We are not attempting to perplex a plain matter of science by introducing into its discussion a metaphysical subtilty. The principle here contended for is one of the first dictates of the inductive philosophy, and as such it has been frankly acknowledged and acted upon by all the great improvers of science in modern days. When Newton discovered that the planets circle round the sun in the same manner in which a stone thrown by the hand describes a curve before reaching the earth, he may be said to have _explained_ the former phenomenon by bringing it into the same class with certain results which have long been familiar to us. But the explanation was only relative, not absolute. The latter phenomenon is, in reality, no more explicable than the former; he did not pretend to know the _cause_ of the stone's falling to the ground, any more than of the revolution of the planets. It was something to be able to arrange these apparently heterogeneous results in the same class, and gravity was a convenient name to apply to the whole. But the supposition, that gravity was an occult cause, inherent in matter, he earnestly repelled, and declared that it was "inconceivable."[2] Franklin showed, that a thunder-cloud and the charged conductor of an electrical machine manifested the same phenomena, and might therefore be classed together; sparks were obtained from both, Leyden jars were charged from them, other bodies were attracted and repelled in a similar way, so that it was reasonable to believe that the same agency was acting in both cases. What this agency was he did not even guess. The _cause_ of electric action, whether in the excited cloud, or the excited tube, was just as obscure as ever. Chemists observed, that different substances, when brought into close contact, sometimes remained distinct, and sometimes united with each other in various but regular proportions; and these capacities of coalescing with one class of bodies, and of remaining unaffected by another, are called chemical "affinities." This is a convenient generalization, and has properly received a specific name; though the common appellation throws no light on the _cause_ of the phenomena, which remains an impenetrable secret. To say that certain action is _caused_ by the operation of chemical affinities is only to arrange it with a large class of other observed appearances, equally obscure as to their origin and essential character.