14 



PHYSICAL PROPERTIES. 



case in any known body. Consequently, all bodies 

 are more or less porous; that is, have interstitial 

 spaces, which are either empty, or filled by some 

 substance of a different nature from the body in 

 question. Volcanic lava is so full of cavities filled 

 with air, that it floats upon water. 



There is a mineral called hydrophane^ which man- 

 ifests its porosity in a very remarkable manner. It 

 is a silicious stone, and in its ordinary state is semi- 

 transparent. On plunging it, however, into water, 

 and then withdrawing it, we find it to be as trans- 

 lucent as glass. The pores, in this case, which 

 were previously filled with air, are now filled with 

 water, which renders the mineral transparent. 



It is common to see water percolating through 

 the sides and roofs of caverns and grottoes, and, 

 being impregnated with calcareous and other earth, 

 form stalactites, hanging like icicles from the roof, 

 and presenting a highly interesting appearance 

 This is owing to the porosity of such rocks. The 

 nearer the particles of which a body is composed 

 approach each other, the harder is it said to be. 



Thus, among minerals, the diamond^ which is con- 

 solidated carbon^ is the hardest ; and naptha, or petro- 

 leum, is one of the softest. We judge of the relative 

 hardness of miner^ils by the readiness with which 

 they receive impressions by a sharp-pointed body. 

 For example, the sharp edge of a piece of flint will 

 scratch glass, and a diamond will scratch both ; 

 quartz scratches feldspar, and topaz scratches 

 quartz ; fluor spar scratches calcareous spar, while 

 the latter scratches gypsum ; so that this character 

 is of great value in determining the species of any 

 unknown mineral.* 



* One of the secondary properties of bodies, which it is proper 

 here to notice, is elasticity. It is by the aid of this property that 

 bodies, when reUeved from the influence of a compressing force, 

 restore themselves to their former dimensions. All elastic bod- 

 ies are therefore compressible, though all compressible bodies 



