358 POPULAR SCIENCE MONTHLY. 



is thus made subject to a double decay — a corrosion by oxygen and 

 a solution by acid. 



There is nothing new about this. It is not even a novel state- 

 ment of a fundamental electro-chemical truth. In times past, how- 

 ever, 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, rail- 

 way 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, gener- 

 ally. 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 



CoppKK Djiii" Pipe aftek Seventeen Days' Exposlre in Salt Watek to the Action 

 OF Electkicity. Hall' size. 



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." 



To produce electrolytic disintegration of pipes, etc., on a scale 

 grand enough to cause apprehension, a bountiful source of elec- 

 tricity 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 elec- 

 tricity 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 imderground electrical conductors except 



