THE SUBSTANCES OF THE EARTH. 11 



Hydrogen and nitrogen are both odorless, but when 

 united they form ammonia (NH 3 ) which is noted for its 

 strong odor. Chlorine alone is poisonous, but with sodium 

 it forms common salt (NaCl). The great variety of dif- 

 ferent kinds of matter is produced by the great number 

 of possible combinations of atoms of the elementary 

 substances. 



Chemical Action. Molecules are the smallest particles 

 into which a substance can be divided without changing 

 its nature. Whatever change is wrought upon matter, 

 its real nature remains the same so long as its individual 

 molecules are not broken up. 



A substance may be melted, or converted into a gas, or 

 mixed with some other substance, and yet it is the same 

 substance ; but if its molecules are divided into their sep- 

 arate atoms, and these unite again with other atoms in 

 different combinations, it is no longer the same. 



The force which divides molecules into their separate 

 atoms, and permits them to form other combinations, so 

 as to produce new substances, is called chemical force, or 

 the chemical action of one substance upon another. 



When two molecules of different kinds are brought to- 

 gether, if some of the atoms of one are more strongly 

 attracted by atoms in the other than by neighboring atoms 

 in their own molecule, one or both of the molecules will 

 be broken up and new molecules will be formed. 



When water is poured upon quicklime, the atoms 

 of the water molecule unite with those of the lime mole- 

 cule, forming molecules of a new substance, or slaked 

 lime. 



When some of the atoms are not needed in making up 

 the new molecule they are set free. If pieces of zinc are 

 placed in hydrochloric acid, the chlorine of the acid will 



