360 POPULAR SCIENCE MONTHLY. 



hard, dense, even-grained cast iron, containing alloyed rather than 

 combined carbon, have not been appreciably corroded. At Day- 

 ton, Ohio, on the other hand, seventy-seven thousand dollars' worth 

 of damage has already resulted. One peculiarity of electrolyzed 



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Lead Service Fii'e aftkk Eight Months' Bi i:iai. in Builders' Sand. Thu collapsed ap- 

 pearance of the pipe is due entirely to the iciuoval of the lead by electrolysis, the bore 

 retainini^ its oriszinal shape. Tlie dark spot 011 the upper surface of the pipe is the point 

 of rupture. One third size. 



cast iron is that the original shape is usually retained, the iron 

 being eaten away and leaving a punky formation of pure or nearly 

 pure graphite. In such a case a superficial examination detects 

 nothing wrong, and it requires a mechanical scraping to show that 

 the strength is not there. For this reason good photographs of 

 cast-iron electrolysis are somewhat hard to obtain. 



The reason for the comparative immunity of cast iron is not 

 as yet definitely understood. It certainly does not lie particularly 

 in the asphaltic varnish usually applied, for this varnish affords 

 little or no protection when used upon wrought iron or other metals. 

 Xor can it be accounted for by the composition of cast iron itself, 

 inasmuch as a fractured or brightly scraped surface of cast iron 

 shows approximately the same symptoms as other metals when 

 acted upon by a given current for a given time. Whether the iron 

 oxide is the saving feature, or whether the " skin " due to the 

 process of casting acts as an insulator, is not yet settled. 



When the trouble first appeared in Boston, in 1891, its cause 

 was promptly identified. The electric-railway construction of those 

 days was so crude, however, that many well-informed electricians 

 fell into the error of assuming that heavier rails, more and larger 

 return feeders, and better bonding (i. e., wire connections from rail 

 to rail, around the joints, designed to decrease the resistance) would 

 prove a panacea for all electrolytic ills. Indeed, this view is still 

 held by a surprisingly large numl^er of men versed in matters 

 electrical. 



I am of the opiiiiun that it is imijossible, from a financial stand- 

 point, to provide so satisfactory a legitimate return that consider- 

 able electricity will not seek a path through pipes, cable covers, 

 etc.; for, in order to confine the electric current to the rails, the 

 resistance of the earth and its contained pipes would have to be 

 infinitely great, and this condition can be realized only by making 

 the resistance of the rail infinitely small as compared with that of 



