28 GENERAL SCIENCE 



speak of an atom of water, for the smallest particle of 

 water that can exist is composed of one atom of oxygen 

 combined with two atoms of hydrogen. This combina- 

 tion of atoms we call a molecule of water. A molecule 

 is the smallest particle of water that can possibly exist; 

 for if the atoms of oxygen and hydrogen are separated, 

 we no longer have water but the two elements in the 

 form of gases. 



Molecules are usually composed of atoms of different 

 kinds ; the atoms may be all of one kind, however. The 

 molecules of some substances are composed of a large 

 number of atoms of different kinds. The molecule of 

 alum is composed of at least one hundred atoms. In 

 our work on electricity we shall have something more 

 to say about atoms and the way they are held together. 



Properties of Matter. With so many different people 

 in the world it would seem at first impossible to dis- 

 tinguish the individuals. However, our experience has 

 shown us that with all the millions of human beings there 

 are always individual characteristics or properties which 

 enable us to tell one from another. It is the same way 

 with the substances which go to make up the earth. 



Those characteristics or properties which are held in 

 common by all substances are called general properties, 

 and those properties which belong only to certain kinds 

 of matter and enable us to distinguish one substance 

 from another are called special properties. 



All matter occupies space to the exclusion of all other 

 matter from that same space. 



Experiment i. Fit a two-hole rubber stopper to a pint bottle 

 (Figure 21). Through one hole of the stopper pass a funnel 

 tube and through the other pass a delivery tube leading to a bowl 

 of water. Pour water into the funnel tube. What causes the 



