The smallest amount of a gas,—­atmospheric air for instance,—­can be compressed into a space a thousand times smaller by mere mechanical pressure, and then its bulk must be to the least measurable surface of a solid body, as a grain of sand to a mountain.  By the mere effect of mass,—­the force of gravity,—­gaseous molecules are attracted by solids and adhere to their surfaces; and when to this physical force is added the feeblest chemical affinity, the liquifiable gases cannot retain their gaseous state.  The amount of air condensed by these forces upon a square inch of surface is certainly not measurable; but when a solid body, presenting several hundred square feet of surface within the space of a cubic inch, is brought into a limited volume of gas, we may understand why that volume is diminished, why all gases without exception are absorbed.  A cubic inch of charcoal must have, at the lowest computation, a surface of one hundred square feet.  This property of absorbing gases varies with different kinds of charcoal:  it is possessed in a higher degree by those containing the most pores, i.e. where the pores are finer; and in a lower degree in the more spongy kinds, i.e. where the pores are larger.

In this manner every porous body—­rocks, stones, the clods of the fields, &c.,—­imbibe air, and therefore oxygen; the smallest solid molecule is thus surrounded by its own atmosphere of condensed oxygen; and if in their vicinity other bodies exist which have an affinity for oxygen, a combination is effected.  When, for instance, carbon and hydrogen are thus present, they are converted into nourishment for vegetables,—­into carbonic acid and water.  The development of heat when air is imbibed, and the production of steam when the earth is moistened by rain, are acknowledged to be consequences of this condensation by the action of surfaces.

But the most remarkable and interesting case of this kind of action is the imbibition of oxygen by metallic platinum.  This metal, when massive, is of a lustrous white colour, but it may be brought, by separating it from its solutions, into so finely divided a state, that its particles no longer reflect light, and it forms a powder as black as soot.  In this condition it absorbs eight hundred times its volume of oxygen gas, and this oxygen must be contained within it in a state of condensation very like that of fluid water.

When gases are thus condensed, i.e. their particles made to approximate in this extraordinary manner, their properties can be palpably shown.  Their chemical actions become apparent as their physical characteristic disappears.  The latter consists in the continual tendency of their particles to separate from each other; and it is easy to imagine that this elasticity of gaseous bodies is the principal impediment to the operation of their chemical force; for this becomes more energetic as their particles approximate.  In that state in which they exist within the pores