and is producing immense changes. And in the future it is destined still more profoundly to alter our mechanical ideas and activities: the great revolution there is only just beginning; another half-century is yet needed fully to develop it.
These two great principles—evolution and the conservation of energy—form the main bulk of our age's addition to the world's accumulated stock of knowledge. But among the separate sciences many wonderful advances have also been made which can not be overlooked in the briefest retrospect of the half-century's gains. To these a few words must next be devoted.
Among sciences of the abstract-concrete group, electricity had hardly got beyond the stage of an elegant amusement at the opening of our epoch. Statical electricity was still the department about which most was known. Galvanism as yet stood apart as a distinct study. Its connection with magnetism had not long been proved by the discoveries of Oersted. In 1837 itself, however, Wheatstone constructed the first telegraph. From that moment, under the fostering care of Faraday, Daniell, Cooke, Morse, Arago, Tyndall, Edison, and Thomson, electric science became a power in the world. The whole theory of electricity as a mode of energy has since been fully explored and expounded. A vast field has been added to science. Units and modes of absolute measurement have been invented. The telephone and microphone have been introduced; secondary batteries have been formed and improved; the dynamo has become a common object of the country; and the electric light has grown under our very eyes into a practical and extremely dazzling reality. Electricity, as we know it, with all its manifold useful applications, is almost entirely a creation of the last half-century.
In physics the present epoch, though chiefly remarkable for the series of investigations which led up to the discovery of the law of conservation, has also illustrated many minor principles of the first importance. The true theory of heat and the laws of radiant energy have been definitely formulated. The undulatory theory of light—a discovery of the previous quarter-century—has been universally adopted and justified. Thermo-dynamics have been elevated into a great and increasing branch of science. Sir William Thomson's law of dissipation of energy has completed and rounded off the theory of conservation. The causes and methods of glacier-motion have been investigated and established. Photography has almost passed through its entire life-cycle. The polarization of light has been observed and studied. Spectrum analysis has come into the front rank as an instrument of research. In short, a greater number of new physical phenomena have been discovered or old ones interpreted than in the whole space of previous time put together.
In chemistry, the advance has been more in detail than elsewhere. Chemical science alone still remains a somewhat fragmentary mass
