318 SCIENCE PROGRESS 



dilute solution (one containing 1*3 grammes of soda per litre) the 

 iron rusted rapidly but not in stronger solutions. He accounts 

 for this result by assuming that in making, filling and sealing up 

 the apparatus he used it was impossible to exclude carbon 

 dioxide entirely and that therefore the soda solution contained 

 a certain amount of carbonate. He then calls attention to 

 Kiister and Gruter's observations showing that aqueous 

 solutions of sodium carbonate are more or less hydrolysed in 

 the sense of the equation 



Na,CO a + H 2 = CO, + 2NaOH, 



the partial pressure of the carbon dioxide in equilibrium with 

 a normal solution of sodium carbonate being 0*000072 atmo- 

 spheres at 90 C. Friend supposes that in the case of the very 

 dilute alkaline solutions he used sufficient carbon dioxide was 

 present in the atmosphere to condition rusting. 



When he used a saturated solution of caustic baryta in place 

 of a strong solution of caustic soda in the apparatus described 

 in the previous article, the iron always rusted in the course of 

 an hour or two. He points out that the baryta was not specially 

 purified and probably contained an appreciable quantity of 

 carbonate in addition to that formed from the carbon dioxide in 

 the flask : in fact, he supposes that the baryta was not capable of 

 withdrawing every trace of carbon dioxide from the atmosphere — 

 being less effective in this respect than soda, 



Friend is therefore to be regarded as an out-and-out defender 

 of the " acid theory of corrosion." In his conclusion he refers 

 to the so-called "electrolytic theory" discussed in the previous 

 article and points out that no proof has been given that 

 Kohlrausch and Heydweiller had pure water at their disposal 

 and that the minute degree of electrical conductivity which they 

 observed water to have may have been and probably was due 

 to the presence of a minute proportion of carbon dioxide. He 

 concludes with the remark : " If such is really the case, the 

 electrolytic theory of corrosion becomes a myth, whereas the 

 acid theory is unaffected by it." 



With reference to this conclusion, it may be pointed out 

 again that to draw a distinction between an electrolytic theory 

 and an acid theory is little short of absurd — the acid theory is 

 necessarily an electrolytic theory: the only question at issue 

 is whether or no water pure and simple can function as an 



