Molecular energies, on the other hand, act with vastly greater force, but at very small distances, and appear sometimes as attractive and sometimes as repulsive forces. Thus solid bodies are held together by a force of cohesion which is very powerful, but acts only at very small distances, as we may see if we break a piece of glass and try to mend it by pressing the broken edges together. We cannot bring them near enough to bring the molecular attraction again into play and make the broken glass solid. But the same glass acts with repellent energy if another solid tries to penetrate it, so that we can walk on a glass floor without sinking into it. Heat also, by increasing the distance between the molecules, first weakens the cohesive force so that the solid becomes fluid, and finally overcomes it altogether, so that it passes into the state of gas in which the centripetal attraction of the molecules is extinguished, and they tend to recede further and further from each other under the centrifugal force of their own proper velocities. The great energy of molecular forces will be apparent from the fact that a bar of iron, in cooling 10° Centigrade, contracts with a force equal to a ton for each square inch of section, as exemplified in the tubular bridge across the Menai Straits, where space has to be allowed for the free contraction and expansion of the iron under changes of temperature. Chemical energy, or the mutual attractions and repulsions of atoms, is even more powerful than that of molecules. It displays itself in their elective affinities, or what may be called the likes and dislikes, or loves and hatreds, of these ultimate particles. Perhaps the best illustration will be afforded by that ‘latest resource of civilisation,’ dynamite. This substance, or to give it its scientific name, nitro-glycerine, is composed of molecules each of which is a complex combination of nine atoms of oxygen, five of hydrogen, three of nitrogen, and three of carbon. Of these, oxygen and hydrogen have a strong affinity for one another, as is seen by their rushing together whenever they get the chance, and by their union forming the very stable compound, water. Oxygen and carbon have also a very strong affinity, and readily form the stable product carbonic dioxide gas. Nitrogen, on the other hand, is a very inert substance; its molecule consists of two atoms of itself which are bound together by a strong affinity, and can only be coaxed with difficulty into combinations with other elements, forming compounds which are, as it were, artificial structures, and very unstable. We see this in the air, which consists mainly of oxygen and nitrogen, but not in chemical combination, the oxygen being simply diluted by the nitrogen, as whisky is with water, with the same object of diluting the too powerful oxygen or too potent alcohol, and enabling the air-breather or whisky-drinker to take them into the system without burning up the tissues too rapidly.