li 



NATURAL PHILOSOPHY. 



[Lesson IV. 



pressing upon the lower ones. So it is 

 with the particles of nir at the surface of 

 the earth : they are pressed upon hy those 

 above them, at a pressure of about 15 Ibs. 

 to every square inch. If you examine the 

 bags, you will perceive that a is very much 

 compressed, b not quite so much, c less, 

 and d almost uncompressed ; so that as we 

 ascend you observe the pressure is less, the 

 same as it is with atmospheric pressure. 



4K T. Can you compress gases the 

 same as you do atmospheric air ? 



P. Yes ; all gases can be compressed ; 

 and one of the most familiar illustrations 

 of the truth of this, is the compression of 

 carbonic acid gas in the manufacture of 

 soda-water. 



45. T. Are fluids or liquids compres- 

 sible ? 



P. Yes ; but in so slight a degree that, 

 in a hydrostatic sense, they are considered 

 incompressible ; yet they are not abso- 

 lutely incompressible, but only yield slightly 

 to very intense pressure. 



46. T. Are you certain that liquids 

 have been compressed ? 



P. Yes. Canton proved this by ex- 

 periment in the year 1761. He placed a 

 tube with a bulb, similar to the one we 

 employed [Experiment 6], in a condenser, 

 and submitted the surface of the liquid to 

 a very intense pressure of condensed air. 

 The result was, that the level of the liquid 

 fell perceptibly, and rose again to its 

 original height upon removing the pressure. 



47. T. Can solids be compressed? 



P. Yes. You observe this piece of 

 lead is round, in fact, it is what is called a 

 bullet; now immediately that it receives a 

 stroke of this hammer it will be partially 

 compressed. [Experiment 9.] You see 

 that it is somewhat flattened, and now that 

 it has received six strokes, the size of it is 

 diminished, but its weight is the same. In 

 the same manner, iron, steel, gold, and 

 other solids may be compressed. 



48. T. How can you account for this ? 

 P. All bodies have interstices between 



the different particles of matter, and it is 



therefore very evident that the atoms arc- 

 not in immediate contiguity wi:i, 

 other. The spaces between the atoms are 

 capable of being compressed or extended, 

 and therefore the volume of the body may 

 be diminished or increased. 



49. T. What do you call the spaces 

 between the particles of bodies ? 



P. They are called pores. 



50. T. Have all bodies spaces or pores 

 between their particles ? 



P. Yes ; every body is porous. 



51. T. Then the property of being 

 porous is a general one ? 



P. It is ; and the quality of being so is 

 called POROSITY. 



GENERAL QUESTIONS ON LESSON IV. 



1. When bodies have their volume en- 

 larged without increasing their ma 

 what term do you express this prop. 



2. What is the volume of a body ? 



3. How is the volume of a body es- 

 timated ? 



4. Prove by experiments that the volume 

 of a body may be enlarged without increasing 

 the mass. 



5. Are all bodies capable of being ex- 

 tended ? 



6. What relation has the volume of a 

 body to its figure ? 



7. Can bodies be diminished in bulk 

 without decreasing their mass ? 



8. What term has been applied to 

 this property of diminishing the bulk of 

 bodies? 



9. Are all bodies capable of being com- 

 pressed ? 



10. Give some familiar examples of the 

 compressibility of bodies, and illustrate 

 the fact by experiments. 



11. How do you account for the com- 

 pressibility and extensibility of bodies ? 



12. What term has been applied to the 

 spaces between the particles of bodies? 



13. What is the name of the general 

 law, " that all bodies have spaces between 

 their particles? " 



