Previous to the explosion this gas was a mixture of molecules of hydrogen and oxygen (2H↓{2} + 2O) at the ordinary temperature, but after the explosion it consists of a smaller number of molecules at a very much higher temperature. What is “energy of position”? The golf ball at the bottom of the hill was at a distance of _R_ feet from the centre of the earth, but at the top of the hill it is at a distance of _R_ + 100 feet from the centre of the earth. In the first case it was free to fall _R_ feet, but in the second case it is free to fall _R_ + 100 feet. The atoms of the constituent molecules of water occupy the position H - O - H, the bonds (-) indicating that the atoms are very close together; but when the water is decomposed by an electric current, the atoms occupy the positions O - O + H - H + H - H, the (+) indicating that the atoms are relatively far apart from each other. Now the golf ball and the earth, or the atoms of hydrogen and oxygen, are physically the same material entities, whether they are close together or far apart, yet when the earth and the ball, or the atoms of oxygen and hydrogen, are separated from each other, their “properties” are different from what they are when they are close together. What is it that makes the difference? It is that which is _between_ them. Is it, in the last case, “the potential energy of chemical affinity”? This dreadful phrase is actually used in a recent book on biology: “In the elements carbon and oxygen, so long as they remain separate, a certain amount of energy remains latent. When the carbon and oxygen atoms are allowed to come together and unite, this potential energy of chemical affinity is liberated as kinetic energy.” What is changed by the tractation and pellation (the terms suggested by Soddy in place of the anthropomorphic ones, “attraction” and “repulsion”)? It is the ether which has become changed in some way. Potential energy resides therefore in the ether of space. ISOTHERMAL AND ADIABATIC CHANGES