THE CORROSION OF IRON AND OTHER METALS 643 



desirable to reopen the discussion from this point of view and 

 to consider the cogency of the arguments advanced, especially 

 as Moody's contention is not accepted by Messrs. Cushman and 

 Gardner in their recent comprehensive book on the subject of 

 the corrosion of metals, a work which will doubtless be re- 

 garded by engineers and builders as authoritative on account 

 of the position one of its authors occupies in the United States 

 Department of Agriculture and the attention he appears to have 

 given to the subject. 



The historian of the future considering the growth of scien- 

 tific knowledge during the latter half of the nineteenth century 

 will be sorely puzzled probably to account for the slowness with 

 which chemists arrived at clear conceptions of the nature of 

 chemical change and of the conditions that determine it. Perhaps 

 he will be forced to conclude that it is not within the compass 

 of average human nature to be scientific — that dogma must 

 necessarily be preferred above science. It is certainly strange 

 that a problem of such importance as that afforded by the rusting 

 of iron should be a subject on which opinions can differ to the 

 extent apparent in recent discussions. 



If the conditions that determine the attack of metals are to 

 be appreciated, there are certain fundamental facts upon which, 

 in the first instance, attention should be centred. 



When common Spelter — the crude Zinc of commerce— is 

 placed in dilute sulphuric or muriatic acid, it is at once attacked 

 and dissolved. In most text-books the interaction is represented, 

 without further remark, by the expression : 



Zn + H,S04 = H2 + ZnSO,. 



The student is allowed to believe that the metal actually dis- 

 places hydrogen from the acid ; usually no account is taken of 

 the part played by the impurities in the zinc. But as zinc is 

 freed from the impurities which are associated with it in spelter 

 — arsenic, lead, graphite, etc. — it becomes less and less readily 

 attackable by acids and ultimately all but insoluble. All who 

 have worked with carefully purified material agree that this is 

 the case : for example, Lieut.-Colonel Reynolds and Professor 

 Ramsay {Cliem. Soc. Trans. 1887, 51, 857) in speaking of a speci- 

 men of the metal which they had separated from most carefully 

 purified zinc sulphate by electrolysis and then volatilised in an 

 exhausted tube of hard glass, say : "The sublimed metal appeared 



