Polytechnic Association Proceedings. 359 



of a ceutury or more. Those which do show injury or decomposi- 

 tion Avithiu a less period, are where iron of an unsuitable quality 

 has been used. " The engineer has therefore only to select a proper 

 quality of metal to insure its durability. With gray iron, having 

 a good surface, no apprehensions of its decay in salt water need 

 be apprehended, and he (Mr. Simpson) was about to use it exten- 

 sively for piles in submarine works." 



Mr. Webb said that from his examination of the question, and 

 of worlvs executed, that strong iron, of proper quality, exposed to 

 the action of salt water, would last an indefinite time. 



The discussion of this subject at the London Institution, and the 

 cuses named, are therefore conclusive on the question, and demon- 

 strate that where corrosion or change of structure does take place, 

 it is due either to the use of an improper quality of iron or in 

 placing it in contact with other metals, so as to produce a galvanic 

 action. 



Cast-iron, to resist corrosion, should be as hard as the case will 

 admit, of an even, close grain, and with the carbon combined and 

 not in the form of graphite. Impure, soft foundery iron, will cor- 

 rode rapidly ; close-grained gray iron, of an even texture, will 

 endure in the most exposed places, more than a quarter of a cen- 

 tury without sensible injury; and white iron of good quality may, 

 for all practical purposes, be considered as imperishable in such 

 situations. 



It is well known to the profession in this counti-y, that many 

 cast-iron water mains have corroded greatly, not only externally, 

 when they are exposed to the alternations of moist and dry eaith 

 and to the mephitic gases from sewers and leaky gas-pipes, but 

 also internally, when they are only exposed to the action of the 

 most pure water, and a small amount of air in combination. In 

 the latter case, the oxydation sometimes takes the tubucular form, 

 where the rust is collected and aoforreofated in hard knobs. 



In the early practice in this country, water-pipes were often 

 made from the worst quality of metal — it could hardly be called 

 iron — but was a composition of iron cinders, burnt sand, and scoria. 

 The iron of this compound was the softest, most open-grained and 

 in large crj^stals; indeed precisel}^ such a, mixture as would offer 

 the least resistance to corrosion or disintegration. 



It is not at all surprising that such iron should corrode rapidly 

 and fearfully, and these cases are often improper!}' r' ferred to, to 

 confirm the assertion that cast-iron rapidly corrodes. 



