\\.\L PHILOSOPHY. 



II 



SOX V. 



ve learned how bodies may be extended, and how 



compressed, and hare also ascertained that all bodies are porous. Now these facts are 

 all tery important, especially when they are associated with other matters which we shall 

 consider hereafter. They are intimately connected with the affairs of every-dav life, and 

 are therefore of great consequence. Hallam had become conversant with the laws which 

 regulate them, and was led on step by step until he applied the knowledge he thus 

 acquired to useful purposes. He learned that even solid bodies may be made to expand 

 or contract without diminishing their mass, and he also learned that solid bodies have 

 pores or interstices between their particles. Hallam had yet to learn more of tho 

 principles of Natural Philosophy ; so have we. 



QUESTIONS. 



52. T. You said that all bodies have 

 pores or interstices between their particles ; 

 now do you know this ? 



P. It has been discovered by experi- 

 ment, and I know it by reading. More 

 than two centuries ago it was proved lu- 

 cent, at the Academy !)! Cimento, 

 .11 Florence, that gold was porous ; the ex- 

 perience was the result of accident, but it 

 established the fact that water may be made 

 to pass through gold. 



53. T. Then you would imply, that 

 because all bodies arc compressible, there 

 are interstices between tlu-:r j>:irticles. 



P. Certainly, but the sire of the pores 

 varies in different substance-. 

 substance may contain 10,000 pores in a 

 square inch, and another 100,000 pores in 

 the same space. In the former case, the 

 pores are considerably larger than in the 

 latter. 



54. 7 i the effect of the ports 

 of a body being closer together I 



P. The substance itself is rendered 



inon- di-r.s.-. 



55. T. What do you mean by being 

 den,.- ? 



P. '} !. -ponds upon 



the pr<> . 



of any sub- 



tancc, the less will be the porosity. The 

 density of a body ia the relation of its 

 weight to its volume, and therefore in- 

 dicates its specific gravity, a property we 

 hall consider on another occasion. 



56. T. How can you prove that bodies 

 have pores ? 



/'. By a very simple experiment. [Ex- 

 periment 10.] 1 have here a piece of wood, 

 with a wire fastened to it, and a tumbler of 

 water. I will plunge the wood into the 

 water, and keep it at the bottom of the 

 tumbler by means of the wire [performing 

 the experiment]. You sec tint 

 bubbles of air are rising to the surface, they 

 have escaped from the pores of the wood. 

 which are bein-j filled with water i: 

 If there were not any int. rst ices it wouM 

 be impossible for the air to be in the sub- 

 stance of the wood, because it is contrary 

 to one of the established general laws of 

 Natural Philosophy. 



/'.Is the knowledge of the porosity 

 of bodies applied to any useful or scientific 



^es? 



P. Yes; filtration is based upon, ami 

 electrotyping is under obliga: 



58. T. Can you adduce any further 



proofs of the jHiroiity of bodies f 



P. Yes ; many bodies are capable of 



compression merely by merit. mi. M 

 and this I will explain by a simple 



ill 1 1 | I li.ive 



basin of water, and a piece of cork floating 



I will take an 

 - (n* it i* commonly 



filled with air) and invert it orcr 

 the cork, so that the edge shall just .. 

 the water; the air is now confined within 

 the tumbler and occupies a given space, 



