FUNDAMENTAL PROPERTIES OF MATTER. 31 



assumption. We shall learn, hereafter, that the law of Avogadro is 

 one of the greatest importance to the science of chemistry. 



Gravitation. Every particle of matter in the universe attracts 

 every other particle ; consequently, all masses attract each other, and 

 this attraction is known as gravitation. The action of gravitation 

 between the thousands of heavenly bodies moving in the universe is 

 to be considered by astronomy, but some of the phenomena caused 

 by the mutual attraction of the substances composing the earth are 

 of importance for our present consideration. 



Such phenomena caused by gravitation are the falling of substances, 

 the flowing of rivers, the resistance which a substance offers on being 

 lifted or carried. A body thrown up into the air or deprived of its 

 support will fall back upon the earth. In this case the mutual attrac- 

 tion between the earth and the substance has caused its fall. It 

 might appear that in this case the attraction was not mutual, but ex- 

 erted by the earth only; it has been proved, however, by most exact 

 experiments, that there is also an attraction of the falling substance 

 for the earth, but the amount of the force of this attraction is directly 

 proportional to th.e mass of the bodies, and consequently too insig- 

 nificant in the above case to be noticed. 



The law of gravitation, known as Newton's law, may thus be stated : 

 All bodies attract each other with a force directly proportional to 

 their masses and inversely proportional to the squares of their distance 

 apart. With regard to the earth and bodies upon it at a given place, 

 the mass of the earth and the distance between the earth and the 

 bodies remain the same, so that the only thing that varies is the mass 

 of the bodies. Hence, according to Newton's Law, the force with 

 which such bodies are attracted to the earth varies directly as their 

 masses. In other words, if a body A has twice the mass or quan- 

 tity of matter as a body B, it will be attracted with twice as much 

 force to the earth as the body B. 



Weight is an expression used to denote the quantity of mutual 

 attraction between the earth and the body weighed. When we weigh 

 bodies on a balance, we primarily compare two forces, namely, the 

 pull of the earth on each of the bodies on the pans of the balance, 

 nevertheless, we can use the balance to measure mass or quantity of 

 matter. For if two bodies are exactly balanced, that is, " weigh " 

 the same, we know that the pull of the earth is the same on each, and 

 since this attraction, as was shown above, is directly proportional to 



