It is proper, however, to observe here, that the various theorems of mechanical science do not rest upon any hypothesis concerning these atoms as a basis. These theorems are not inferred from this or any other supposition, and therefore their truth would not be in anywise disturbed, even though it should be established that matter is physically divisible _in infinitum_. The basis of mechanical science is _observed facts_, and, since the reasoning is demonstrative, the conclusions have the same degree of certainty as the facts from which they are deduced. (19.) _Porosity._--The _volume_ of a body is the quantity of space included within its external surfaces. 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 completely 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 contact. 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 kind from the body in question. These interstitial spaces are called _pores_. In bodies which are constituted uniformly throughout their entire dimensions, 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. (20.) The proportion of the quantity of matter to the magnitude is called the _density_. Thus if of two substances, one contain 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 subtle nature. If the pores of a body on the surface of the earth, and exposed to the atmosphere, be greater than the atoms of air, then the air may pervade the pores. This is found to be the case with many sorts of wood which have an open grain. 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 entering 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.