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HOW CROPS GROW. 



We distinguish a number of forces, which, acting on or 

 through matter, produce all material phenomena. In the 

 subjoined scheme the recognized forces are to some ex 

 tent classified and defined, in a manner that may prove 

 useful to the reader. 



The sciences that more immediately relate to agricul 

 ture are: 



I. Physics or natural philosophy, the science which 

 considers the general properties of matter and such of its 

 phenomena as are not accompanied by essential change 

 in its obvious qualities. All the forces in the preceding 

 scheme, save the last two, manifest themselves through 

 matter without destroying or masking the matter itself. 

 Iron may be hot, luminous, or magnetic, may fall to the 

 ground, be melted, welded, and crystallized ; but it remains 

 iron, and is at once recognized as such. The forces whose 

 play does not disturb the evident characters of substances 

 are physical. 



II. Chemistry, the science which studies the proper 

 ties peculiar to the various kinds of matter, and those 

 phenomena which are accompanied by a fundamental 

 change in the matter acted on. Iron rusts, wood burns, 

 and both lose all the external characters that serve for 

 their identification. They are, in fact, converted into other 

 substances. Affinity, or chemical affinity, unites two or 

 more elements into compounds, unites compounds together 

 into more complex compounds j and, under the influence of 



