The doctrine called the correlation, persistence, equivalence, transmutability, indestructibility of force, or the conservation of energy, is a generality of such compass that no single form of words seems capable of fully expressing it; and different persons may prefer different statements of it. My understanding of the doctrine is, that there are five chief powers or forces in Nature: one _mechanical_, or _molar_, the momentum of moving matter; the others _molecular_, or embodied in the molecules, also supposed in motion--these are, heat, light, chemical force, electricity. To these powers, which are unquestionable and distinct, it is usual to add vital force, of which, however, it is difficult to speak as a whole; but one member of our vital energies, the nerve-force, allied to electricity, fully deserves to rank in the correlation. Taking the one mechanical force, and those three of the molecular named heat, chemical force, electricity, there has now been established a definite rate of commutation, or exchange, when any one passes into any other. The mechanical equivalent of heat, the 772 foot-pounds of Joule, expresses the rate of exchange between mechanical momentum and heat: the equivalent or exchange of heat and chemical force is given (through the researches of Andrews and others) in the figures expressing the heat of combinations; for example, one pound of carbon burnt evolves heat enough to raise 8,080 pounds of water one degree, C. The combination of these to equivalents would show that the consumption of half a pound of carbon would raise a man of average weight to the highest summit of the Himalayas. It is an essential part of the doctrine, that force is never absolutely created, and never absolutely destroyed, but merely transmuted in form or manifestation. As applied to living bodies, the following are the usual positions. In the growth of plants, the forces of the solar ray--heat and light--are expended in decomposing (or deoxidizing) carbonic acid and water, and in building up the living tissues from the liberated carbon and the other elements; all which force is given up when these tissues are consumed, either as fuel in ordinary combustion, or as food in animal combustion. It is this animal combustion of the matter of plants, and of animals (fed on plants)--namely, the reoxidation of carbon, hydrogen, etc.--that yields all the manifestations of power in the animal frame. And, in particular, it maintains (1) a certain warmth or temperature of the whole mass, against the cooling power of surrounding space; it maintains (2) mechanical energy, as muscular power; and it maintains (3) nervous power, or a certain flow of the influence circulating through the nerves, which circulation of influence, besides reacting on the other animal processes--muscular, glandular, etc.--has for its distinguishing concomitant the MIND.