Porosity
Pores are small interstices between the particles of matter which compose bodies, and are either empty or filled with some insensible medium. Condensation and rarefaction are only performed by opening and closing the pores. What shape the atoms of different bodies are, we have no means of determining. Pores are often visible to the naked eye, as in sponge and pumice-stone; but in gold and granite they are too minute to be detected. If we place a little water on chalk or cloth it disappears; in a certain sense it penetrates them, but it does not enter into the solid particles; it only passes into the vacant places or pores. A piece of iron is made smaller by hammering. This proves its porosity. Its particles could not be brought into closer contact if there were no interstices between them. Mercury passes through lead, and water may be forced through the pores of gold. So that, though matter is essentially impenetrable, it is also universally porous. See PARTICLE.