70 HOW CROPS FEED. 
ozone actually produced in the atmosphere and consumed 
in it, or at the surface of the soil. We have as yet indeed 
no satisfactory means of information on this point, but 
may safely conclude from the foregving considerations that 
ozone performs an important part in the economy of 
nature. 
Relations of Ozone to Vegetable Nutrition.—Of the 
direct influence of atmospheric ozone on plants, nothing 
is certainly known. Theoretically it should be consumed 
by them in various processes of oxidation, and would have 
ultimately the same effects that are produced by ordinary 
oxygen. 
Indirectly, ozone is of great significance in our theory 
of vegetable nutrition, inasmuch as it is the cause of chem- 
ical changes which are of the highest importance in main- 
taining the life of plants. This fact will appear in the 
section on Nitric Acid, which follows. 
§ 8, 
COMPOUNDS OF NITROGEN AND OXYGEN IN THE ATMOS- 
PHERE. 
Nitric Acid, NO,H.—Under the more common name 
Aqua fortis (strong water) this highly important sub- 
stance is to be found in every apothecary shop. It is, 
when pure, a colorless, usually a yellow liquid, whose 
most obvious properties are its sour, burning taste, and 
power of dissolving, or acting upon, many metals and other 
bodies. ; 
When pure, it isa half heavier than its own bulk of 
water, and emits pungent, suffocating vapors or fumes; in 
this state it is rarely seen, being in general mixed or di- 
luted with more or less water; when very dilute, it evolves 
no fumes, and is even pleasant to the taste. 
It has the properties of an acid in the most eminent de- 
gree; vegetable blue colors are reddened by it, and it 
