There is nothing new about this. It is not even a novel statement of a fundamental electro-chemical truth. In times past, however, we were wont to consider this matter as pertaining solely to the laboratory or to the electroplating industry; now we are forced to see that the reproduction of this experiment on a grand scale is attended with results as disagreeable as they are widespread. Hidden beneath our highways lie gas pipes, water pipes, railway tracks, Edison tubes, cement-lined iron subway ducts, and lead-covered cables. These are the electrodes. In contact with these conductors is the soil, containing an electrolyzable salt--chloride, nitrate or sulphate of ammonia, potash, soda, or magnesia, generally. In the presence of moisture this soil becomes an electrolyte, or salt solution. In the absence of electricity no appreciable damage occurs; but the passage of an electric current, no matter how small, from one pipe to another is sure, sooner or later, to leave its traces upon the positive conductor in the form of a decay other than mere oxidation. It is to this decay that has been given the name of _electrolysis_; so that when this heading appears in the daily press or in technical journals one may interpret the term popularly as “the electrolytic corrosion of metals buried in the soil.” [Illustration: COPPER DRIP PIPE AFTER SEVENTEEN DAYS’ EXPOSURE IN SALT WATER TO THE ACTION OF ELECTRICITY. Half size.] To produce electrolytic disintegration of pipes, etc., on a scale grand enough to cause apprehension, a bountiful source of electricity is essential. Unfortunately, this condition is not lacking to-day in any town in which the usual overhead trolley electric railway is in operation. This system of electric propulsion is based upon the use of a “ground return”--that is to say, the electricity passes out from the power house to the bare trolley wire, thence to the pole on the roof of the car, thence through the motors to the wheels, whence it is expected to return to the power house, _via_ the rails. As a matter of fact, however, the released electricity by no means confines itself to the rails and the copper return feeders--legitimate paths provided for it. It avails itself, on the other hand, of what may be termed, for brevity’s sake, the illegitimate return--comprising all underground electrical conductors except the rails and return feeders, and including subterranean water-courses, sewers, and metallic earth veins. [Illustration: WROUGHT-IRON SERVICE PIPE FOR WATER AFTER ONE YEAR’S BURIAL BENEATH A TROLLEY TRACK. The fibrous appearance of the surface is characteristic of wrought iron and steel.]