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 sihcious 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 minerals 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 
