NATURAL HISTORY. 23 



of her laws, can be produced by art, whether for 

 the purposes of investigation, or for the formation 

 of new substances necessary or useful to man. 



Thus we become acquainted with the consti- 

 tuent parts of all animals, vegetables, and mine- 

 rals of the atmosphere and of the surrounding 

 waters of the uses to which each can be applied, 

 and of the changes and results which their mutual 

 action upon each other is constantly effecting. 

 And from this we learn, that the work of de- 

 struction and composition of natural substances 

 is constantly taking place, reducing compound 

 into simple bodies, and associating the particles 

 of simple bodies so as to render them compound. 



These operations are effected by what has 

 been termed the laws of chemical attraction ; or 

 that affinity of bodies for each other which leads 

 to their approximation and union. All substances 

 are governed by the laws of attraction ; though 

 those laws are varied according to the circum- 

 stances of the case. Thus when a smaller body 

 is simply attracted by a larger one, it is deno- 

 minated the attraction of gravitation, as illus- 

 trated by the solar system, or more familiarly, by 

 the falling of an apple from the tree to the ground. 

 When bodies of similar bulk and weight, and 

 homogeneous in their constituent parts, are at- 

 tracted and unite, forming only an addition of 

 mass to the same materials, it is termed the 

 attraction of cohesion. But when heterogeneous 



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