CHAPTEK II. 



LIMESTONE. 



A large part of the structure of the earth's 

 crust is formed of a substance called lime- 

 stone. Ordinary limestone is a compound of 

 common lime and carbon dioxide, a gas that is 

 found mixed with the air to a very small de- 

 gree. Carbon dioxide will be better known by 

 the older people as carbonic acid. It is a gas 

 that is given off whenever wood and coal are 

 burned, or any substance containing carbon. 

 It is composed of one atom of carbon to two of 

 oxygen. Every ton of coal that is burned 

 sends off three and two-thirds tons of this gas. 

 The increase in weight comes from the fact 

 that every atom of carbon unites with two of 

 oxygen, which it takes from the air, and the 

 oxygen is heavier than the carbon. 



In comparing the relative weights of atoms 

 (the smallest combinable particle of a solid, 

 liquid, or gas) we use the hydrogen atom as 

 the unit of comparison and call it " one," be- 

 cause it is the lightest of all atoms. The car- 

 is 



