And yet inventors have not learned the meaning of this law. They continue trying to invent perpetual-motion machines--machines that will produce work from nothing. This is what a perpetual-motion machine would be if such a machine were possible. For a machine without friction is impossible, and friction means wasted work--work changed into heat. A machine to keep itself running and supply the work wasted in friction must produce work from nothing. The great law of nature is that you cannot get something for nothing. Whether you get work, heat, electricity, or light, something must be used up to produce it. For whatever you get out of a machine you must give an equivalent. This law cannot be evaded, and from it there is no appeal. Chapter V GREAT INVENTIONS OF THE NINETEENTH CENTURY The discoveries of Faraday prepared the way for the great inventions of the nineteenth century. By the middle of the century men knew how to control the wonderful power of electricity. They did not know what electricity is, nor do we know to-day, though we have made some progress in that direction; but to control it and make it furnish light, heat, and power was more important. Before the inventions of James Watt made it possible to use steam-power, factories were built near falling water, so that water-power could be used. But the steam-engine made it possible to build great factories wherever a supply of water for the boilers could be obtained. Cities were built around the factories. Cities already great became greater. With the growth of cities the need of a new means of producing light and power made itself felt. Electricity promised to become the Hercules that should perform the tasks of the modern world. Discovery gave way to invention. During the second half of the nineteenth century many great inventions were made and industries were developed, while discoveries were few until near the close of the century. Within this period the great industries which characterize our modern civilization, and which arose out of the discoveries that science had made in the centuries preceding, attained a high degree of development. In this chapter we shall trace the applications of some of the discoveries with which we have now become familiar. This will lead us into the field of electrical invention, for we are dealing now with the beginning of the world's electrical age. Electric Batteries