ACTION OF WATER ON IRON. 27& 



inside with Roman cement. Tills mode would no doubt for a 

 time diminish corrosive action, but it is much to be feared that 

 it could have but little permanence when the current was rapid ; 

 and, should the water contain much earthy matter, the tendency 

 of this to deposit and adhere to the pipes must be fatally 

 increased. 



53. The Academy appends a note to M. Vicat's communi- 

 cation, in which an opinion is expressed, that the tubercles of 

 Grenoble have attained their lai'gest size, and are stationary ; 

 and it is questioned, Will they always remain so ? It must be 

 obvious, indeed, that the rate of increment of these must be a 

 decreasing one ; but I do not perceive anything to set a limit to 

 their accretion, except the stoppage of corrosive action. 



54. In the same volume of the Comptes Rendus, p. 462, a 

 letter is published, from M. Prunelle, stating a case in which 

 tubercles had formed in conduit pipes where the water passing 

 was found not to contain a trace of iron. He did not chemically 

 examine the tubercles, which were friable, and as large as eggs. 

 From this it would seem to be the author's view that these 

 masses originate from iron contained in the water ; that this, 

 however, is not the nature of their formation, has been already 

 shown, and is evidenced by the fact, that no tubercles are found 

 in any of the pipes conveying the waters of Grenoble except 

 those made of iron. — (Payen, Ann de Chim. vol. Ixiii. p. 409.) 



55. The explanation of the phenomena of tubercular cor- 

 rosion given by M. Payen and the reporters to the Academy, 

 seems to lose clearness in proving too much. Mere want of 

 homogeneity of structure or of surface is alone sufficient ground 

 to explain the results ; and that the peculiar preservative action 

 of alkaline solutions is not a necessary adjunct, I have lately 

 had an opportunity of proving. In experimenting on the action 

 of very dilute hydrochloric acid, on wrought and cast iron par- 

 tially coated with zinc, I found that, after a time, local con- 

 cretions or tubercles were formed at the points of contact of the 

 zinc and iron ; thus the effect is produced indifferently in acid 

 and in alkaline solutions. And I have since found tubercles 

 formed in iron pipes at Curraghmore, the seat of the Marquis of 

 Waterford, by water, which appears free from alkaline or earthy 

 matter. The peculiar effect, too, is confined to chilled or un- 

 equally-cooled cast iron, to mottled cast iron, and to damasked 

 wrought iron, or that of mixed constitution, and in all appears 

 to result from heterogeneity of composition ; it is therefore un- 

 necessary to call in to the aid of the explanation the distinct 

 and curious phenomena of the preservative action of alkaline 

 solutions. 



