the middle of the nineteenth century, when it gave birth to the principle of conservation of energy, a generalization which grew immediately and inevitably out of the mechanical theory of heat as it lay in the minds of Rumford, Joule and their co-workers. Between 1860 and 1890 the proofs of the kinetic hypothesis came in so rapidly through the brilliant work of such masters as Joule, Clausius, Maxwell, Kelvin and Boltzmann that the scientific world began to be convinced, and only here and there was found a man of standing among the scoffers. Then, about 1887, a reaction set in, and the school of energetics arose in Germany, which attempted to force the principle of conservation of energy to devour its own mother. The most spectacular of the onslaughts was made by Ostwald, who in 1895 wrote a widely circulated essay, entitled "The Demolition of Scientific Materialism," elsewhere printed under the title, "The Route of Modern Atomism." Led by such a bell-wether, the sheep began to jump back over the wall, and the results of that backward movement are still felt in the United States, particularly in high-school texts, despite the fact that to-day the opposition among scientific men to the kinetic hypothesis is absolutely gone, and even Ostwald has admitted his error. Indeed, so direct and so convincing is now the evidence that it is not too much to say that any one who wishes can now have immediate ocular demonstration of the perpetual dance of the molecules of matter.
But since we are here as much concerned with the atomic or granular theory of electricity as with the kinetic theory of matter, let us turn for a moment to consider the present status of our knowledge as to the nature of electricity. Unlike the kinetic theory of matter, the granular theory of electricity can, boast of no great antiquity. Indeed, in its present form it is but ten or fifteen years old, and in no form is it more than one or two hundred years old. For there are no electrical theories of any kind which go back of our own Benjamin Franklin. It is true that the Greeks discovered that rubbed amber had the power of attracting to itself light objects placed in its neighborhood, but this is all until A.D. 1600, when Queen Elizabeth's surgeon, Gilbert, found that a glass rod and some twenty other bodies, when rubbed with silk, acted like the rubbed amber of the Greeks, and he consequently decided to describe the phenomenon by saying that the glass rod had become electrified (amberized, electron being the Greek word for amber) or had acquired a charge of electricity. In 1733, Dufay, a French physicist, further found that sealing wax, when rubbed with cat's fur, was also electrified, but that it differed from the electrified glass rod, in that it strongly attracted any electrified body which was repelled by the glass, while it repelled any electrified body which was attracted by the glass. About 1847, Benjamin Franklin adopted the following purely arbitrary convention, and said. We will consider that there are