“Limestone,” answered his uncle, “contains two different substances: first, lime, and then an invisible substance, impalpable as air itself, in fact, a gas, carbonic acid gas. The name of carbonate of lime given to the limestone denotes precisely this combination. As it is when taken from the ground, the stone contains the two substances closely united, so incorporated indeed as no longer to have the qualities characterizing them when apart. Heat destroys this union: the lime stays in the kiln, and the carbonic acid gas is dissipated in the atmosphere with the smoke from the burnt fuel. After this liberation of the gas the lime is left in its pure state, no longer masked by the presence of another substance, but just as it is needed by the mason for making mortar.” “Then all that the fire does,” queried Jules, “is just to break apart the limestone and drive out the carbonic acid gas that it contained?” “What takes place in the lime-kiln,” replied his uncle, “is nothing but the separation of the lime and the gas. Now let us turn our attention to the mortar. When lime is watered, it gets very hot, swells, cracks open, and crumbles into a fine powder like flour. The heat that is generated comes from the violence with which the two substances rush together. Before absorbing water lime is called quicklime; after this absorption, which has reduced it to powder, it is called slaked lime. This slaked lime is reduced to a paste with water, and then well mixed and kneaded with sand. The result is the mortar used in laying stone and brick in order to hold the courses firmly together and give solidity to the building. “There is one thing I advise you to note, if you have not already done so, since it will explain to you the part played by mortar in masonry. Look at the water that for several days has covered a bed of lime slaked by the masons. You will see floating on the surface small transparent particles resembling ice. Well, these tiny fragments of crust are nothing but stone like that from which the lime was obtained; in a word, they are limestone or carbonate of lime. To make stone of that kind two substances are necessary, as I have just told you: lime and carbonic acid gas. The lime is furnished by the water, in which it must be present in solution, since the water covers a thick bed of this material; and as to the carbonic acid gas, it is furnished by the air, where it is always to be found, though in small quantities. Lime, then, has this peculiarity, that it slowly incorporates the small amount of carbonic acid gas present in the atmosphere, and so once more becomes the limestone that it was before.