10 HOW CROPS GROW. 



Physics, or "natural philosophy," is the science 

 which considers the general properties of matter and such 

 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- re- 

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

 whose play does not disturb the evident characters of sub- 

 stances are physical. 



Chemistry is 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 bums, 

 and both lose all the 'external characters that serve for 

 their identification. They are, in fact, converted into 

 other substances. Chemical attraction, affinity, or chem- 

 ism, as it is variously termed, unites two or more ele- 

 ments into compounds, unites compounds together into 

 more complex compounds ; and, under the influenco of 

 heat, light, and other agencies, is annulled or overcome, 

 so that compounds resolve themselves into simpler com- 

 binations or into their elements. Chemistry is the science 

 of composition and decomposition ; it considers the laws 

 and results of affinity. 



Biology, or physiology, unfolds the laws of the 

 propagation, development, sustenance, and death of liv- 

 ing organisms, both plants and animals. 



When we assert that the object of agriculture is to de- 

 velop from the soil the greatest possible amount of cer- 

 tain kinds of' vegetable and animal produce at the least 

 cost, we suggest the topics which are most important for 

 the agriculturist to understand. 



The farmer deals with the plant, with the soil, with 

 manures. These stand in close relation to each other, 



