226 CHEMISTRY. 



25 grammes of water. Lime has also a considerable affinity 

 for carbonic acid. When, therefore, it is exposed for some 

 time to the air, it unites with the carbonic anhydride of 

 the air as well as its moisture, and air-slaked lime is there- 

 fore a mixture of hydrate and carbonate of lime. 



313. Solubility of Lime. While lime has so great an af- 

 finity for water, the dry substance which results after its 

 thirst is slaked is very sparingly soluble. It is in strong 

 contrast in this respect with the alkalies. While potas- 

 sium hydrate is soluble in half its weight of water, the 

 hydrate of lime requires one thousand times its weight of 

 water to dissolve it. It is very remarkable that cold 

 water will dissolve more of it than warm. Lime-water, as 

 the solution of lime and water is called, is sometimes used 

 as a medicine. With sweet-oil it makes a soapy mixture 

 which is often used as an application to burns. 



314. Mortar. The most important use of lime is in mak- 

 ing mortar. As glue holds pieces of wood together, so does 

 mortar bricks and stones. In the making of mortar w r e stir 

 sand, lime, and water together, and the sand becomes in- 

 timately mixed with the hydrate of lime while it is form- 

 ing. As the mortar becomes dry by the evaporation of 

 all the water that is not used up in the formation of the 

 hydrate, there occurs also another chemical change car- 

 bonic anhydride is attracted from the air, and unites with 

 a portion of the lirne ; so that we have in the mortar a 

 mixture of carbonate and hydrate of lime, which has more 

 firmness than either of these substances separately. Then, 

 again, mortar becomes hard gradually, from a chemical 

 action between the sand and lime, whereby silicate of lime 

 is formed. The sand serves to give both body and firm- 

 ness to the mortar. When mortar is used as plastering, 

 hair is added, the fibres serving to hold the mortar more 

 thoroughly together. 



