Acids mixed with such alkalies as soda, saleratus, potash, marble dust and lime, when wetted, begin to change into gas which rises into the atmosphere and is lost, but if the change takes place within a lump of dough that becomes light with innummerable bubbles of the gas, which expanding still more with the heat in baking make a light and spongy loaf. Soda fountains are charged by placing marble dust, which is one form of lime, in the generator, pouring dilute sulphuric acid upon it and immediately screwing down the lid. The acid and lime change into gas which can only escape through a pipe into another tight vessel nearly filled with water, where it remains imprisoned until drawn from the soda fount. Whether in bread or in a soda fountain, if the acic and alkali are not properly matched in quantity, a portion of either one or the other will remain behind unused and unchanged in the bread or in the generator. When they are rightly proportioned they still do not all go off in the gas, but leave a remainder, a new compound called a salt, which may be only common salt or may be something hurtful, according to the kinds of acids and alkalis employed.

According to one of the stories from history, Cleopatra owned the largest and most valuable pearl in the world of her day, and dissolved it in a cup of vinegar and drank it. Strong vinegar would dissolve the pearl, considerable time being allowed, nitric acid would have consumed it in a few minutes, yet Cleopatra did not drink the pearl; it passed off in the form of gas. If she drank during the effervescense she drank a sort of soda water. If the pearl and vinegar were just sufficient to neutralize each other, when the pearl disappeared the vinegar had lost its sourness and Cleopatra drank only water containing a nearly tasteless tartrate.