40 HOW CROPS FEED. 
a burning taper in the second bottle, when, if the experiment was right- 
ly conducted, the flame will be extinguished, 
c. Into a bottle filled as in the last experiment with earbonie acid, 
some lime-water is poured and agitated. The previously clear lime-wa- 
ter immediately becomes turbid and milky from the formation of carbon- 
ate of lime, which is nearly insoluble in water. 
Carbonic Acid in the Atmosphere.—To show the pres 
ence of carbonic acid in the atmosphere, it is only neces- 
sary to expose lime-water in an open vessel. But a little 
time elapses before the liquid is covered with a white film 
of carbonate. As already stated, the average proportion 
of carbonic acid in the atmosphere is 6—10000ths 
(1-1600th nearly) by weight, or 4-10000ths (1-2500th) 
by bulk. Its quantity varies somewhat, however. Among 
over 300 analyses made by De Saussure in Switzerland, 
Verver in Holland, Lewy in New Granada, and Gilm in 
Austria, the extreme range was from 47 to 86 parts by 
weight in 100,000. 
Deportment of Carbonic Acid towards Water,—W ater 
dissolves carbonic acid to a greater or less extent, accord- 
ing to the temperature and pressure. Under the best or- 
dinary conditions it takes up about its own volume of the 
gas. At the freezing point it may absorb nearly twice as 
much, This gas is therefore usually found im spring, well, 
and river waters, as well as in dew and rain. The consid- 
erable amount held in solution in cold springs and wells 
is a principal reason of the refreshing quality of their wa- 
ter. Under pressure the proporticn cf carbonic acid ab- 
sorbed by water is much larger, and when the pressure is 
removed, a portion of the gas escapes, resuming its gase- 
ous form and causing effervescence. The liquid that flows 
from a soda-fountain is an aqueous solution of carbonic 
acid, made under pressure. Bottled cider, ale, champagne, 
and all effervescent beverages, owe their sparkle and much 
of their refreshing qualities to the carbonic acid they con- 
tain. 
