If any one to-day should still wish to think of heat as a substance, we might allow that person this liberty with little ado. He would only have to assume that that which we call quantity of heat was the energy of a substance whose quantity remained unaltered, but whose energy changed. In point of fact we might much better say, in analogy with the other terms of physics, energy of heat, instead of quantity of heat. When we wonder, therefore, at the discovery that heat is motion, we wonder at something that was never discovered. It is perfectly indifferent and possesses not the slightest scientific value, whether we think of heat as a substance or not. The fact is, heat behaves in some connexions like a substance, in others not. Heat is latent in steam as oxygen is latent in water. V. THE CONFORMITY IN THE DEPORTMENT OF THE ENERGIES. The foregoing reflexions will gain in lucidity from a consideration of the conformity which obtains in the behavior of all energies, a point to which I called attention long ago.[56] A weight _P_ at a height _H₁_ represents an energy _W₁ = PH₁_. If we suffer the weight to sink to a lower height _H₂_, during which work is done, and the work done is employed in the production of living force, heat, or an electric charge, in short, is transformed, then the energy _W₂ = PH₂_ is still _left_. The equation subsists _W₁/H₁ = W₂/H₂_, (2) or, denoting the _transformed_ energy by _W' = W₁-W₂_ and the _transferred_ energy, that transported to the lower level, by _W = W₂_, _W'/(W' + W) = (H₁-H₂)/H₁_, (3) an equation in all respects analogous to equation (1) at page 165. The property in question, therefore, is by no means peculiar to heat. Equation (2) gives the relation between the energy taken from the higher level and that deposited on the lower level (the energy left behind); it says that these _energies_ are proportional to the _heights of the levels_. An equation analogous to equation (2) may be set up for _every_ form of energy; hence the equation which corresponds to equation (3), and so to equation (1), may be regarded as valid for every form. For electricity, for example, _H₁_, _H₂_ signify the potentials. When we observe for the first time the agreement here indicated in the transformative law of the energies, it appears surprising and unexpected, for we do not perceive at once its reason. But to him who pursues the comparative historical method that reason will not long remain a secret.