DIVISIBILITY--POROSITY--DENSITY--COMPRESSIBILITY--ELASTICITY--DILATABILITY. (10.) Besides the qualities of magnitude and impenetrability, there are several other general properties of bodies contemplated in mechanical philosophy, and to which we shall have frequent occasion to refer. Those which we shall notice in the present chapter are, 1. Divisibility. 2. Porosity--Density. 3. Compressibility--Elasticity. 4. Dilatability. (11.) _Divisibility._--Observation and experience prove that all bodies of sensible magnitude, even the most solid, consist of parts which are separable. To the practical subdivision of matter there seems to be no assignable limit. Numerous examples of the division of matter, to a degree almost exceeding belief, may be found in experimental enquiries instituted in physical science; the useful arts furnish many instances not less striking; but, perhaps, the most conspicuous proofs which can be produced, of the extreme minuteness of which the parts of matter are susceptible, arise from the consideration of certain parts of the organised world. (12.) The relative places of stars in the heavens, as seen in the field of view of a telescope, are marked by fine lines of wire placed before the eye-glass, and which cross each other at right angles. The stars appearing in the telescope as mere lucid points without sensible magnitude, it is necessary that the wires which mark their places should have a corresponding tenuity. But these wires being magnified by the eye-glass would have an apparent thickness, which would render them inapplicable to this purpose, unless their real dimensions were of a most uncommon degree of minuteness. To obtain wire for this purpose, Dr. Wollaston invented the following process:--A piece of fine platinum wire, _a b_, is extended along the axis of a cylindrical mould, A B, _fig. 1._ Into this mould, at A, molten silver is poured. Since the heat necessary for the fusion of platinum is much greater than that which retains silver in the liquid form, the wire _a b_ remains solid, while the mould A B is filled with the silver. When the metal has become solid by being cooled, and has been removed from the mould, a cylindrical bar of silver is obtained, having a platinum wire in its axis. This bar is then wire-drawn, by forcing it successively through holes C, D, E, F, G, H, diminishing in magnitude, the first hole being a little less than the wire at the beginning of the process. By these means the platinum _a b_ is wire-drawn at the same time and in the same proportion with the silver, so that whatever be the original proportion of the thickness of the wire _a b_ to that of the mould A B, the same will be the proportion of the platinum wire to the whole at the several thicknesses C, D, &c. If we suppose the mould A B to be ten times the thickness of the wire _a b_, then the silver wire, throughout the whole process, will be ten times the thickness of the platinum