272 KIGHTH REPORT 1838. 



stalline form. We see here, also, the close analogy to the pheno- 

 mena and properties of unannealed glass, that is, of silicates 

 which have been compelled suddenly to pass from the semi-fluid 

 state to that of solidity, without passing through the pasty con- 

 dition. In these the crystalline arrangement is distinctly pro- 

 duced, as Sir David Brewster has determined by the optical 

 examination of Prince Rupert's Drops, and yet these, by being 

 annealed, are brought back to the state of ordinary glass. 



48. Hence, then, the superior hardness and greater specific 

 gravity of chilled cast iron. But while it accrues, from the 

 chilling of cast iron, that it corrodes less rapidly, a singular 

 and in some cases disadvantageous circumstance occurs in the 

 manner of its corrosion, which needs further consideration, as it 

 may sometimes happen to be of much greater practical im- 

 portance than any amount of mere decay of substance. 



49. When a piece of cast iron moulded in sand is exposed to 

 corrosion, this takes place with somewhat variable, but yet with 

 very considerable uiiiformity over its entire exposed surface. 

 Not so, however, with chilled castings ; in these, each nucleus 

 of exudation of its uncombined carbon forms with the iron in its 

 immediate vicinity a voltaic couple ; and its results are, that, 

 in place of the uniform action as before, the largest portion of 

 the surface remains unchanged, and corrosion is nearly wholly 

 confined to these spots, as so many local centres of action. The 

 oxides of iron are formed as usual ; but, from the texture of the 

 casting, and its constitutional carbon being all in a state of 

 combination, little or no carbonaceous or plumbaginous matter 

 is produced ; hence, as the spots of carbon form, with the rest 

 of the casting, so many voltaic couples, the oxides formed are 

 rapidly transferred to these points, and gradually produce large 

 tubercular concretions, which, M. Payen states, alu'ays consist 

 of (F e O) + (F e^ O3 + F e O) + (F Ca O3), and which in course of 

 time contain crystals of the octohedral iron ore thus artificially 

 produced. 



50. M. Payen has presented several memoirs to the Academy 

 of Sciences on the sulDJect of these local actions on iron, and has 

 applied to their explanation the facts he had previously disco- 

 vered, as to the effects of alkaline and saline solutions in retard- 

 ing or accelerating the corrosive action of water on iron. The 

 principal facts he has established are the following, as given by 

 the Commission of the Institute, in reporting on his memoir. — 

 {Coj)iptes Rendus, Feb. 1837, No. VI.) 



" He has found solutions, containing an alkali and sea salt in 

 such proportions, that, in place of being at all preserved, iron 

 placed therein was rapidly attacked. 



