SUBSTITUTION. 



hydrogen to produce hydrochloric acid. In all these 

 cases an external force is required to bring the atoms 

 within the range of mutual affinity : flame, the electrical 

 spark, actinism, or the interposition of a third body, 

 is necessary in each case. There are other examples in 

 which no such influence is required. Potassium and 

 oxygen instantly unite : chlorine, iodine, and bromine 

 immediately, and with much violence, combine with the 

 metals to form chlorides, iodides, or bromides. 



With compound molecules the action is in many 

 cases equally active, and combination is readily effected, 

 as in the cases of the acids and the oxides of some 

 metals, which are all instances of the most common 

 chemical attraction. 



An elementary or simple molecule and molecules of a 

 compound and different constitution are brought 

 together, and a new compound results from an inter- 

 change of their atoms, whilst an element is liberated. 

 These are essentially illustrations of analytical che- 

 mistry. Sulphuretted hydrogen is mixed with chlorine ; 

 the chlorine combines with the hydrogen, and sulphur 

 is set free. Potassium is put into water, and it com- 

 bines with the oxygen of the water, whilst the hydrogen 

 is liberated. 



Two compound molecules being brought together 

 may decompose each other, and form two new com- 

 pounds by an interchange of their elements. 



One element may be substituted for another under 

 certain circumstances. Gold may be replaced by 

 mercury ; copper will take the place of silver ; and iron 

 will occasion the separation of copper from its solutions, 

 the iron itself being dissolved to supply its place; 

 chlorine will substitute hydrogen in the carburetted 

 hydrogen gases; and many other examples might be 

 adduced. 



Chemical phenomena very frequently become of a 

 complex character ; and one, two, or three of these cases 



