582 MAN IS MASTERING HIS MATERIAL WORLD in cases where at least one of the molecules has been activated. Ions may be regarded as activated molecules which combine immediately upon contact. As we have pointed out above, the conditions determine whether these ions will remain combined or separate again at once. The activation of covalent molecules is thought to consist of the addition of an unusual amount of energy to the molecule. This may be accomplished by collision with photons of radiant energy or with swiftly moving electrons, atoms, molecules, or ions. The energy thus added produces an increase in kinetic energy, a displacement of electrons from relatively stable orbits to less stable orbits, an increased energy of vibration of the atoms within the molecules, or an increased energy of rotation of the whole molecules. This added energy may be emitted in the form of radiant energy as fluorescence or phosphorescence as the electrons return to their more stable orbits. In other cases this energy of activation is removed by collision with other particles, thus increasing their kinetic energy; this represents an increase in temperature. The energy represented by the vibration of the atoms may be transmitted from atom to atom within the molecule until it reaches a weak or ruptured bond, when the molecule is either decomposed or internally rearranged. Sometimes activated molecules lose their extra energy by direct union with other molecules; in such cases energy is released. This released energy then activates other molecules, producing what is known as a chain reaction. In case the amount of energy released is greater than that required for activation, the reaction will be exothermic. On the other hand, the amount of energy evolved may be less than that required for activation, and the reaction can be made to continue only by adding sufficient energy to activate the reacting molecules. Such reactions are endothermic. In the explosion of an explosive mixture of gases, an electric spark furnishes all of the energy necessary to activate a few molecules. These activated molecules react to produce sufficient energy to activate many more molecules, and so the reaction proceeds. Explosions are typical chain reactions. In the starting of a fire, the heat of a burning match is sufficient to activate a few molecules; these, in turn, activate many more molecules; soon a whole city or forest may burn up unless the reaction can be stopped. The usual method of stopping such reactions is by pouring on water, which uses up the energy evolved by the reactions as the water is heated and finally vaporized. On the other hand, it is necessary to keep on adding energy when one chars some sugar or bakes a cake because the changes in this case are endothermic.
THE NATURE OF MOLECULES 583 Photography Is an Application of the Activation of Atoms by Radiant Energy. The development of photography has been of inestimable value to Science and modern life. For example, the scientist has used photography to study the internal structure of the atoms, the nature of fardistant nebulae, flaws in metals, and in the diagnosis of disease by use of X rays. Photography has made possible our modern illustrated books, newspapers, and magazines, and the motion-picture industry. It is only necessary to compare silent motion pictures made twenty years ago with the modern sound motion pictures in color to realize the tremendous progress that has been made in photography during the past generation. It was known for a long time (/. H. Schulze, 1727) that light would blacken silver nitrate or silver chloride, but for practical purposes early photography had two great drawbacks: first, no means of fixing the images were known; second, time required for the exposures was quite long. Louis Jacques Daguerre, in 1839, made public the details of his daguerreotype process of sensitizing a silver plate with iodine and developing with mercury vapor. Fox Talbot (1835) found that if he treated paper with successive washings of a solution of common salt (sodium chloride) and a solution of silver nitrate and exposed the wet paper, he could obtain a much more rapid blackening in light than with either silver nitrate or silver chloride alone. Unfortunately the silver chloride left on the paper unchanged by the light will soon darken unless removed, and such a picture will soon become black all over when exposed to light. Later it was found that a silver-salt emulsion could be exposed to light without producing any visible change but that the portion of the salt thus activated, even after a period of thirty or more years, would be reduced by certain substances which develop this latent image and are therefore called developers; the portions of the emulsion not activated are not appreciably reduced during the short period of time required to reduce the activated portion, but continued exposure to the developer would eventually reduce nearly all of the silver salts in the emulsion. Fox Talbot discovered the use of a developer (gallic acid) and reported it to the Royal Society in 1841. The most common developers are pyrogallic acid, hydroquinone, and methyl-para-aminophenol sulfate, otherwise known as "Pictol," "Metol," or "Elon." It was found that larger crystalline particles were more readily activated than the smaller particles and that the sensitiveness to light could also be controlled by the use of the bromide and iodide of silver, as well as the chloride. Silver bromide is more sensitive to light than