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 cement is 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.

54. Calcination of Lime-stone. The effect of heat on limestones varies with the constituent elements of the stone. The pure lime-stones 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 temperature much exceeds a red heat. The action of heat on the impure lime-stones, 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. 55. The calcination of the hydraulic lime-stones, 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.

56. The more or less facility with which the impure lime-stones 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 lime-stones, which, when in large lumps, vitrify at the surface before the interior is thoroughly burnt.

57. 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. This 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 at

mospheric current. In burning the impure lime-stones, 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.

58. Wood, charcoal, peat, the bituminous and anthracite coals are used for fuel in lime-burning. M. Vicat states, that wood is the best fuel for burning hydraulic lime-stones; 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 lime-stone 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.

59. 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.

60. 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 propor

Fig. 1 represents a vertical section through the axis and centre lines of the entrances communicating with the interior of a kiln for burning lime with coal.

A, solid masonry of the kiln, which is built up on the exterior like a square tower, with two arched entrances at B, B on opposite sides.

C, interior of the kiln, lined with fire-brick or stone. h

c, c, openings between B, B and the interior through which the burnt lime is drawn.

tions determined by experiment, are 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 bottom. Kilns used in this way are called perpetual kilns; they are more economical in the consumption of fuel than those in which the burning is intermitted, and which are, on this account, termed intermittent kilns. Wood

may also be used as fuel in perpetual kilns, but not with such economy as coal; it moreover presents many inconveniences, in supplying the kiln with fresh stone, and in regulating its discharge. The inverted conical-shaped kiln is generally adopted for coal, and the ovoidal-shaped for wood.

61. Some care is requisite in filling the kiln with stone when a wood fire is used. A dome (Fig. 2) is formed of the largest blocks

of the broken stone, which either rests on the bottom of the kiln or on the ash-grate. The lower diameter of the dome is a few feet less than that of the kiln; and its interior is made sufficiently capacious to receive the fuel which, cut into short lengths, is placed up endwise around the dome. The stone is placed over and around the courses which form the dome, the largest blocks in the centre of the kiln. The management of the fire is a matter of experiment. For the first eight or ten hours it should be carefully regulated, in order to bring the stone gradually to a red heat. By applying a high heat at first, or by any sudden increase of it until the mass has reached a nearly uniform temperature, the stone is apt to shiver, and choke the kiln, by stopping the voids between the courses of stone which form the dome. After the stone is brought to a red heat, the supply of fuel should be uniform until the end of the calcination. The practice sometimes adopted, of abating the fire towards the end, is bad, as the last portions of carbonic acid retained by the stone, require a high degree of heat for their expulsion. The indications of complete calcination are generally manifested by the diminution which gradually takes place in the mass, and which, at this stage, is about one sixth of the primitive volume; by the broken appearance of the stone which forms the dome, the interstices between which being also choked up by fragments of the burnt stone; and by the ease with which an iron bar may be forced down through the burnt stone in the kiln. When these indications of complete calcination are observed, the kiln should be closed for ten or twelve hours, to confine the heat and finish the burning of the upper strata.

62. The form and relative dimensions of a kiln for wood can

be determined only by careful experiment. If too great height be given to the mass, the lower portions may be overburned before the upper are burned enough. The proportions between the height and mean horizontal section, will depend on the texture of the stone; the size of the fragments into which it is broken for burning; and the more or less facility with which it vitrifies. In the memoir of M. Petot, already cited, it is stated as the results of experiments made at Brest, that large-sized kilns are more economical, both in the consumption of fuel and in the cost of attendance, than small ones; but that there is no notable economy in fuel when the mean horizontal section of the kiln exceeds sixty square feet.

63. The circular seems the most suitable form for the horizontal sections of a kiln, both for strength and for economizing the heat. Were the section the same throughout, or the form of the interior of the kiln cylindrical, the strata of stone, above a certain point, would be very imperfectly burned when the lower were enough so, owing to the rapidity with which the inflamed gases, arising from the combustion, are cooled by coming into contact with the stone. To procure, therefore, a temperature throughout the heated mass which shall be nearly uniform, the horizontal sections of the kiln should gradually decrease from the point where the flame rises, which is near the top of the dome of broken stone, to the top of the kiln. This contraction of the horizontal section, from the bottom upward, should not be made too rapidly, as the draft would be injured, and the capacity of the kiln too much diminished; and in no case should the area of the top opening be less than about one fourth the area of the section taken near the top of the dome. The best manner of arranging the sides of the kiln, in the plane of the longitudinal section, is to connect the top opening with the horizontal section through the top of the dome, by an arc of a circle whose tangent at the lower point shall be vertical.

64. Lime-kilns are constructed either of brick, or of some of the more refractory stones. The walls of the kiln should be sufficiently thick to confine the heat, and, when the locality admits of it, they are built into a side hill; otherwise, it may be necessary to use iron hoops, and vertical bars of iron, to strengthen the brick-work. The interior of the kiln should be faced either with good fire-brick or with fire-stone.

65. M. Petot prefers kilns arranged with fire-grates, and an ash-pit under the dome of broken stone, for the reason that they give the means of better regulating the heat, and of throwing the flame more in the axis of the kiln than can be done in kilns without them. The action of the flame is thus more uniformly felt through the mass of stone above the top of the dome, while that

of the radiated heat upon the stone around the dome, is also more uniform.

66. M. Petot states, that the height of the mass of stone above the top of the dome should not be greater than from ten to thirteen feet, depending on the more or less compact texture of the stone, and the more or less ease with which it vitrifies. He proposes to use kilns with two stories, (Fig. 3,) for the purpose

Fig. 3 represents a vertical section through the axis and centre line of the entrance of a lime-kiln with two stories for wood.

A, solid masonry of the kiln.

B, dome shown by the dotted line.
C, interior of lower story.

D, dome of upper story.

E, interior of upper story.
a, arched entrance to kiln.

b, receptacle for water to furnish a
current of aqueous vapor.

c, doorway for drawing kiln, &c., closed by a fire-proof door.

d, ash-pit under fire-grate.

e, upper doorway for drawing kiln, &c.

B

C

b

of economizing the fuel, by using the heat which passes off from the top of the lower story, and would otherwise be lost, to heat the stone in the upper story; this story being arranged with a side-door, to introduce fuel under its dome of broken stone, and complete the calcination when that of the stone in the lower story is finished.

M. Petot gives the following general directions for regulating the relative dimensions of the parts of the kiln. The greatest horizontal section of the kiln is placed rather below the top of the dome of broken stone; the diameter of this section being 1.82, the diameter of the grate. The height of the dome above the grate is from 3 to 6 feet, according to the quantity of fuel to be consumed hourly. The bottom of the kiln, on which the piers of the dome rest, is from 4 to 6 inches above the top of the grate; the diameter of the kiln at this point being about 2 feet 9 inches greater than that of the grate. The diameter of the horizontal section at top is 0.63, the diameter of the greatest horizontal section. The horizontal sections of the kiln diminish from the section near the top of the dome to the top and bottom of the kiln; the sides of the kiln receiving the form shown in Fig. 3: the object of contracting the kiln towards the bottom being to allow the stone near the bottom of the kiln to be thoroughly burned by the radiated heat. The grate is formed of cast-iron bars of the usual form;