THE CORROSION OF IRON AND OTHER METALS 649 



appeared on the bright metal surfaces in five minutes or less 

 and in one hour was deep and heavy. 



There is no reason to dispute the conclusion arrived at by 

 Cushman and Gardner that whilst some iron is dissolved no 

 rusting takes place under conditions such as were realised 

 in their experiments until oxygen was admitted : it may well 

 be that boiling the water in the manner described would have 

 the effect of so nearly removing the whole of the oxygen 

 dissolved in the water that there should be no obvious rusting 

 under such conditions. It is known, however, that the complete 

 displacement of carbon dioxide from water is a very difficult 

 task, if not an impossibility ; it cannot be admitted that the 

 treatment described would be effective in removing it entirely. 



Cushman and Gardner themselves admit that it may be 

 doubted whether it is possible to boil out all carbonic acid 

 from the water contained in the apparatus described. They 

 contend, however, that "granting this is the case in regard to 

 the last traces, it is easily shown that the hydrogen ions which 

 would be supplied by a minute quantity of carbonic acid are 

 of no more importance than the hydrogen ions supplied by the 

 normal dissociation of pure water and that the assumption that 

 carbonic acid must be present is quite unnecessary." 



They then call attention to Whitney's argument that the 

 concentration of " hydrogen ions " in a solution of carbon 

 dioxide in equilibrium with ordinary air at 15° is 16 grams 

 per 10,000,000 litres of water — " or only sixteen times as many 

 as perfectly pure water contains ; and that, at the boiling 

 temperature, the carbon dioxide dissolved would probably 

 yield a concentration of hydrogen ions even less than in pure 

 water, for not only is the solubility of the gas greatly 

 diminished but the dissociation of water is greatly increased 

 by rise of temperature." 



We are here brought into the region of pure speculation, 

 not to say obfuscation ; a definite issue is raised, however, that 

 can be discussed. 



Nothing could be more complete than the following state- 

 ment by Dr. Cushman : 



" It appears to the writer to be demonstrated that Whitney 

 was right in his assertion that iron goes into solution up to a 

 certain maximum concentration in pure water without the aid 

 of oxygen, carbonic acid or other reacting substances, 



