18 Hature'0 Afracles. 



bon and two atoms of the oxygen, while all of 

 the calcium is retained with one atom of oxy- 

 gen, leaving ordinary lime. Lime, then, is 

 simply oxide of calcium. 



As all know, it is used almost exclusively for 

 making mortar for building purposes. In 

 order to do this we have to put it through the 

 process of " slacking/' by pouring water upon 

 it, and here another chemical change takes 

 place. The water unites with the lime, when 

 immediately the heat that was expended iri 

 throwing off the carbon dioxide and was stored 

 in the lime as energy is now given up again in 

 the form of heat. When a considerable bulk 

 of lime is slacked very rapidly the heat that is 

 given off is so great that it will produce com- 

 bustion. Here is a beautiful illustration of 

 what has been erroneously called "latent heat." 

 It is " heat stored as potential energy," that is 

 released by the combination of lime with 

 water. Slackened lime, then, is called calcic 

 hydrate. 



Very little of the limestone that we find is 

 absolutely pure. It is considered good when 

 it does not contain over five or six per cent, of 

 foreign substance. When more than this is 

 present the lime is considered poor, and when 

 it reaches fifteen per cent, or more of impuri- 

 ties it assumes the property of hardening 

 under water and is called cement. 



Carbonate of lime is found in several other 



