i. The former relies on the new scientific conception of the _Unity of Force_. When Newton established the composition of Light in his treatise on Optics, and the law of Gravitation in his Principia, he conceived himself to be treating of two separate powers of nature, between which, quick as he was to seize unexpected relations, he dreamt of no interchange. Yet now it is understood that when collisions occur of bodies gravitating on opposite lines, the momenta that seem to be killed simply burst into light and heat. When Priestley's experiments detected the most important chemical element on the one hand, and the fundamental electrical laws on the other, he seemed to move on paths of research that had no contact. Yet, in the next generation, chemical compounds were resolved by electricity; which again turns up in exchange for magnetism, and can pass into motion, heat, and light. To see the transmigration of natural agency, trace only through a few of its links the effect of the sunshine on the tropic seas. So far as it warms the mass of waters, either directly or through the scorched shores that they wash, it stirs them into shifting layers and currents, and creates _mechanical_ power. But it also removes the superficial film; and thus far spends itself, not in raising the temperature, but in changing the form from liquid to vapor, and so altering the specific gravity as to transfer what was on the deep to the level of the mountain-tops. It is the Pacific that climbs and crowns the Andes, resuming on the way the liquid state in the shape of clouds, and as it settles crystallizing into solid snow and ice. The original set of solar rays have now played their part, and made their escape elsewhere. But there is sunshine among the glaciers too, which soon begins to resolve the knot that has been tied, and restore what has been stolen. It sets free the waters that have been locked up, and lets their gravitation have its play upon their flow. As they dash through ravines, or linger in the plains, they steal into the roots of grass and tree, and by the tribute which they leave pass into the new shape of _vital_ force. And if they pass the homesteads of industry, and raise the food of a civilized people, who can deny that they contribute not only to the organic, but to the _mental_ life, and so have run the whole circuit from the lowest to the highest phase of power? That the return back may be traced from the highest to the lowest, is shown by every effort of thought and will; which through the medium of nervous energy in one direction sets in action the levers of the limbs, and in another works the laboratory of the organic life, and forms new chemical compounds, of which some are reserved for use, while others pass into the air as waste. Still further: all doubt of identity in the force which masks itself in these various shapes is said to be removed by the test of direct measurement before and after the change.