28 MATTER AND ITS PHYSICAL PROPERTIES. 



truth would not. be in anywise disturbed, even though it should be established 

 that matter is physically divisible ad infmitum. The basis of mechanical sci- 

 ence is observed facts ; and since the reasoning is demonstrative, the con- 

 clusions have the same degree of certainty as the facts from which they are 

 deduced. 



FOROSITY. 



The volume of a body is the quantity of space included within its external 

 surface. The mass of a body is the collection of atoms or material particles 

 of which it consists. Two atoms or particles are said to be in contact, when 

 they have approached each other until arrested by their mutual impenetrability. 

 If the component particles of a body were in contact, the volume would be com- 

 pletely occupied by the mass. But this is not the case. We shall presently 

 prove that the component particles of no known substance are in absolute con- 

 tact. Hence it follows that the volume consists partly of material particles and 

 partly of interstitial spaces, which spaces are either absolutely void and empty 

 or filled by some substance of a different species from the body in question. 

 These interstitial spaces are called pores. 



In bodies which are constituted uniformly throughout their entire dimen- 

 sions, the component particles and the pores are uniformly distributed through 

 the volume ; that is, a given space in one part of the volume will contain the 

 same quantity of matter and the same quantity of pores as an equal space in 

 another part. 



The proportion of the quantity of matter to the magnitude is called the den- 

 sity. Thus, if of two substances, one contains in a given space twice as much 

 matter as the other, it is said to be " twice as dense." The density of bodies 

 is therefore proportionate to the closeness or proximity of their particles, and 

 it is evident that the greater the density, the less will be the porosity. 



The pores of a body are frequently filled with another body of a more subtile 

 nature. If the pores of a body on the surface of the earth, and exposed to the 

 atmosphere, be greater than the particles of air, then the air will pervade the 

 pores. This is found to be the case of many sorts of wood which have open 

 grains. If a piece of such wood, or of chalk, or of sugar, be pressed to the 

 bottom of a vessel of water, the air which fills the pores will be observed to 

 escape in bubbles, and to rise to the surface, the water pervading the pores and 

 taking its place. 



If a tall vessel or tube, having a wooden bottom, be filled with quicksilver, 

 the liquid metal will be forced by its own weight through the pores of the 

 wood, and will be seen escaping in a silver shower from the bottom. 



The process of filtration in the arts depends on the presence of pores of such 

 a magnitude as to allow a passage to the liquid, but to refuse it to those impu- 

 rities from which it is to be disengaged. Various substances are used as filters ; 

 but whatever be used, this circumstance should always be remembered, that no 

 substance can be separated from a liquid by filtration except one whose parti- 

 cles are larger than those of the liquid. In general, filters are used to separate 

 suiid impurities from a liquid. The most ordinary filters are soft-stone paper 

 and charcoal. 



All organized substances in the animal and vegetable kingdoms are, from 

 their very nature, porous in a high degree. Minerals are porous in various 

 degrees. Among the silicious stones is one called hydrophone, which mani- 

 fests its porosity in 'a very remarkable manner. The stone in its ordinary 

 state is semi-transparent. If, however, it be plunged in water, when it is with- 

 drawn it is as translucent as glass. The pores in this case previously filled 



