TIIK COMPOSITION OK WATKK. HYDROGEN 119 



of forming oxides (rusts or earths, as Stahl called them) in air that is, 

 which are capable of burning or combining with oxygen. The capacity 

 of metals for combining with oxygen, and therefore for decomposing 

 water, or for the evolution of hydrogen, is very dissimilar. 7 Among 

 metals, potassium and sodium have the greatest energy in this respect. 

 The first occurs in potash, the second in soda. They are both lighter than 

 water, soft, and easily change in air. By bringing one or the other of 

 them in contact with water at the ordinary temperature, 8 a quantity of 



7 In order to demonstrate the difference of the .affinity of oxygen for different 

 elements, it is enough to compare the amounts of heat which are evolved in their combi- 

 nation with 16 parts by weight of oxygen ; in the case of sodium (when Na 2 O is formed, 

 or 46 parts of Na combine with 16 parts of oxygen, according to Beketoff) 100,000 calories 

 (or units of heat) are evolved, for hydrogen (when water, H 2 O, is formed) 69,000 calories, 

 for iron (when the oxide, FeO, is formed) 69,000, and if the oxide FeoO 3 is formed, 

 64,000 calories, for zinc (ZnO is formed) 86,000 calories, for lead (when PbO is formed) 

 51,000 calories, for copper (when CuO is formed) 38,000 calories, and for mercury (HgO is 

 formed) 31,000 calories. 



These figures cannot correspond directly with the magnitude of the affinities, for the 

 physical and mechanical side of the matter is very different in the different cases. 

 Hydrogen is a gas, and, in combining with oxygen, gives a liquid ; consequently it changes 

 its physical state, and, in doing so, evolves heat. But zinc and copper are solids, and, 

 in combining with oxygen, give solid oxides. The oxygen, previously a gas, now passes 

 into a solid or liquid state, and, therefore, also must have given up its store of heat in 

 forming oxides. As we shall afterwards see, the degree of contraction (and conse- 

 quently of mechanical work) was different in the different cases, and therefore the 

 figures expressing the heat of combination cannot directly depend on the affinities, on 

 the loss of internal energy previously in the elements. Nevertheless, the figures above 

 cited correspond, in a certain degree, with the order in which the elements stand hi 

 respect to their affinity for oxygen, as may be seen from the fact that the mercury oxide, 

 which evolves the least heat (among the above examples), is the least stable, is easily 

 decomposed, giving up its oxygen ; whilst sodium, the formation of whose oxide is accom- 

 panied by the greatest evolution of heat, is able to decompose all the other oxides, taking 

 up their oxygen. In order to generalise the connection between affinity and the evolu- 

 tion and the absorption of heat, which is evident in its general features, and was firmly 

 established by the researches of Favre and Silberman (about 1840), and then of Thomsen 

 (in Denmark) and Berthelot (in France), many investigators, especially the one last 

 mentioned, established the law of maximum work. This states that only those chemical 

 reactions take place of their own accord in which the greatest amount of chemical 

 (latent, potential) energy is transformed into heat. But, in the first place, we are not 

 able, judging from what has been said above, to distinguish that heat which corresponds 

 with purely chemical action from the sum total of the heat observed in a reaction (in the 

 calorimeter) ; in the second place, there are evidently endothermal reactions which 

 proceed under the same circumstances as exothermal (carbon burns in the vapour of 

 sulphur with absorption of heat, whilst in oxygen it evolves heat) ; and, in the third 

 place, there are reversible reactions, which when taking place in one direction evolve 

 heat, and when taking place in the opposite direction absorb it ; and, therefore, the 

 principle of maximum work in its elementary form is not supported by science. But the 

 subject continues to be developed, and will probably lead to a general law, such as 

 thermal chemistry does not at present possess. 



8 If a piece of metallic sodium be thrown into water, it floats on it (owing to its light- 

 ness), keeps in a state of continual movement (owing to the evolution of hydrogen on 

 nil sides), and immediately decomposes the water, evolving hydrogen, which can be 



