394 SCIENTIFIC RECREATIONS. 



a considerable amount of heat when mixed with water, combining to make 

 hydrate of lime, or " slaked lime." This soon crumbles into powder, and 

 as a mortar attracts the carbonic acid from the air, by which means it 

 assumes the carbonate and very solid form, which renders it valuable for 

 cement and mortar, which, when mixed -with sand, hardens. Caustic lime is 

 used in whitewashing, etc. 



Carbonate of Lime (CaCo 3 ) occurs in nature in various forms, as lime- 

 stone, chalk, marble, etc. Calc-spar (arragonite) is colourless, and occurs as 

 crystals. Marble is white (sometimes coloured by metallic oxides), hard, 

 and granular. Chalk is soft and pulverizing. It occurs in mountainous 

 masses, and in the tiniest shells, for carbonate of lime is the main component 

 of the shells of the Crustacea, of corals, and of the shell of the egg ; it enters 

 likewise into the composition of bones, and hence we must regard it as one 

 of the necessary constituents of the food of animals. It is an almost invari- 

 able constituent of the waters we meet with in Nature, containing, as they 

 always do, a portion of carbonic acid, which has the power of dissolving 

 carbonate of lime. But when gently warmed, the volatile gas is expelled, 

 and the carbonate of lime deposited in the form of white incrustations upon 

 the bottom of the vessel, which are particularly observed on the bottoms 

 of tea-kettles, and if the water contains a large quantity of calcareous matter, 

 even our water-bottles and drinking-glasses become covered with a thin film 

 of carbonate of lime. These depositions may readily be removed by pouring 

 into the vessels a little dilute hydrochloric acid, or some strong vinegar, which 

 in a short time' dissolves the carbonate of lime. 



Sulphate of Lime (CaSO 4 ) is found in considerable masses, and is 

 commonly known under the name of Gypsum. It occurs either crystallized 

 or granulated, and is of dazzling whiteness ; in the latter form it is termed 

 Alabaster, which is so soft as to admit of being cut with a chisel, and is 

 admirably adapted for various kinds of works of art. Gypsum contains 

 water of crystallization, which is expelled at a gentle heat. But when 

 ignited, ground, and mixed into a paste with water, it acquires the property 

 of entering into chemical combination with it, and forming the original 

 hydrate, which in a short time becomes perfectly solid. Thus it offers to the 

 artist a highly valuable material for preparing the well-known plaster of Paris 

 figures, and by its use the noblest siatues of ancient and modern art have 

 now been placed within the reach of all. Gypsum, moreover, has received a 

 valuable application as manure. In water it is slightly soluble, and imparts 

 to it a disagreeable and somewhat biiterish, earthy taste. It is called 

 " selenite " when transparent. 



Phosphate of Lime constitutes the principal mass of the bones of animals, 

 and is extensively employed in the preparation of phosphorus ; in the form 

 of ground bones it is likewise used as a manure. It appears to belong to 

 those mineral constituents which are essential to the nutrition of animals. 

 It is found in corn and cereals, and used in making bread; so we derive 

 the phosphorus which is so useful to our system. 



