146 PRINCIPLES OF CHEMISTRY 



from water ; so, for example, hydrochloric acid, which is formed 

 directly by the combination of hydrogen with chlorine, gives hydrogen 

 by the action of a great many metals, just as sulphuric acid does. 

 Potassium and sodium also displace hydrogen from its compounds with 

 nitrogen ; it is only from its compounds with carbon that hydrogen is 

 not displaced by metals. Hydrogen, in its turn, is able to replace 

 metals ; this is accomplished most easily on heating, and with those 

 metals which do not themselves displace hydrogen. If hydrogen be 

 passed over the compounds of many metals with oxygen at a red heat, 

 it takes up the oxygen from the metals and displaces them just 

 as it is itself displaced by metals. If hydrogen be passed over the 

 compound of oxygen with copper at a red heat, then metallic copper 

 and water are obtained CuO-fH 2 =H 2 O + Cu. This kind of double 

 decomposition is called reduction with respect to the metal, which is 

 thus reduced to a metallic state from its combination with oxygen. 

 But it must be recollected that all metals do not displace hydrogen 

 from its compound with oxygen, and, conversely, hydrogen is not able 

 to displace all metals from their compounds with oxygen ; thus it does 

 not displace potassium, calcium, or aluminium from their compounds 

 with oxygen. If the metals be arranged in the following series : 

 K, Na, Ca, Al . . . . Fe, Zn, Hg . . . . Cu, Pb, Ag, Au, then 

 the first are able to take up oxygen from water that is, displace 

 hydrogen whilst the last do not act thus, but are, on the contrary, 

 reduced by hydrogen that is, have, as is said, a less affinity for 

 oxygen than hydrogen, whilst potassium, sodium, calcium have more. 

 This is also expressed by the amount of heat evqlved in the act of 

 combination with oxygen, and is shown by the fact that potassium and 

 sodium and other similar metals evolve heat in decomposing water : but 

 copper, silver, and the like do not do this, because in combining with 

 oxygen they evolve less heat than hydrogen does, and therefore it hap- 

 pens that when hydrogen reduces these metals heat is evolved. Thus, 

 for example, if 16 grams of oxygen combine with copper, 38000 units of 

 heat are evolved ; and when 16 grams of oxygen combine with hydrogen, 

 forming water, 69000 units of heat are evolved ; whilst 23 grams of 

 sodium, in combining with 16 grams of oxygen, evolve 100000 units of 

 heat. This example clearly shows that chemical reactions which pro- 

 ceed directly and unaided evolve heat. Sodium decomposes water and 

 hydrogen reduces copper, because they are exothermal reactions, or 

 those which evolve heat ; copper does not decompose water, because 

 such a reaction would be accompanied by an absorption (or secretion) 

 of heat, or belongs to the class. of endothermal reactions, in which heat 

 is absorbed ; and such reactions do not generally proceed directly, 



