restored if all the couples in a direct line between the two walls change partners and thus absorb the two partnerless persons again into the general dancing throng. Our observer on the gallery would then not notice any difference in the assembly; all remain properly paired.
The electrolysis of water is commercially utilised for the production of pure oxygen and hydrogen. The electrodes are of iron, and the electrolyte is a 16% soda solution. The cost of the process, including cost of power, labour, interest and depreciation, varies from 9d. to 1s. 2d. per one cubic meter oxygen + two cubic meters hydrogen, according to the magnitude of the plant and local conditions.
Electrolysis is not confined to liquids; it can also be produced in a solid, provided it is a conductor. Thus, if a current is sent through a lump of caustic potash, it is decomposed into oxygen and the metal potassium. Let, in Fig. 7, P be a platinum plate, C a lump of caustic potash, and M a globule of mercury placed into a cavity hollowed out of the solid electrolyte; then, on the passage of a current in the direction shown by the arrows, oxygen will collect on the surface of the platinum, and
