16 



NATURAL PHILOSOPHY. 



[Lesson V. 



but if I plunge the tumbler below the sur- 

 face, and keep it tbere.it will be found that 

 the water rises to a certain height above the 

 level of the brim, and the deeper that it is 

 plunged the more the cork rises in the tum- 

 bler : but as the pressure is removed and the 

 goblet rises, it will be found that the water 

 descends and the cork with it, because the 

 air expands. Thus you will see that air is 

 capable of being compressed a sufficient 

 proof of its porosity. 



59. T. Why did not the water fill the 

 tumbler when you plunged it below the 

 surface ? 



P. Because, as the air was in the tum- 

 bler, the water could not occupy the same 

 space at the same time, and, therefore, the 

 experiment also proves the IMPENETRA- 

 BILITY of the air. 



60. T. What do you mean by impene- 

 trability r 



P. By the impenetrability of bodies is 

 meant, that no two particles of matter can 

 occupy the same identical portion of space 

 at the same moment. 



61. T. How can you prove this ? 



P. By experiment. [Experiment 12.] 

 I have here a piece of clay, and a bullet, 

 which 1 will enclose within the clay. Now 

 it is quite impossible to make another bullet 

 occupy the cavity that contains the first 

 bullet as long as it is there. This you will 

 readily understand, because it is like trying 

 to pour a pint of water into 

 a pint measure already full 

 of water. Again, if I drive 

 a nail into a piece of wood, 

 the effect is only to compress 

 the wood, because it is im- 

 possible that the wood and 

 nail can be in the same 

 identical space at the same 

 precise time. 



62. T. Has this know- 

 lodge of the impenetrability 

 of bodies been usefully em- 

 ployed ? 



P. Yes ; the principle of the wedge 

 is founded upon it. In the above diagram 

 you will see the explanation of the law ; the 

 point of the wedge has been inserted into 



a block of wood by a blow from a ham- 

 mer, and has displaced the wood by com- 

 pression. The substance of the woo;l is 

 dividing, because it cannot be compressed 

 any more. 



03. T, If you remove the wedge, does 

 the wood resume its former shape, and 

 occupy the space it did before the wedge 

 was driven in ? 



P. No; because, unlike the air in the 

 tumbler, it is not elastic, otherwise it would 

 resume its former dimensions. 



64. T. Are not all bodies elastic ? 



P. No ; lead or iron may be com- 

 pressed or diminished in size, but they 

 cannot resume their former volume; and 

 therefore we learn, that elasticity does not 

 always accompany compressibility. 



65. T. Then am I to understand that 

 elasticity is the power by which a body 

 resumes its figure or volume, after that 

 figure or volume has been altered by the 

 action of any force ? 



P. Yes, undoubtedly, after the force 

 that caused the alteration of the figure has 

 ceased to act ; not otherwise, and tint 

 power is found in solid and fluid bodies. 



GENERAL QUESTIONS ON LESFON V. 



1. How do you know that all bodies have 

 pores between their particles ? 



2. What is the result of the greater 

 proximity of the pores of bodies ? 



3. What is meant by the density of 

 bodies ? 



4. Can you prove the porosity of bodies? 



5. Is it possible for two bodies to occupy 

 the same space at the same time ? 



6. What do you call this natural law of 

 bodies ? 



7. How can you prove the impenetra- 

 bility of bodies ? 



8. Has the knowledge of the impenetra- 

 bility of bodies been practically applied ? 



9. Do all bodies possess the property of 

 resuming their former volume after being 

 compressed ? 



10. What is this property of matter 

 named ? 



