It has long been known that heat and light are closely connected together. The accumulation of a certain amount of heat is always accompanied by the appearance of light. But when it was found that the light could be separated from the heat by various means, it seemed possible that the two phenomena were simply associated. It is now, however, ascertained that light and heat are identical in their nature, and that a vast number of other phenomena-- electricity, galvanism, magnetism, chemical action, and gravitation, as well as light and heat, are different manifestations of one and the same thing, which is called force or energy. In a great number of cases it is possible for us, by the use of appropriate means and apparatus, to transform these manifestations, so as to make the same force assume a variety of forms. Thus motion suddenly arrested becomes heat. A rifle-ball when it strikes the target becomes very hot. The heat produced by the concussion against an iron shield is found sufficient to ignite the powder in some of the newly invented projectiles. The best illustration, however, is to be obtained from galvanism. By means of the Voltaic battery we set free a certain amount of force, and we can employ it at pleasure to produce an intense light in the electric lamp, or to melt metals which resist the greatest heat of our furnaces; it will convert a bar of iron into a magnet, or decompose water into its constituents, oxygen and hydrogen, or separate a metal from its combination with oxygen. But in all these processes no new force is produced--the force set free is unchangeable in itself, and we cannot increase its amount. Owing to the imperfection of our instruments and our skill a part of it will always escape from our control, and be lost to us, but not destroyed. When, however, due allowance is made for this loss, the results produced are always in exact proportion to the amount of force originally set free. Thus, if we employ it to decompose water, the amount of water decomposed always bears an exact proportion to the amount of metal which has been oxidized in the cells of the battery.