CHEMISTRY. 29 



decomposed, and one of them goes to unite with the new 

 substance, and form a compound, by means of elective affinity. 

 Two bodies which have no affinity for each other, may 

 sometimes be made to unite by means of a third : thus, oil and 

 water will not unite alone, but by the medium of the alkali 

 potash, which has an affinity for both, they unite and form the 

 well known compound, soap.* Chemical union is usually 

 attended with the evolution of heat. Some substances unite 

 without any apparent action, while others have an affinity so 

 strong that union takes place with an explosion. Chemical 

 affinity manifests itself iu a more complex form under the 

 name of double elective affinity. 



When nitrate of ammonia and carbonate of potash are 

 mixed together in solution, a double decomposition and reunion, 

 take place : the potash leaves the carbonic acid to go to the 

 nitric acid, and the nitric acid leaves the ammonia to go to 

 the potash, the carbonic acid and ammonia, finding them- 

 selves deserted and alone, unite and form carbonate of ammo- 

 nia. Thus nitrate of ammonia and carbonate of potash are 

 decomposed, and nitrate of potash and carbonate of ammonia 

 formed. This may be more clearly shown by arranging the 

 four elements thus : 



Nitrate of ( 1. Nitric Acid. 3. Potash. ) Carbonate 



Ammonia. ( 2? Ammonia. 4. Carbonic Acid. ) of Potash. 



This change of elements took place because a stronger 

 affinity existed between 1 and 3, than between 1 and 2, and 

 a stronger affinity existed between 2 and 4, than between 3 

 and 4. These compounds might again be decomposed by 

 others, having affinities sufficiently powerful to overcome that 

 which holds them together. In order that bodies may be 



* This example is taken from Comstock's Chemistry on account of 

 its plainness; but is nevertheless not strictly true, as the idea of an 

 intermediate substance is now abandoned by chemists. The truth is, 

 that the alkali and oil unite and form soap, which it itself dissolved in 

 the water. 



