By comparing the constituents of these last two stones with the analyses of the cement-stones of New York, and the magnesian hydraulic limestones of Prof. Rogers, it will be seen that they consist, respectively, of nearly the same combinations of lime, magnesia, and silica. Although not brought out in the analysis of the preceding stones, there is probably none in which the alkaline salts do not occur, and, in some, of sufficient amount to injure mortar made from them, by their efflorescence. 50. PHYSICAL CHARACTERS AND TESTS OF HYDRAULIC LIMESTONES. The simple external characters of a limestone, as color, texture, fracture, and taste, are insufficient to enable a person to decide whether it belongs to the hydraulic class; although they assist conjecture, particularly if the rock, from which the specimen is taken, is found in connection with the clay deposits, or if it belong to a stratum whose general level and characteristics are the same as the argillo-magnesian rocks. These rocks are generally some shade of drab, or of gray, or of a dark grayish-blue; have a compact texture; fracture even or conchoidal; with a clayey or earthy smell and taste. Although the hydraulic limestones are usually colored, still it may happen that the stone may be of a pure white, arising from the combination of lime with a pure clay. The difficulty of pronouncing upon the class to which a limestone belongs, from its physical properties alone, renders it necessary to resort to a chemical analysis, and even to direct experiment to decide the question. 51. A prejudice exists among lime manufacturers and builders in favor of the dark-colored products of calcined hydraulic limestones, but without any foundation, so far as experiment goes, as some of the most celebrated cements are light colored. As a general rule, a dark-colored material is an unfavorable sign, as showing the presence of some foreign ingredient. 52. In making a complete chemical analysis of a limestone, more skill in chemical manipulations is requisite than engineers usually possess; but a person who has the ordinary elementary knowledge of chemistry can readily ascertain the quantity of clay or of magnesia contained in a limestone, and from these two elements can pronounce, with tolerable certainty, upon its hydraulic properties. To arrive at this conclusion, a small portion of the stone to be tested-about five drachms is taken and reduced to a powder; this is placed in a capsule, or an ordinary watch crystal, and slightly diluted muriatic acid is poured over it until it ceases to effervesce. The capsule is then gently heated, and the liquor evaporated, until the residue in the capsule has acquired the consistence of thin paste. This paste is thrown into a pint of pure water and well shaken up, and the mixture is then filtered. The residue left on the filtering paper is thoroughly dried, by bringing it to a red heat; this being weighed will give the clay, or insoluble matter, contained in the stone. It is important to ascertain the state of mechanical division of the insoluble matter thus obtained; for if it be wholly granular, the stone will not yield hydraulic lime. The granular portion must therefore be carefully separated from the other before the latter is dried and weighed. 53. If the sample tested contains magnesia, an indication of this will be given by the slowness with which the acid acts; if the quantity of magnesia be but little, the solution will at first proceed rapidly and then become more sluggish. To ascertain the quantity of magnesia, clear lime-water must be added to the filtered solution as long as any precipitate is formed, and this precipitate must be quickly gathered on filtering paper, and then be washed with pure water. The residue from this washing is the magnesia. It must be thoroughly dried before being weighed, to ascertain its proportion to the clay. 54. Having ascertained, by the preceding analysis, the probable hydraulic energy of the stone, a sample of it should also be submitted to direct experiment. This may be likewise done on a small scale. A sample of the stone must be reduced to fragments about the size of a walnut. A crucible, perforated with holes for the free admission of air, is filled with these fragments, and placed over a fire sufficiently powerful to drive off the carbonic acid of the stone. The time for effecting this will depend on the intensity of the heat. When the heat has been applied for three or four hours, a small portion of the calcined stone may be tried with an acid, and the degree of the calcination may be judged of by the more or less copiousness of the effervescence that ensues. If no effervescence takes place, the operation may be considered completed. The calcined stone should be tried soon after it has become cold; otherwise, it should be kept in a glass jar made as air tight as practicable until used. 55. When the calcined stone is to be tried, it is first slaked by placing it in a small basket, which is immersed for five or six seconds in pure water. The stone is emptied from the basket so soon as the water has drained off, and is allowed to stand until the slaking is terminated. This process will proceed more or less rapidly, according to the quality of the stone, and the degree of its calcination. In some cases, it will be completed in a few minutes; in others, portions only of the stone will fall to powder, the rest crumbling into lumps which slake very sluggishly; while other varieties, as the true cement stones, give no evidence of slaking. If the stone slakes either completely or partially, it must be converted into a paste of the consistence of soft putty, being ground up thoroughly, if necessary, in an iron mortar. The paste is made into a cake, and placed on the bottom of an ordinary tumbler, care being taken to make the diameter of the cake the same as that of the tumbler. The vessel is filled with water, and the time of immersion noted. If the lime is only moderately hydraulic, it will have become hard enough at the end of fifteen or twenty days, to resist the pressure of the finger, and will continue to harden slowly, more particularly from the sixth or eighth month after immersion; and at the end of a year it will have acquired the consistency of hard soap, and will dissolve slowly in pure water. A fair hydraulic lime will have hardened so as to resist the pressure of the finger, in about six or eight days after immersion, and will continue to grow harder until from six to twelve months after immersion; it will then have acquired the hardness of the softest calcareous stones, and will be no longer soluble in pure water. When the stone is eminently hydraulic, it will have become hard in from two to four days after immersion, and in one month it will be quite hard and insoluble in pure water; after six months, its hardness will be about equal to the more absorbent calcareous stones; and it will splinter from a blow, presenting a slaty fracture. As the hydraulic cements do not slake perceptibly, the burnt stone must first be reduced to a fine powder before it is made into a paste. The paste, when kneaded between the fingers, becomes warm, and will generally set in a few minutes, either in the open air or in water. Hydraulic cements are far more sparingly soluble in pure water than the hydraulic lime; and the action of pure water upon them ceases, apparently, after a few weeks' immersion in it. 56. Calcination of Limestone. The effect of heat on lime-stones varies with the constituent elements of the stone. The pure limestones will stand a high degree of temperature without fusing, losing only their carbonic acid and water. The impure stones containing silica fuse completely under a great heat, and become more or less vitrified when the tem perature much exceeds a red heat. The action of heat on the impure limestones, besides driving off their carbonic acid and water, modifies the relations of their other chemical constituents. The argillaceous stones, for example, yield an insoluble precipitate when acted on by an acid before calcination, but are perfectly soluble afterwards, unless the silex they contain happens to be in the form of grains. 57. The calcination of the hydraulic limestones, from their fusible nature, requires to be conducted with great care; for, if not pushed far enough, the under-burnt portions will not slake; and, if carried too far, the stone becomes dead or sluggish; slakes very slowly and imperfectly at first; and, if used in this state for masonry, may do injury by the swelling which accompanies the after-slaking. 58. The more or less facility with which the impure limestones can be burned depends upon several causes; as the compactness of the stone, the size of the fragments submitted to heat, and the presence of a current of air, or of aqueous vapor. The more compact stones yield their carbonic acid less readily than those of an opposite texture. Stones which, when broken into very small lumps, can be calcined under the red heat of an ordinary fire in a few hours, will require a far greater degree of temperature, and for a much longer period, when broken into fragments of six or eight inches in diameter. This is particularly the case with the impure limestones, which, when in large lumps, vitrify at. the surface before the interior is thoroughly burnt. 59. If a current of vapor is passed over the stone after it has commenced to give off its carbonic acid, the remaining portion of the gas which, under ordinary circumstances, is expelled with great difficulty, particularly near the end of the process of calcination, will be carried off much sooner. The influence of an aqueous current is attributed, by M. Gay-Lussac, purely to a mechanical action, by removing the gas as it is evolved, and his experiments go to show that a like effect is produced by an atmospheric current. In burning the impure limestones, however, an aqueous current produces the farther beneficial effect of preventing the vitrification of the stone when the temperature has become too elevated; but as the vapor, on coming in contact with the heated stone, carries off a large portion of the heat, this, together with the latent heat contained in it, may render its use in some cases far from economical. 60. Wood, charcoal, peat, the bituminous and the anthracite coals are used for fuel in lime-burning. M. Vicat states, that wood is the best fuel for burning hydraulic limestones; that charcoal is inferior to bituminous coal; and that the results from this last are very uncertain. When wood is used, it should be dry and split up, to burn quickly and give a clear blaze. The common opinion among lime-burners, that the greener the fuel the better, and that the limestone should be watered before it is placed in the kiln, is wrong; as a large portion of the heat is consumed in converting the water in both cases into vapor. Coal is a more economical fuel than wood, and is therefore generally preferred to it; but it requires particular care in ascertaining the proper quantity for use. III. LIME KILNS. LIME KILNS. Great diversity is met with in the forms and proportions of lime-kilns. Wherever attention has been paid to economy in fuel, the cylindrical, ovoidal, or the inverted conical form has been adopted. The two first being preferred for wood and the last for coal. 61. The whole of the burnt lime is either drawn from the kiln at once, or else the burning is so regulated, that fresh stone and fuel are added as the calcined portions are withdrawn. The latter method is usually followed when the fuel used is coal. The stone and coal, broken into proper sizes (Fig. 1), and in proportions determined by experiment, are Fig. 1 represents a vertical section through the axis and centre lines of A, solid masonry of the kiln, which is built up on the exterior like a c, c, openings between B, B and the interior through which the burnt lime is drawn. placed in the kiln in alternate layers; the coal is ignited at the bottom of the kiln, and fresh strata are added at the top, as the burnt mass settles down and is partially withdrawn at the |