THE COMPOSITION <>K WATKK. HYDROGEN 121 



may be expressed by the equation : H 2 O 4- Na=NaHO + H ; the mean- 

 ing of this is clear from what has been already said.' 1 



Sodium and potassium act on water at the ordinary temperature. 

 < >ther heavier metals only act on it with a rise of temperature, and 

 then not so rapidly or vigorously. Thus magnesium and calcium only 

 liberate hydrogen from water at its boiling point, and zinc and iron only 

 at a red heat, whilst a whole series of heavy metals, such as copper, lead, 

 mercury, silver, gold, and platinum, do not in the least decompose 

 water at any temperature, and do not replace its hydrogen. 



From this it is clear that hydrogen may be obtained by the decom- 

 position of steam by the action of iron (or zinc) with a rise of tempera- 

 ture. The experiment is conducted in the following manner : pieces 

 of iron (tilings, nails. Arc.), are laid in a porcelain tube, which is then 



9 This reaction is vigorously exothermal. If a sufficient quantity of water be taken 

 the whole of the sodium hydroxide, NaHO, formed is dissolved, and about 42,500 units of 

 heat are evolved per 23 grams of sodium taken. As 40 grams of sodium hydroxide 

 arc produced, and they in dissolving, judging from direct experiment, evolve about 10,000 

 calories ; therefore, without an excess of water, and without the formation of a solution, 

 the reaction Xa + H 2 O = H + NaHO would evolve about 32,500 calories. We shall after- 

 wards learn that hydrogen contains in its smallest isolable particles H 2 and not H, 

 and therefore it follows that the reaction should be written thus 2Na + 2H 2 O = H 2 + 

 'JXaHO, and it then corresponds with an evolution of heat of 4- 05,000 calories. And as 

 X. X. Beketoff showed that Na^O, or anhydrous oxide of sodium, forms the hydrate, or 

 sodium hydroxide (caustic soda), 2NaHO, with water, evolving about 35,500 calories, there- 

 fore the reaction 2N a + H 2 O = H 2 + NaoO corresponds to 29,500 calories. This quantity 

 of heat is less than that which is evolved in combining with water, in the formation 

 of caustic soda, and therefore it is not to be wondered at that the hydrate, NaHO, is always 

 formed and not the anhydrous substance Na^O. That such a conclusion, which agrees 

 with facts, is inevitable is also seen from the fact that, according to Beketoff, the anhy- 

 drous sodium oxide, NaoO, acts directly on hydrogen,with separation of sodium Na^O -t- H = 

 NaHO + Na. This reaction is accompanied by an evolution of heat equal to about 

 3,000 calories, because Na2O + H 2 O gives, as we saw, 35,500 calories and Na + H>O evolves 

 32,500 calories. However, an opposite reaction also takes place XaHO + Na = NaoO + H 

 (both with the aid of heat) consequently, in this case heat is absorbed. In this we see 

 an example of calorimetric calculations and the small use of the law of maximum work 

 for the general phenomena of reversible reactions, to which the case just considered 

 belongs. But it must be remarked that all reversible reactions evolve or absorb but 

 little heat, and judging from what has been said in Note 6 (and in Note 25 of Chap. I.), 

 the reason of the discrepancy between the law of maximum work and reality must 

 before all be looked for in the fact that we have no means of separating the heat which 

 corresponds with the purely chemical process from the sum total of the heat observed, 

 and as the structure of a number of substances is altered by heat alone and also by 

 contact, we can scarcely hope that the time approaches when such a distinction will be 

 possible. A heated substance, in point of fact, has no longer the original energy of its 

 atoms that is, the act of heating not only alters the store of movement of the molecule^ 

 but also of the atoms forming the molecules, in other words, it makes the beginning of or 

 preparation for chemical change. From this it must be concluded that thernio-chemistry, 

 or the study of the heat accompanying chemical transformations, cannot be identified 

 with chemical mechanics. Thermo-chemical data form a part of it, but they alone 

 cannot give it. 



