If nitrogen had more affinity for oxygen it would combine chemically with it, and we should live in an atmosphere of nitrous oxide, or laughing gas. The molecule, therefore, of nitro-glycerine resembles a house of cards, so nicely balanced that it will just stand, but will fall to pieces at the slightest touch. When this is supplied by a slight percussion the molecule falls to pieces and is resolved into its constituent atoms, which rush together in accordance with their natural affinities, forming an immense volume of gas, partly of water in the form of steam where oxygen has combined with hydrogen, and partly of carbonic dioxide where it has combined with carbon, leaving the nitrogen atoms to pair off, and revert to their original form of two-atom molecules of nitrogen gas. It is as if ill-assorted couples, who had been united by matrimonial bonds tied by the manœuvres of Belgravian mothers, found themselves suddenly freed by a decree of divorce _a vinculo matrimonii_, and rushed impetuously into each other’s arms, according to the laws of their respective affinities. So striking is the similitude that one of Goethe’s best-known novels, the ‘Wahlverwandschaften,’ takes its title from the human play of these chemical reactions. The enormous energy developed when these atomic forces are let loose and a vast volume of gas almost instantaneously created, is attested by the destructive force by which the hardest rocks are shattered to pieces and the strongest buildings overthrown. These loves and hatreds, or, as they are termed, chemical affinities and repulsions of the atoms, are the principal means by which the material structure of the universe is built up from the original elements. The earth, or solid crust of the planet we inhabit, consists mainly of oxidised bases, and is due to the affinity of oxygen for silicon, calcium, aluminium, iron, and other primary elements of what are called metals. This affinity enables them to make stable compounds, which, under the existing conditions of temperature and otherwise, hold together and are not readily decomposed. Water in like manner, in all its forms of waves, seas, lakes, rivers, clouds, and invisible vapour, is due to the affinity between oxygen and hydrogen forming a stable compound. Salt again is owing to the affinity of chlorine for sodium, and so for nearly all the various products with which we are familiar, oxygen and nitrogen in the air we breathe being almost the only elements which exist in their primary and uncombined state in any considerable quantities, and form an essential part of the conditions which render our planet a habitable abode for man and other forms of life. We shall see presently something more of the nature of these affinities, and the laws by which they act; but before entering on this branch of the subject we must consider the remaining form in which the one indestructible energy of the universe manifests itself, viz. that of electricity.