46 HOW CROPS GROW. 
Our attention may now be directed to the study of savk 
compounds of these elements as constitute the basis of 
plants in general; since a knowledge of them will prepare 
us to consider the remaining elements with a greater de 
gree of interest. 
Previous to this, however, we must, first of all, gam a 
lear idea of that force or energy, in virtue of whose action, 
chiefly, these elements are held in, or separated from their 
combinations. 
§ 3, 
CHEMICAL AFFINITY. 
Chemical attraction or affinity ¢s the force which unites 
or combines two or more substances of unlike character, to 
a new body different from its ingredients. 
Chemical combination differs 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, differing 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 jess than five distinct compounds, one of which is the 
so-called laughing gas, while the others form suffocating 
and corrosive vapors that are totally irrespirable. 
