“Plaster is not all prepared beforehand for the reason that it hardens very quickly, turns to stone, and is then unfit for use. Accordingly, to have it in a suitable state of softness, it must be prepared at the moment of using.” “And what do they make that powder of that turns to stone when it is mixed with water?” “Plaster is made from a stone called gypsum, which, always the same as to its nature, varies much in appearance according to its state of purity. Sometimes it is a shapeless rock, whitish and more or less grained; sometimes a fine fibrous mass with a silky luster; or, again, a substance as transparent as glass and splitting into very thin scales which show, here and there, the superb colors of the rainbow. Struck by their beauty, workmen engaged in quarrying gypsum have given the name of ‘Jesus-stone’ to these brilliant laminæ. Also, from their brilliance and their cheapness, they are called ‘donkey’s mirrors.’ In ancient times these beautiful sheets of transparent gypsum were used as window-panes. “Impure gypsum, in the form of shapeless rock, is used for ordinary plaster, while pure gypsum, which comes in glass-like sheets or in blocks of a silky appearance, is used for fine plaster, as in all sorts of molding. The stone from which plaster is obtained occurs in abundance in several departments of France, where it forms hills and even whole mountains, as for example in the departments of the Seine, the Mouths of the Rhone, and Vaucluse. For conversion into the usual plaster of Paris this stone must be subjected to a moderate heat. To this end it is the practice to build with gypsum blocks a row of small vaults, and on these vaults to pile fragments of smaller size. Then the firing is done by burning fagots and brushwood under these vaults.” “And is it carbonic acid gas this time, too, that is driven out by the heat, as in the manufacture of lime?” asked Jules. “No, my friend: gypsum does not contain any carbonic acid gas. It is made of lime, as in limestone, but united with sulphuric acid, which heat is powerless to drive out. Besides this it contains water, which forms a fifth of the total weight of the stone. This water, and nothing further, escapes under the action of heat. With this expelled the gypsum is turned to plaster. “But this latter has a strong tendency to take on again the moisture parted with in the kiln, and thus to become once more what it was in the beginning—primitive stone. It is this peculiarity that renders gypsum suitable for plaster. Moistened in the trough, the powdery matter quickly incorporates the water that is thus restored to it, and the whole hardens into a block having the solidity of gypsum that has not yet passed through the kiln. Lime turns to stone by being permeated with carbonic acid gas, which restores it to its limestone state. Plaster becomes stone by absorbing water, which brings it back to the state of gypsum. The transformation of lime is slow, of plaster very rapid.