HOME MAIS^URE. 81 



added, the carbonic acid is set free, and we see it pass ofE 

 as bubbles rising through the water. Limestone is so 

 slightly dissolved by water that it is tasteless. It takes 

 one thousand six hundred parts of water to dissolve one 

 part of limestone. Water, in which there is much car- 

 bonic acid, dissolves a considerable amount of carbonate 

 of lime. If a small piece of limestone be kept at a strong 

 red heat for some hours, it will be only about half as 

 heavy as the original stone. What has it lost in the 

 burning? If tested with acid, as before, no bubbles of 

 gas will be given off. The heat has driven out all the 

 carbonic acid; it is no longer a carbonate of lime, but 

 simply lime (an oxide of the metal calcium, or calcic 

 oxide, as the chemists have it). Limestone burned in 

 kilns produces lime, often called quick-lime. If a lump 

 of freshly burned lime have water gradually put upon it, 

 it soon becomes hot; in a little while it swells up, cracks, 

 and falls into a very white powder; though much water 

 has been added, the powder is quite dry. The water has 

 united with the lime, making a solid, caustic or slaked 

 lime. Lime exposed takes up moisture from the air, and 

 we have air-slaked lime. Slaked lime with enough water 

 forms whitewash, or '^milk of lime." On standing, the 

 greater part of the lime will settle, leaving clear lime- 

 water — a saturated solution of lime; that is, the water 

 has taken up all it can dissolve, for at ordinary tempera- 

 ture it requires several hundred parts of water to dissolve 

 one part or quick-lime. If clear lime-water be placed in 

 a glass, and with a straw or pipe-stem the breath be 

 forced into it, the lime-water will soon become cloudy, 

 and then milky. Set the glass aside, and a fine white 

 powder will settle at the bottom, leaving the water clear 

 above. The breath contains carbonic acid; this, when 

 forced into the lime water, unites with the lime, forming 

 carbonate, the same as unburned limestone, which, being 

 little soluble, separates as a white powder. If we con- 



