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 remaining elements with a greater de- 
gree of interest. 
Previous to this, however, we must, first of all, gain a 
clear idea of that force or energy, in virtue of whose action, 
chiefly, these elements are held in, or separated from their 
combinations, 
B 2. 
CHEMICAL AFFINITY. 
Chemical attraction or affinity is the force which unites 
or combines two or more substances of unlike character, to 
anew 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 less 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. 
