SULPHATE OP LIME.

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when heated, is able when cold to take it from either ofthe alkalies. (129.)

164. When quicklime is mixed with vegetable oranimal substances, it greatly accelerates their decay: it,however, renders the process of putrefaction far lessnoisome than it is under ordinary circumstances, becausethe lime absorbs many of the products of decay.

165. When quicklime is slaked with water, it crum-bles down to a powder, and is found to have combinedwith a quantity of water; although it appears quite dry,for the water is chemically combined with the lime: ifthis lime thus slaked is left exposed to the air, it com-bines with carbonic acid, and becomes changed into car-bonate of lime, and at the same time parts with part ofthe water with which it had previously combined.

166. Sulphate of lime is less common and abundantthan the carbonate; it is however a frequent ingredientin the soil, in spring and mineral waters, and is foundsometimes in large beds constituting what is called plas-ter-stone, gypsum, and alabaster; these are all compoundsof lime and sulphuric acid, and precisely similar in com-position. Common plaster of Paris is dry sulphate oflime, or gypsum deprived by heat of the water whichit naturally always contains, and when mixed with asmall quantity of water it recombines with it and becomesa hard, dry, solid substance.

167. The burning of gypsum to make plaster of Parisis quite different from the burning of chalk to makequicklime: in the former case the native sulphate of