To make mortar with which masonry is held in place it is customary to use lime. In a sort of trough lined with sand are placed lumps of stone having a calcined appearance, and on these stones water is poured. In a few moments the pile becomes heated to high temperature, cracks and splits and finally crumbles into dust, at the same time absorbing the water, which disappears little by little as it is taken up by the solid matter or vaporized by the heat. More water is added to reduce it all to paste, which is finally mixed with sand. The product of the mixture is mortar. Such is the process often witnessed by Emile and Jules, who are always surprised, that stone, by having water poured on to it, should become hot and turn the water into jets of steam. “Lime,” Uncle Paul explained to them, “is obtained from a widely diffused stone called limestone or, in more learned language, carbonate of lime. The process is of the simplest sort. It consists of heating the stone in kilns built in the open air in the vicinity of both limestone and fuel, so as to avoid the expense of transportation in the manufacture of a product that it is desirable to furnish at a low price. “A lime-kiln is about three meters high, and is lined with fire-proof brick. An opening at the bottom serves for taking out the lime when the firing has continued long enough. In filling the kiln it is the usual practice to begin by laying large pieces of limestone so as to form a sort of rude vault over the fireplace, and on this vault are piled smaller fragments until the entire cavity is filled. The fuel used may be fagots, brushwood, turf, or coal. After the firing has gone on long enough, operations are suspended and the lime is withdrawn by breaking down the vault supporting the entire mass, which crumbles and comes crowding out at the lower opening, whence it is usually removed. “Another method still followed in some localities and of more ancient origin consists of filling the kiln with alternate layers of fuel and limestone. The whole rests on a bed of fagots that serves for starting the fire. As soon as the fire has spread throughout the mass, the opening at the top is closed with pieces of sod in order to make the combustion slower and more even.” “Nothing could be simpler,” said Jules, “than lime-making. Now I should like to know what effect the heat of the kiln has on the limestone. How does it happen that stone turns into lime by passing through fire?”