46 HOW CROPS GROW. 



Our attention may now be directed to the study of such 

 compounds of these elements as constitute the basis of 

 plants in general ; since a knowledge of them will prepare 

 us to consider the remaruing elements with a greater de- 

 gree of interest. 



Previous to this, however, we must, first of all, gam a 

 clear idea of that force or energy, in vii'tue of whose actbn, 

 chiefly, these elements are held in, or separated from their 

 combinations. 



§ 3. 



CHEMICAL AFFINITY. 



Chemical attraction or affinity is the force which unites 

 or combines two or more substances of unlike character, to 

 a new body different from its ingredients. 



Chemical combination diifers essentially from mere mix- 

 ture. Thus we may mix together in a vessel the two gases 

 oxygen and hydrogen, and they will remain uncombined 

 for an indefinite time, occupying their original volume; 

 but if a flame be brought into the mixture they instantly 

 unite with a loud explosion, and in place of the light and 

 bulky gases, we find a few drops of water, which is a liquid 

 at ordinary temperatures, and in winter weather becomes 

 solid, which does not sustain combustion like oxygen, nor 

 itself burn as does hydrogen ; but is a substance having its 

 own peculiar properties, diflering from those of all other 

 bodies with which we are acquainted. 



In the atmosphere we have oxygen and nitrogen in a 

 state of mere mixture, each of these gases exhibiting its 

 own characteristic properties. When brought into chemi- 

 cal combination, they are capable of yielding a series of 

 no less than five distinct compounds, one of which is the 

 so-called laughing gas, while the others form suflTocating 

 and corrosive vapors that are totally irrespirable. 



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