28 INTRODUCTION. 



The heat required to raise one pound of water one degree centi- 

 grade is usually taken as the unit of comparison. On thus comparing 

 olive oil, we find its specific heat to be J. If we say the specific 

 heat of mercury is -^ we indicate that equal quantities of heat will 

 be required to raise one pound of water or 32 pounds of mercury one 

 degree, or that the heat which raises one pound of water one degree 

 will raise One pound of mercury 32 degrees. 



3. GRAVITATION. 



Action of 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 the mass of the bodies, and consequently .too insig- 

 jrificant in the above case to be noticed. 



The law of gravitation, known as N^toT^slaw, may thus be stated : 

 / All bodies attract each other with a force directly proportional to 

 I their masses and inversely proportional to the squares of their distance 

 \ apart. 



QUESTIONS. 11. What two kinds of divisibility of matter do we distinguish, 

 and by what actions are they accomplished ? 12. Explain the term molecule. 

 13. Mention one of the facts which prove that a gas consists of particles with 

 intervals between them. 14. State the law of Avogadro. 15. Mention the 

 effects produced by increased velocity of the molecules of a mass. 16. Give an 

 explanation of the expressions latent heat, free heat, and specific heat. 17. 

 Explain the construction of a mercury thermometer. 18. How many degrees 

 of Fahrenheit are equal to 50 C. ? 19. How many degrees of centigrade are 

 equal to 167 F. ? 20. What is distillation, and what is sublimation ? 



