50 Van Beek on the Supposed Property of Tin 



the same quantity in every closed gasometer, and can, when 

 mixed with steam, by no means cause an explosion. When the 

 bottom or the sides of a steam-boiler are incandescent for want 

 of water, the boiler may undoubtedly burst, as dreadful expe- 

 rience has often taught ; but in this case it is only the suddenly 

 increased tension of the steam that makes the danger imminent, 

 even without the presence of any inflammable or explosive gas. 



On the contrary, I perfectly agree with Dr Davy in his 

 statement, that the protection of iron-boilers of steam-vessels is 

 not a matter of absolute necessity, since experience has taught 

 us that the oxidation of the iron cannot take place after the 

 sea-water boils, a circumstance which generally occurs. Ac- 

 cording to his experiments, which we found perfectly confirmed, 

 the iron in sea-water is not oxidated by decomposition of sea- 

 water, but chiefly by the air contained in it, and when this is ex- 

 pelled by means of an air-pump, or by boihng, the oxidation of 

 the iron immediately ceases. I can, nevertheless, by no means 

 conceive why Dr Davy finds in this circumstance, which he 

 seems to consider as a peculiar case, the satisfactory explana- 

 tion of the result of an -experiment perfectly according with my 

 experiments above described, but in apparent contradiction to 

 his former investigations ; in which expei'iment he observed, to 

 his great surprise, that steel was not preserved by tin from cor- 

 rosion in sea-water, whilst repeated galvanometrical experiments 

 had taught him that iron in this fluid must be necessarily pre- 

 served by tin. I can, moreover, by no means agree with Dr 

 Davy in his statement, that iron in sea-water would be in a 

 similar case as when this metal is exposed to the action of hu- 

 mid air or acid vapours, since in this latter case, as he affirms, 

 it cannot be perfectly preserved from oxidation by the con- 

 tact of a more positive metal (zinc). I cannot participate in 

 this opinion, firstly, because decisive experiments have taught 

 me, that iron can certainly be completely preserved from oxida- 

 tion in sea-water by a more positive metal (zinc) ; and, secondly, 

 because copper, whose oxidation in sea-water proceeds exactly 

 in the same manner, is also perfectly preserved from oxidation 

 in this fluid by more positive metals (iron or zinc). The late 

 Sir Humphrey Davy had already observed, that the copper 

 sheeting of sea-vessels, to whose preservation he chiefly in- 



