The rarest metals can now be separated from their oxides with perfect ease, and new compounds, obtainable in no other way, the furnace has placed at our disposal. This field of research is now barely opened; from it the twentieth century should gather a rich harvest.
With electricity also chemistry is nearly allied, and along some lines the two branches of science have been curiously intertwined. Like the other physical forces, electricity may either provoke or undo combination, and, like heat, it may itself be generated as a product of chemical action. The voltaic pile and the galvanic battery owe their currents to chemical change, and it is only since the middle of the century that any other source of electric energy has become available for practical purposes. It is not surprising, therefore, that many thinkers should have sought to identify chemical and electric force, the two have so much in common. It was with the galvanic current that Davy decomposed the alkalies, and since his day other electro-chemical decompositions have been studied in great number, to the development of important industries. To the action of the current upon metallic solutions we owe the electrotype and all our processes for electroplating, and these represent only the beginnings of usefulness. Even now, almost daily, advances are being made in the practical applications of electrolysis, and the forward movement is likely to continue throughout the coming century. From the curiously reversible chemical reactions of the secondary battery the automobile derives its power, and here again we find a field for invention so large that its limits are beyond our sight. From every peak that science can scale new ranges come into view. The solution of one problem always creates another, and this fact gives to scientific investigation its chief interest. We gain, only to see that more gain is possible; the opportunity for advance is infinite. Forever and ever thought can reach out into the unknown, and never need to weep because there are no more worlds to conquer.
It was the study of electro-chemical changes which led Berzelius to his electro-chemical theory of combination, and then to the dualistic theory, which has already been mentioned. In or about the year 1832, when the Berzelian doctrines were at the summit of their fame, Faraday showed that the chemical power of a current was directly proportioned to the quantity of electricity which passed, and this led him to believe that chemical affinity and electric energy were identical. Electrolysis, the electrical decomposition of compounds in solution, was a special object of his attention, and by quantitative methods he found that the changes produced could be stated in terms of chemical equivalents or combin-
