3i 4 SCIENCE PROGRESS 



is difficult to understand why so few metals can be substituted 

 for platinum and why combustible gases such as methane and 

 carbonic oxide cannot be coupled with oxygen in a gas battery. 



Hydrogen apparently is peculiar in its relationship to 

 platinum, peculiar in forming with it a hydride, perhaps of the 

 binary class, such as is not producible from other metals, at all 

 events by a direct process. Methane presumably has no such 

 " combining power " ; carbonic oxide, instead of forming binary 

 compounds, apparently has a pronounced tendency to form 

 " cycloid " compounds, as in the case of nickel carbonyl. In 

 any case, some explanation is needed of the differences observed 

 between combustible gases ; it may well be that the differences 

 are more or less of the order now suggested. 



The results obtained by Lambert and Thomson, referred 

 to in the previous article, merit discussion from this point of 

 view. Assuming that acid (carbonic acid) was present together 

 with the oxygen and water, the fact that the highly purified 

 iron they prepared did not rust unless coupled with an electro- 

 negative conductor may be regarded as proof that two con- 

 tiguous portions of a mass of iron cannot respectively play 

 the parts of the electro -positive and the electro -negative 

 components of a voltaic couple in presence of a depolariser 

 — in other words, that oxygen cannot act as a depolariser at 

 or on a simple iron surface. 



It has been customary to assume that when copper dissolves 

 in dilute sulphuric acid in the presence of oxygen, different 

 more or less adjacent parts of the metal form the positive and 

 negative terminals and that at one of these the acid is active 

 whilst the depolariser (oxygen) is active at the other. 



It is of course conceivable, indeed probable, that there are 

 impurities in copper which are active in promoting depolari- 

 sation. The difference between iron and copper, however, 

 may arise from the fact that copper oxide has a lower and iron 

 oxide a higher heat of formation than that of water. Or it 

 may be that iron lacks the power of merely absorbing oxygen 

 at its surface ; iron, we know, is rendered passive with peculiar 

 facility : apparently it is easy to " polarise " it permanently 

 with oxygen. 



When ordinary iron rusts, most of the hydrogen it displaces 

 from whatever acid may be present becomes oxidised ; the 



