“As soon as it comes from the kiln plaster is ground under vertical millstones and then sifted. The powder must be kept in a very dry place, since it contracts moisture easily and then will not harden or set, as they say, when mixed with water. You will perceive clearly enough that after being more or loss impregnated with moisture plaster cannot have the same tendency to absorb the water necessary to change it into a solid mass; the substance being already somewhat soaked will not show the same thirst when the time comes for using it. All damp and, still more, all wet plaster is of no further use. “Statues, busts, medallions, and various other ornamental objects are made by casting with fine plaster of Paris. This is prepared from the purest gypsum, those beautiful transparent scales I told you about a little while ago. It is heated in ovens similar to those used by bakers, and cut off from contact with the burning fuel, so as to preserve its whiteness. The powder, which looks like fine flour, is mixed with water and reduced to a smooth paste, which is then poured into molds. When the plaster has set, the mold, which is in several pieces, all joined together, is taken apart and the finished cast withdrawn.” CHAPTER XII PLASTER OF PARIS IN AGRICULTURE “In agriculture plaster of Paris has by no means the importance of lime; nevertheless it produces excellent results on clover, sainfoin, and lucerne. It is used in the spring for sprinkling the young leaves when they are still damp with the morning dew. Still, foggy weather is the most favorable for this work. Plaster also acts well on rape, flax, buckwheat, and tobacco, but has no effect on cereals. “The intelligent farmer puts plaster of Paris to still another use. In every dunghill there is always going on a slow combustion, or fermentation, giving forth ammonia in vaporous form; and this ammonia escapes into the air as a total loss, whereas it ought to be retained as far as possible in the manure, since the compounds of ammonia constitute the source whence plants obtain nitrogen. Therefore to prevent this waste, plaster is sprinkled over the dunghill. Sometimes, too, it is sprinkled over each layer of manure as the pile rises. The plaster absorbs the ammoniac vapors, gives them a little of its sulphuric acid, and converts them into a compound, sulphate of ammonia, which is proof against vaporization. Hence we say that plaster of Paris fixes ammonia, that is to say prevents its being dissipated.