It is necessary to sketch in a few sentences that field of creation with which the argument from design has to do. The world presents to us four kingdoms or classes of facts. One of these, and the first in point of order, is the mineral kingdom. A few so-called elements, as metals, earthy bases, and the like, acted upon by certain forces, known to us as gravitation, motion, heat, electricity, magnetism, chemical affinity, have formed the mountain and the valley, the wind and the clouds, the sea margin and the cave; in a word, all the grand substructure on which the higher kingdoms are to take their places. Modern science has discovered however, that these physico-chemical forces are interchangeable or convertible; that retarded motion turns to heat, as in the railway break, that heat generates electricity, and the electric current magnetises the iron round which it passes. Not only this, but each force generates a certain equivalent of another--so much and no more; and no force is lost, though a force may pass from an active to a potential state. For example, two tuns of water are raised by evaporation from the sea, and one of them falls in rain in a valley drained by a river, and in its downward motion back to the sea it will turn the water-wheel, lift the tilt-hammer, bear the barge swiftly in its current, leap over the rocky ledge a foaming cataract, and in all these it is only sending back a portion of the force which was spent upon its evaporation; and the real source of all this work is, and must be, the sun's heat. And ere the water rests again in the sea it will have accounted for the whole of the force, neither less nor more, that had operated upon it; part of it in friction on its bed and in consequent heat; part of it in tasks imposed by human skill. The other tun of water shall fall into some land-locked tarn, high in the hills, where it cannot at once render back its force in work or duty, but the force is there, held in suspense or in reserve. Water lifted from the sea level to the valley of the Engadine, a mile higher, has used much of the sun's heat; it will restore that heat or some equivalent force, as soon as you make a way for it to the sea level again; and it will have parted with all the force, neither more nor less, which raised it to that height. That forces are convertible, and that whether converted or not they are conserved, so that nothing is lost, are propositions demonstrated. It is not, I believe, demonstrated, but it is a probable supposition, that all forces are but one force manifested in different modes.