, but one more highly calcined than the other. When this lime has been burned just enough, it is of a yellow-fawn colour; when a little more burned, it is of an ashy-gray, and when too much calcined, of a decided blue. It was upon the two extremes of calcination that I made the above experiments, they show that the degree of calcination has a sensible influence on the swelling of this hydraulic lime. As the swelling of lime, shown in the above table, was obtained with quite small quantities, and with pulverized quicklime, I caused experiments to be made at the mortar beds on a large scale, with fat lime and with Obernai lime; these being the two kinds of lime ordinarily used upon the works. The following results were obtained. Fat lime was taken immediately from the kiln, and measured in the boxes in use at the mortar beds; care being taken to break up a portion of the lumps of quick lime into smaller pieces, in order to occupy the interstices between the larger pieces, and to have the measure well filled: water, in quantity sufficient to bring the lime at once to paste of the consistence of mortar, was thrown on without delay, and the quantity of paste thus obtained was measured. Proceeding thus-one measure of quicklime, just from the kiln, required two measures of water to produce the state of paste, and yielded 1.83 of paste, which differs but little from Table No. 1, wherein the produce is 1.75. The same operation was repeated with Obernai lime, after having rejected vitrified pieces, and those which had not been sufficiently calcined: one measure of this lime absorbed 1.30 of water in being reduced to paste, and in this state gave 1.30 of lime. This differs somewhat from the result in the Table. The difference may be owing to this, that in the experiments of the Table, the lime was pulverized, and was twice slaked; that is to say had two successive applications of water, while in the larger experiment the lime was not broken up, and had water poured on but once. The degree of calcination might, also, have had some influence. Many metallic oxides are susceptible of absorbing and solidifying a certain quantity of water forming compounds which possess peculiar properties. It is to these compounds that the term hydrate has been assigned. It has been seen, above, that lime is a metallic oxide, and that this substance absorbs and solidifies a large quantity of water; but the quantity of water absorbed by lime in forming its hydrate is not exactly known. Berzelius asserts that the hydrates are formed of water and oxides in such proportions that the quantity of oxygen contained in the oxide is equal to the quantity of oxygen contained in the water; but Mr. Thenard does not admit this law: he says that the experiments on which it is founded are not numerous enough, nor sufficiently precise, to allow its definitive admission. It is certain nevertheless, says this celebrated chemist, that amongst the hydrates which have as yet been examined, those which contain the most water, are those, also, of which the oxides contain the most oxygen. According to Berzelius, the hydrate of lime is obtained by throwing upon quick lime the water necessary to reduce it to thin paste (bouille,) and exposing this paste in a silver or platina crucible to the heat of a spirit-of-wine lamp. After having dried the hydrate of lime in this manner, it is weighed, and the quantity of water it has absorbed is known by the augmentation of weight. Berzelius made two experiments, one with 10 grammes of lime and the other with 30 grammes. He found in the first experiment, that the lime had increased in weight 32.1 per cent., and in the second, 32.5: in this second experiment there was, therefore, an augmentation of four-tenths more than in the first. He attributes this difference to an absorption of carbonic acid, and he admits, as good, only the first experiment, in which 100 parts of pure lime containing 28.16 parts of oxygen, are combined with 32.1 parts of water containing 28.3 parts of oxygen; whence Berzelius concludes that the water absorbed by pure lime contains a quantity of oxygen equal to that contained in the lime. I have repeated the experiment of Berzelius by operating on 20 grammes of pure lime, using, as he did, a spirit-of-wine lamp, and a platina crucible. I was surprised at obtaining an augmentation of only 22.5 per cent. I repeated the experiment several times, successively diminishing the thickness of the wick, and as I did this, the lime retained more and more water. I inferred, therefore, that the hydrate of lime decomposes with a feeble heat; and that, if Berzelius obtained a greater result in the second experiment than in the first, it was not all due to the absorption of carbonic acid, seeing that the operation lasts only a short time; but to this, that heating with an equal flame, two volumes of hydrate of lime, of which one was triple the other, the smaller volume should lose most water by the heat. But there is a fact which proves with how great facility the hydrate of lime abandons a part of its water. All those who have made mortar of lime newly slaked, have perceived that it becomes very dry in a short time. If, when in this state, it be worked for some time without adding water, it will be brought back nearly to the same moist state it had at first; and drops of water may be seen on the mortar. The same result is obtained with lime alone. It follows from this, that simple friction (working) decomposes the hydrate of Jime, and that a feeble heat produces the same effect. To know, therefore, the quantity of water which enters into the hydrate of lime, it appears to me that other means of drying should be resorted to than fire. The various kinds of lime are used in constructions, only after having been brought to the condition of hydrate: nothing, therefore, that relates to the properties of this compound, is a matter of indifference. As yet, few experiments have been made to determine the quantity of water that should be given to lime in making mortar. I proposed undertaking several experiments on this point, but time failed me. The matter should be attended to, because, opinions are much divided thereon, for want of exact experiments. The following are the principal properties of hydrate of lime: it is white, pulverulent, and much less caustic than quick lime; it easily abandons to heat the first portion of water, but it requires a high temperature to drive off all the water entering into its composition. This hydrate absorbs carbonic acid; experiments which follow show that it has, also, the property of absorbing oxygen, and that lime sustains important modifications in consequence of this absorption of oxygen. According to the chemists, lime is incapable of absorbing a fresh quantity of oxygen: but according to my observations, there is no doubt that the hydrate of lime absorbs a considerable quantity. I shall give, in the following chapter, experiments which I made on this subject. CHAPTER III. Experiments on several hydraulic limes of the environs of Strasburg, on the Metz lime, and on Boulogne pebbles. When I was sent to Strasburg in 1816, they were in the habit, there, of using fat lime only. One of the two dikes enclosing the navigable canal which passes from the town to the Rhine across the ditch of the place, requir ing repairs, I had occasion to remark that while the two facings, which were of cut stone, were much disjointed, the interior of the dike, which was a mass of concrete, was in good condition, and not a drop of water passed through it. The concrete being very hard, and, as it seemed, not made of brick, or tile dust, I conjectured that it had been composed of hydraulic lime, and I made researches accordingly. I learned that the Millers along the Bruche had used, for a long time, in the repairs of their works, a particular species of lime, which they obtained from a village, at the foot of the Vosges, called Obernai. Mr. Mossère, Engineer of roads and bridges, told me that he had used this lime in the works of the canal of "Monsieur," and that he was well satisfied with it; and, on making the essay, I found it to be eminently hydraulic: it appeared to me to be in no respect inferior to the Metz lime, which I had seen employed with success, at that place, in 1800 and 1801. At other stations, I had several times made hydraulic mortars of fat lime and brick, or tile, dust. At the great works of Vésel, where I was employed three years, considerable use was made of trass, which was brought from Andernach by the Rhine; and on the experience I had acquired of hydraulic limes, I introduced the use of the Obernai lime in all the constructions of the works of Strasburg, both in and out of water. I have already observed that all the revetments comprised between porte de Pierre and pont Royal, giving a development of about 1650 yards, were made with hydraulic lime. Later researches have shown me that hydraulic lime is to be found at the foot of the Vosges, from Belfort to Vissembourg. *This village is situated between Schelestadt and Strasburg: upon the map of Cassini it is written Ober-Ehnheim, but it is pronounced Obernai, and is so written on some maps. I shall give experiments which I made with hydraulic lime from Altkirch, Obernai, Rouxviller, Ingviller, Oberbronn, Verdt, &c. There are no certain means of knowing by inspection whether a limestone will give fat lime or hydraulic lime, for there are hydraulic limestones of several colours: those of Alsace are generally of a slate blue, like those of Metz, but those of Altkirch, and the hydraulic chalk of Vitry are white; the Boulogne pebbles, and the English lime-stone of which Parker's cement is made, are red. I may say, however, that when a lime-stone is blue, it is a presumption that it will afford hydraulic lime. It is remarkable that in lime-stone giving fat lime, the iron exists as red oxide, while it exists in the lowest state of oxidation in a great proportion of the hydraulic lime-stones, even when left for a long time exposed to the air; the same thing happens with good slate, and it seems to me to be owing to the presence of clay in the hydraulic lime-stones and in the slate, with which the iron is probably in combination. As I have before said, the Boulogne pebbles, and Parker's cement stone, were exceptions; but this might be due to some peculiar cause: the sea-water might have some influence on the oxidation of the iron contained in the Boulogne pebbles. All the blue hydraulic lime-stones which I have caused to be calcined, gave a yellow ochre colour whenever the heat was not great. If the calcination was raised, the colour passed, successively, to fawn, ash-gray, and, finally, if the heat had been very great, to slate blue. I cannot satisfy myself as to the cause of the blue colour which the lime-stone takes by a high calcination, because it seems necessary to suppose that this effect is the bringing back of the iron to its first degree of oxidation, which is possible, but does not seem to me easy to explain. A like effect appears to take place in the calcination of clays that contain oxide of iron. Some clays submitted to calcination are blackish, because of the iron being present at the lowest state of oxidation: at a certain degree of heat, the iron passes to the state of red oxide, and the clay takes the decided colour commonly seen in bricks: if the heat be augmented, the red colour weakens, and becomes fawn or straw colour; if the heat be pushed to a high degree, the clay becomes of an ash colour and afterwards of a very decided slate-blue colour-the iron having then lost a great part of its oxygen. If slate, and the clay just described, be calcined at the same time, the iron will pass with much more difficulty to the state of red oxide in the slate than in the clay. The degree of heat necessary to bring the iron in the slate to the condition of red oxide, will have caused the iron in the clay to pass through all the stages of oxidation before mentioned, and have imparted the blue colour to the clay. Are these different phenomena due to this, that while, in the slate, the iron is combined with the alumine, there is no such combination in this clay? In clays which are strongly calcined, has the alumine the property of abstracting the oxygen from the iron? It is for chemistry to clear up these points. M. Berthier has given the following method of detecting hydraulic limestones. "The stone must be pulverized and sifted through a silk sieve: 10 grammes of the powder being put in a capsule, muriatic acid, diluted with a little water, must be poured on, little by little. (In want of muriatic acid, nitric acid, or even vinegar, may be used.) The whole should be constantly stirred with a glass or wooden rod, as long as there may be any effervescence; this having ceased, the solution must be evaporated, in a gentle heat, to the consistence of paste; about one pint of water must be added and the whole filtered; the clay will remain upon the filter; it must be dried in |