The size of the ultimate particles of matter must be small in the extreme. Organised beings, possessing life and all its functions, have been discovered so small, that a million of them would occupy less space than a grain of sand. The malleability of gold, the perfume of musk, the odour of flowers, and many other instances might be given of the excessive minuteness of the atoms of matter. Supposing the density of the air at the surface of the earth to be represented by unity, Sir John Herschel has shown that, under any hypothesis as to its atoms, it would require a fraction having at least 1370 figures in its denominator to express its tenuity in the interplanetary space; yet the definite proportions of chemical compounds afford a proof that divisibility of matter has a limit. The cohesive force, which has been the subject of the preceding considerations, only unites particles of the same kind of matter; whereas affinity, which is the cause of chemical compounds, is the mutual attraction between particles of different kinds of matter, generally producing a compound which has no sensible property in common with its component parts except that of their combined gravity, as, for example, water, which is a compound of oxygen and hydrogen gases. It is merely a result of the electrical state of the particles, chemical affinity and electricity being only forms of the same power. In most cases it produces electricity, as in the oxidation of metals and combustion, and in every case without exception heat is evolved by bodies while combining chemically; and as heat is an expansive force, chemical action is changed into mechanical expansion, but it is not known in this case why heat is produced, nor the manner in which the particles act.