T1IK COMPOSITION OF AVATKK. HYDROGEN 123 



insoluble in water, are capable of combining with (have an affinity for) 

 acid oxides (as we shall afterwards fully consider), and form saline and 

 soluble substances, with acids, or hydrates having acid properties, hence 

 by the action of such hydrates, or of their aqueous solutions, 13 iron 

 and zinc are able to liberate hydrogen with great ease at the ordinary 

 temperature that is, they act on solutions of acids just as sodium acts 

 on water. 14 Sulphuric acid, or oil of vitriol, H 2 S0 4 , is usually chosen 



because at this temperature zinc sulphate and sulphuric acid split up, and the action must 

 take place between the water and zinc. From the preceding proposition results proceed 

 which are in some cases verified by experiment. If the action of zinc or iron on a solu- 

 tion of sulphuric acid presents a non-reversible reaction, then we may by this means 

 obtain hydrogen in a very compressed state, and compressed hydrogen will not act on 

 solutions of sulphates of the above-named metals. This is verified in reality as far as 

 was possible in the experiments to keep up the compression or pressure of the hydro- 

 gen. Those metals which do not evolve hydrogen with acids, on the contrary, should, at 

 least at an increase of pressure, be displaced by hydrogen. And in fact Brunner showed 

 that gaseous hydrogen displaces platinum and palladium from the aqueous solutions of 

 their chlorine compounds, but not gold, and Beketoff succeeded in showing that silver 

 and mercury, under a considerable pressure, are separated from the solutions of certain 

 of their compounds by means of hydrogen. Keaction already commences under H pres- 

 sure of six atmospheres, if a weak solution of silver sulphate be taken ; with a stronger 

 solution a much greater pressure is required, however, for the separation of the silver. 



15 For the same reason, many metals in acting on solutions of the alkalis displace 

 hydrogen. Aluminium acts particularly clearly in this respect, because its oxide gives a 

 soluble compound with alkalis. For the same reason tin, in acting on hydrochloric acid, 

 evolves hydrogen, and silicon does the same with hydrofluoric acid. It is evident that 

 in such cases the sum of all the affinities plays a part ; for instance, taking the action of 

 zinc on sulphuric acid, we have the affinity of zinc for oxygen (forming zinc oxide, ZnO), 

 the affinity of its oxide for sulphuric anhydride, S0 5 (forming zinc sulphate, ZnSO 4 ), and 

 the affinity of the resultant salt, ZnSO 4 , for water. It is only the first-named affinity that 

 acts in the reaction between water and the metal, if no account is taken of those forces 

 (of a physico-mechanical character) which act between the molecules (for instance, the 

 cohesion between the molecules of the oxide) and those forces (of a chemical character) 

 which act between the atoms forming the molecule, for instance, between the atoms of 

 hydrogen giving the molecule H 2 containing two atoms. I consider it necessary to 

 remark, that the hypothesis of the affinity or endeavour of heterogeneous atoms to enter 

 into a common system and in harmonious movement (i.e., to form a compound molecule) 

 must inevitably be in accordance with the hypothesis of forces inducing homogeneous 

 atoms to form complex molecules (for instance, H 2 ), and to build up the latter into 

 solid or liquid substances, in which the existence of an attraction between the homo- 

 geneous particles must certainly be admitted. Therefore, those forces which bring about 

 solution must also be taken into consideration. These are all forces of one and the same 

 series, and in this may be seen the great difficulties surrounding the study of mole- 

 cular mechanics and its province chemical mechanics. 



14 The representation given above of the cause of the easy action of iron or zinc on 

 sulphuric acid, naturally forms a hypothesis which explains only what is observed. 

 It is only at first sight that this hypothesis exhibits any similarity to the hypothesis of 

 predisposing affinity which reigned in past times. According to that, it was supposed that 

 reaction takes place (and hydrogen is evolved) by reason of the affinity for the sulphuric- 

 acid of the oxide of zinc which might be produced, and that decomposition could 

 not take place without this. The influence of a force in respect to a substance \shirh lias 

 not been produced, but which is capable of being formed, is not clear. In the repre- 

 sentation introduced by me, it is acknowledged that zinc already acts on water by 



