224 



CHEMISTRY. 



the carbonic acid being driven off. The operation is car- 

 ried out on a large scale in a kind of furnace called a 

 lime-kiln, shown in Fig. 91. The decomposition takes places 

 at a lower temperature in a current of air than otherwise, 

 and this is effected by building a tall kiln. 



Fig. 91. 



How simply heating their bodies effects their decom- 

 position we will explain : Heat expands all bodies, or, in 

 other words, puts the particles in them farther apart. But, 

 as you have already learned, it is necessary that particles 

 of different substances should be in immediate contact, or 

 exceedingly near to each other, in order that the attractive 

 force may come into action. Now it is supposed that in 

 the case of the carbonate of lime the heat, in expanding it, 

 puts the particles of the carbonic anhydride at such a dis- 



