The latest inventions of electrical science—the dynamo and the accumulator—afford remarkable instances of this convertibility of one primitive energy into different forms. In the instance just quoted of obtaining light from electricity by the voltaic battery, the cost has hitherto proved an obstacle to its adoption. The electrical energy is all obtained from the transformation of the heat produced in the cells by the chemical action on the metal used, which is commonly zinc. Now, the heat of combination of zinc with oxygen is only about one-sixth of that of coal, while the cost of zinc is about twenty times as great. Theoretically, therefore, energy got by burning zinc costs 120 times as much as that got by burning coal. Practically the difference is not nearly so great, for there is very little loss of energy in the battery by the process of conversion, while the best steam-engine cannot convert into work as much as twenty per cent, of the heat energy in the coal consumed. Still, after making every allowance, the cost of energy from zinc remains some twenty times as great as from coal, so that unless some process is found for obtaining back the zinc as a residual product, there is no prospect of this form of electricity being generally available for light or for mechanical power. The dynamo is an instrument invented for the mechanical generation of electricity by taking advantage of the principle that electrical energy is produced by moving magnets near coils of wire, or coils of wire near magnets. A current is thus started by induction, and, once started, the mechanical power exerted in making the magnet or coils revolve is continually converted into electricity until the accumulated electrical energy becomes very powerful. The original energy comes of course from the coal burned in the steam-engine which makes the magnet or coils revolve.