the area of the spaces between the bars being one fourth the total area of the grate. The bottom of the ash-pit, which may be on the same level as the exterior ground, is placed 18 inches below the grate; and at the entrance to the ash-pit is placed a reservoir for water, about 18 inches in depth, to furnish an aqueous current. The draft through the grate is regulated by a lateral airchannel to the ash-pit, which can be totally or partially shut by a valve; the area of the cross section of this channel is one tenth the total area of the grate. A square opening 16 inches wide, the bottom of which is on a level with the bottom of the kiln, leads to the dome for the supply of the fuel. This opening is closed with a fire-proof and air-tight door.

In arranging a kiln with two stories, M. Petot states, that the grates of the upper story are so soon destroyed by the heat, that it is better to suppress them, and to place the fuel for completing the calcination of the stone of this story, on the top of the burnt stone of the lower story.

67. Slaking Lime. Quick-lime may be slaked in three dif ferent ways. By pouring sufficient water on the burnt stone to convert the slaked lime into a thin paste, which is termed drowning the lime. By placing the burnt stone in a basket, and immersing it for a few seconds in water, during which time it will imbibe enough water to cause it to fall, by slaking, into a dry powder; or by sprinkling the burnt stone with a sufficient quantity of water to produce the same effect. By allowing the stone to slake spontaneously, from the moisture it imbibes from the atmosphere, which is termed air-slaking.

68. Opinion seems to be settled among engineers, that drowning is the worst method of slaking lime which is to be used for mortars. When properly done, however, it produces a finer paste than either of the other methods; and it may therefore be resorted to whenever a paste of this character, or a whitewash is wanted. Some care, however, is requisite to produce this result. The stone should be fresh from the kiln, otherwise it is apt to slake into lumps or fine grit. All the water used should be poured over the stone at once, which should be arranged in a basin or vessel, so that the water surrounding it may be gradually imbibed as the slaking proceeds. If fresh water be added during the slaking, it checks the process, and causes a gritty paste to form.

69. In slaking by immersion, or by sprinkling with water, the stone should be reduced to small-sized fragments, otherwise the slaking will not proceed uniformly. The fat limes should be in lumps, about the size of a walnut, for immersion; and, when withdrawn from the water, should be placed immediately in bins, or be covered with sand, to confine the heat and vapor. If left exposed to the air, the lime becomes chilled and separates into a

coarse grit, which takes some time to slake thoroughly when more water is added. Sprinkling the lime is a more convenient process than immersion, and is equally good. To effect the slaking in this way, the stone should be broken into fragments of a suitable size, which experiment will determine, and be placed in small heaps, surrounded by sufficient sand to cover them up when the slaking is nearly completed. The stone is then sprinkled with about one fourth its bulk of water, poured through the rose of a watering-pot, those lumps which seem to slake most sluggishly receiving the most water; when the process seems completed, the heap is carefully covered over with the sand, and allowed to remain a day or two before it is used.

70. Slaking either by immersion or by sprinkling is considered the best. The quantity of water imbibed by lime when slaked by immersion, varies with the nature of the lime; 100 parts of fat lime will take up only 18 parts of water; and the same quantity of meager lime will imbibe from 20 to 35 parts. One volume, in powder, of the burnt stone of rich lime yields from 1.50 to 1.70 in volume of powder of slaked lime; while one volume of meager lime, under like circumstances, will yield from 1.80 to 2.18 in volume of slaked lime.

71. Quick lime, when exposed to the free action of the air in a dry locality, slakes slowly, by imbibing moisture from the atmosphere, with a slight disengagement of heat. Opinion seems to be divided with regard to the effect of this method of slaking on fat limes. Some assert, that the mortar made from them is better than that obtained from any other process, and attribute this result to the re-conversion of a portion of the slaked lime into a carbonate; others state the reverse to obtain, and assign the same cause for it. With regard to hydraulic limes, all agree that they are greatly injured by air-slaking.

72. Air-slaked fat limes increase two fifths in weight, and for one volume of quick lime yield 3.52 volumes of slaked lime. The meager limes increase one eighth in weight, and for one volume of quick lime yield from 1.75 to 2.25 volumes of slaked lime.

73. The dry hydrates of lime, when exposed to the atmosphere, gradually absorb carbonic acid and water. This process proceeds very slowly, and the slaked lime never regains all the carbonic acid which is driven off by the calcination of the lime-stone. When converted into a thick paste, and exposed to the air, the hydrates gradually absorb carbonic acid; this action first takes place on the surface, and proceeds more slowly from year to year towards the interior of the exposed mass. The absorption of gas proceeds more rapidly in the meager than in the fat limes. Those hydrates which are most thoroughly slaked become hardThe hydrates of the pure fat limes become in time very

est.

hard, while those of the hydraulic limes become only moderately hard.

74. The fat limes, when slaked by drowning, may be preserved for a long period in the state of paste, if placed in a damp situation and kept from contact with the air. They may also be preserved for a long time without change, when slaked by immersion to a dry powder, if placed in covered vessels. Hydraulic limes, under similar circumstances, will harden if kept in the state of paste, and will deteriorate when in powder, unless kept in perfectly air-tight vessels.

75. The hydrates of fat lime, from air-slaking or immersion, require a smaller quantity of water to reduce them to the state of paste than the others; but, when immersed in water, they gradually imbibe their full dose of water, the paste becoming thicker, but remaining unchanged in volume. Exposed in this way, the water will in time dissolve out all the lime of the hydrate which has not been re-converted into a sub-carbonate, by the absorption. of carbonic acid before immersion; and if the water contain carbonic acid, it will also dissolve the carbonated portions.

76. The hydrates of hydraulic lime, when immersed in water in the state of thin pastes, reject a portion of the water from the paste, and become hard in time; if the paste be very stiff, they imbibe more water, set quickly, and acquire greater hardness in time than the soft pastes. The pastes of the hydrates of hydraulic lime, which have hardened in the air, will retain their hardness when placed in water.

77. The pastes of the fat limes shrink very unequally in drying, and the shrinkage increases with the purity of the lime; on this account it is difficult to apply them alone to any building purposes, except in very thin layers. The pastes of the hydraulic limes can only be used with advantage under water, or where they are constantly exposed to humidity; and in these situations they are never used alone, as they are found to succeed as well, and to present more economy, when mixed with a portion of sand.

78. Manner of Reducing Hydraulic Cement. As the cement stones will not slake, they must be reduced to a fine powder by some mechanical process, before they can be converted into a hydrate. The methods usually employed for this purpose consist in first breaking the burnt stone into small fragments, either under iron cylinders, or in mills suitably formed for this purpose, which are next ground between a pair of stones, or else crushed by an iron roller. The coarser particles are separated from the fine powder by the ordinary processes with sieves. The powder is then carefully packed in air-tight casks, and kept for use.

79. Hydraulic cement, like hydraulic lime, deteriorates by exposure to the air, and may in time lose all its hydraulic prop

erties. On this account it should be used when fresh from the kiln; for, however carefully packed, it cannot be well preserved when transported to any distance.

80. The deterioration of hydraulic cements, from exposure to the air, arises, probably, from a chemical disunion between the constituent elements of the burnt stone, occasioned by the absorption of water and carbonic acid. When injured, their energy can be restored by submitting them to a much slighter degree of heat than that which is requisite to calcine the stone suitably in the first instance. From the experiments of M. Petot, it appears that a red heat, kept up for a short period, is sufficient to restore damaged hydraulic cements.

81. Artificial Hydraulic Limes and Cements. The discovery of the argillaceous character of the stones which yield hydraulic limes and cements, connected with the fact that brick reduced to a fine powder, as well as several substances of volcanic origin having nearly the same constituent elements as ordinary brick, when mixed in suitable proportions with common lime, will yield a paste that hardens under water, has led, within a recent period, to artificial methods of producing compounds possessing the properties of natural hydraulic lime-stones.

82. M. Vicat was the first to point out the method of forming an artificial hydraulic lime, by mixing common lime and unburnt clay, in suitable proportions, and then calcining them. The experiments of M. Vicat have been repeated by several eminent engineers with complete success, and among others by General Pasley, who, in a recent work by him, Observations on Limes, Calcareous Cements, &c., has given, with minute detail, the results of his experiments; from which it appears that an hydraulic cement, fully equal in quality to that obtained from natural stones, can be made by mixing common lime, either in the state of a carbonate or of a hydrate, with clay, and subjecting the mixture to a suitable degree of heat. In some parts of France, where chalk is found abundantly, the preparation of artificial hydraulic lime has become a branch of manufacture.

83. Different methods have been pursued in preparing this material, the main object being to secure the finest mechanical division of the two ingredients, and their thorough mixture. For this purpose the lime-stone, if soft like chalk or tufa, may be reduced in a wash-mill, or a rolling-mill, to the state of a soft pulp; it is then incorporated with the clay, by passing them through a pug-mill. The mixture is next moulded into small blocks, or made up into balls between 2 and 3 inches diameter, by hand, and well dried. The balls are placed in a kiln,-suitably calcined, and are finally slaked, or ground down fine for use.

84. If the lime-stone be hard, it must be calcined and slaked

in the usual manner, before it can be mixed with the clay. The process for mixing the ingredients, their calcination, and farther preparation for use, are the same as in the preceding case.

85. Artificial hydraulic lime, prepared from the hard limestones, is more expensive than that made from the soft; but it is stated to be superior in quality to the latter.

86. As clays are seldom free from carbonate of lime, and as the lime-stones which yield common or fat lime may contain some portion of clay, the proper proportions of the two ingredients, to produce either an hydraulic lime or a cement, must be determined by experiment in each case, guided by a previous analysis of the two ingredients to be tried.

If the lime be pure, and the clay be free from lime, then the combinations in the proportions given in the table of M. Petot will give, by calcination, like results with the same proportions when found naturally combined.

87. Puzzolana, &c. The practice of using brick or tile-dust, or a volcanic substance known by the name of puzzolana, mixed with common lime, to form an hydraulic lime, was known to the Romans, by whom mortars composed of these materials were extensively used in their hydraulic constructions. This practice has been more or less followed by modern engineers, who, until within a few years, either used the puzzolana of Italy, where it is obtained near Mount Vesuvius, in a pulverulent state, or a material termed Trass, manufactured in Holland, by grinding to a fine powder a volcanic stone obtained near Andernach on the Rhine.

Experiments by several eminent chemists have extended the list of natural substances which, when properly burnt and reduced to powder, have the same properties as puzzolana. They mostly belong to the feldspathic and schistose rocks, and are either fine sand, or clays more or less indurated.

The following Table gives the results of analyses of Puzzolana, Trass, a Basalt, and a Schistus, which, when burnt and powdered, were found to possess the properties of puzzolana.