122 PRINCIPLES OF CHEMISTRY 



subjected to a strong heat and steam passed through it. The steam, 

 coming into contact with the iron, gives up its oxygen to it, and thus 

 the hydrogen is set free and passes out at the other end of the tube 

 together with undecomposed steam. This method, which is historically 

 very significant, 10 is practically inconvenient, as it requires a rather 

 high temperature. Further, this reaction, as a reversible one (a red- 

 hot mass of iron decomposes a current of steam, forming oxide and 

 hydrogen ; and a mass of oxide of iron, heated to redness in a stream 

 of hydrogen, forms iron and steam), does not proceed in virtue of the 

 comparatively small difference between the affinity of oxygen for iron 

 (or zinc), and for hydrogen, but only because the hydrogen escapes, as 

 it is formed, in virtue of its elasticity. 11 If the oxygen compounds that 

 is, the oxides which are obtained from the iron or zinc, be able to pass 

 into solution, then the affinity acting in solution is added, and the 

 reaction may become non-reversible, and proceed with comparatively 

 much greater facility. 12 As the oxides of iron and zinc, by themselves 



10 The composition of water, as we saw above, was determined by passing steam over 

 red-hot iron ; the same method has been used for making hydrogen for filling balloons. 

 An oxide having the composition FesC^ is formed in the reaction, so that it is expressed 

 by the equation 3Fe + 4H^O = Fe 5 O4+8H. It is very important to remark that this re- 

 action is reversible. By heating the scoria in a current of hydrogen, water and iron 

 are obtained. From this it follows, from the principle of chemical equilibria, that if 

 there be taken iron and hydrogen, and also oxygen, but in such a quantity that 

 it is insufficient for combination with both substances, then it will divide itself 

 between the two ; part of it will combine with the iron and the other part with the 

 hydrogen, but a portion of both will remain in an uncombined state. Here again (see 

 note 9) the reversibility is connected with the small heat effect, and here again both re- 

 actions (direct and reverse) proceed at a red heat. But if, in the above-described re- 

 action, the hydrogen escapes as it is evolved, then its partial pressure does not increase 

 with its formation, and therefore all the iron can be oxidised by the water, which could 

 not take place were the iron and water heated to the temperature of reaction in a closed 

 vessel. In this we see the elements of that influence of mass to which we shall have 

 occasion to return later. 



11 Therefore, if iron and water be placed in a closed space, decomposition of the water 

 will proceed on heating to the temperature at which the reaction 3Fe + 4H...O = Fe 3 O 4 + 8H 

 commences ; but it ceases, does not go on to the end, because the conditions for a 

 reverse reaction are attained, and a state of equilibrium will ensue after the decomposi- 

 tion of a certain quantity of water. Judging from what has been said in Note 9, 

 something of the same kind takes place if the iron be replaced by sodium, only 

 then the mass of the water decomposed will be greater, and equilibrium will ensue, 

 with the formation of the hydrate, NaHO, and not of anhydrous oxide, NaoO that is, 

 the water will remain in the form of hydrate only. With copper and lead there will be 

 no decomposition, either at the ordinary or at a high temperature, because the affinity of 

 these metals for oxygen is much less than that of hydrogen. 



12 In general, if reversible as well as non-reversible reactions can take place between 

 substances acting on each other, then, judging by our present knowledge, the non- 

 reversible reactions take place in the majority of cases, which obliges one to acknowledge 

 the action, in this case, of comparatively strong affinities. The reaction, Zn + H 3 SO 4 

 H 2 + ZnSO 4 , which takes place in solutions at the ordinary temperature, is scarcely re- 

 versible under these conditions, but at a certain high temperature it becomes reversible, 



