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INTRODUCTION. 



The influence which chemistry exerts may be seen by means 

 of the chemical forces which are acting both upon dead and liv- 

 ing matter. The principal agents, by which chemistry produ- 

 ces its beneficial results, are affinity, heat, light and electricity. 



1. Chemical Affinity. This is the great agent or cause of 

 all chemical changes on the surface of the earth. It is an at- 

 traction which one kind of matter has for matter of an oppo- 

 site kind. In this respect, it differs from cohesion which acts 

 between matter of the same kind, as between two smooth pie- 

 ces of lead, and it differs from gravitation, which cnly acts upon 

 matter in masses, while affinity effects changes within imper- 

 ceptible distances. It tends to draw together different kinds 

 of matter, and to continue the compound until some force acts 

 upon it to produce decomposition. This force may be heat, 

 light or electricity, but generally it is affinity itself ; for the 

 most important law of its action is, that one kind of matter 

 does not manifest the same desire to unite with every other kind 

 indiscriminately, but the force of affinity is different between 

 different bodies ; so that when two simple bodies are unhed 

 by its force, some third body may have a stronger attraction 

 for each of the constituents, or for one of them, than they have 

 for each other, and the consequence will be that the compound 

 will be decomposed ; the third body will unite with one con- 

 stituent of the compound, and form a new and different sub- 

 stance. Thus, for example, the well known substance ro/> 

 peras, is found in some soils ; it is composed of sulphuric acid 

 (oil of vitriol) , and oxide of iron (iron 7mbt), and these two 

 bodies are held together by the force of affinity ; but when 

 carbonate of lime, common limestone, is scattered over such a 

 soil the copperas is decomposed, the sulphuric acid leaves the 

 oxide of iron, being drawn away by its stronger affinity for 

 lime, and sulphate of lime is formed, which is commonly 

 called gypsum or plaster of Paris. 



In consequence of this election of one body in preference 

 ,^ to another, the composition and decomposition of bodies are 



