is the bubbling up of a gas from a
liquid. The gas that bubbled up from your lemonade was carbon dioxid
(CO_2), and this is the gas that usually bubbles up out of things when
they effervesce.

When you make bread, the yeast turns the sugar into carbon dioxid
(CO_2) and alcohol. The carbon dioxid tries to bubble up out of the
dough, and the bubbles make little holes all through the dough.
This makes the bread light. When bread rises, it really is slowly
effervescing.

HOW SODA WATER IS MADE. Certain firms make pure carbon dioxid
(commercially known as _carbonic acid_ _gas_) and compress it in
iron tanks. These iron tanks of carbon dioxid (CO_2) are shipped to
soda-water fountains and soda-bottling works. Here the compressed
carbon dioxid is dissolved in water under pressure,--this is called
"charging" the water. When the charged water comes out of the faucet
in the soda fountains, or out of the spout of a seltzer siphon, or out
of a bottle of soda pop, the carbon dioxid that was dissolved in
the water under pressure bubbles up and escapes,--the soda water
effervesces.

Sometimes there is compressed carbon dioxid down in the ground. This
dissolves in the underground water, and when the water bubbles up from
the ground and the pressure is released, the carbon dioxid foams
out of the water; it effervesces like the charged water at a soda
fountain.

But the most useful and best-known effervescence is the kind you got
when you stirred the baking soda in the lemonade. Baking soda is made
of the same elements as caustic soda (NaOH), with carbon dioxid (CO_2)
combined with them. The formula for baking soda could be written
NaOHCO_2, but usually chemists put all of the O's together at the end
and write it NaHCO_3. Whenever baking soda is mixed with any kind
of acid, the caustic soda part (NaOH) is used up in neutralizing
the acid. This leaves the carbon dioxid (CO_2) part free, so that it
bubbles off and we have effervescence. Baking soda mixed with an acid
always effervesces. That is why sour milk muffins and pancakes are
light as well as not sour. The effervescing carbon dioxid makes
bubbles all through the batter, while the caustic soda (NaOH) in the
baking soda neutralizes the acid of the sour milk.

[Illustration: FIG. 185. Making a glass of soda lemonade.]

EFFERVESCENCE GENERALLY DUE TO THE FREEING OF CARBON DIOXID. Since
baking soda is so much used in the home for neutralizing acids, people
sometimes get the ide