26 



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 manneT that may prove 

 useful to the reader. 



- i Radiant 



Act at sensi- 

 ble and in- 

 sensible 

 distances 



Act only at 

 insensible 

 distances 



LIGHT 

 Repulsive HEAT 

 Attractive j ELECTRICITY 



LRep'ulsivel^^^'^"^^ 

 - - GRAVITATION 



COHESION 



CKYSTAlIilZATION 



Attractive -I ADHESION 



SOLtlTION 



Osmose 



AFFINITY 



VITALITY 



\ Inductive 

 Cosmical 



Molecular 



Atomic 

 Organic 



■ Physical 



Chemical 

 Physiological 



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 liot, 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. 



IL — 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. Afiinity, or chemical affinity, unites two or 

 more elements into compounds, unites compounds together 

 into more complex compoimds ; and, under the influence of 



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