Effervescence (Lat. effervescere, from ex, out of, and fervescere, to begin boiling), a state of ebullition, without vaporization, caused by the setting free of gases. It may be effected by chemical decomposition, or by diminishing the solvent capacity of a fluid which contains a gas in solution, either by raising its temperature, by diminishing the pressure upon it, or by adding some other body which in dissolving will cause a part of the gas to be expelled. Thus, when a solution of carbonate of soda is mixed with a solution of tartaric acid, the acid and soda unite and carbonic acid gas is set free, causing effervescence. If the solutions are very weak and very cold, the gas may be all absorbed by the water and no effervescence produced. At 65° F., and under the ordinary pressure of the atmosphere, water will absorb about its own volume of the gas. By lowering the temperature or increasing the pressure the amount of the gas absorbed will be increased. In the ordinary soda-water and mineral-water fountains the carbonic acid gas is held in solution under great pressure. When the water is drawn from the fountain into an open glass, the pressure being removed, the gas immediately begins to escape, causing effervescence.

The gas is usually made by dissolving marble or chalk, which are carbonates of lime, in hydrochloric or sulphuric acid. It is then cleansed from impurities and forced by great pressure into a condenser, whence it is transferred to the fountains. Effervescence may also be caused by dissolving zinc in dilute sulphuric acid, or zinc or iron in hydrochloric acid. In both cases hydrogen gas is set free. - Effervescing Powders are preparations of acid and alkaline powders, put up in differently colored papers in order to distinguish them. They are administered by first dissolving the contents of two different papers in separate glasses, then mixing the solutions,,and drinking while the chemical reaction is taking place with effervescence. The common soda and Seidlitz powders form effervescing draughts, the acid in one of the papers combining with the alkali of the carbonate in the other, and expelling the carbonic acid. This gas, continuing to be evolved in the stomach, acts as a refrigerant and diaphoretic, while the alkaline salt is slightly laxative. The drink is especially adapted to febrile complaints. The common soda powders consist of 25 grains of tartaric acid in one paper, and 30 grains of bicarbonate of soda in the other.

An equivalent proportion of bicarbonate of potash is sometimes substituted for the soda. The following are the proportions given in the " American Dispensatory": tartaric acid 1 oz., bicarbonate of soda 1 oz. and 54 grains, or bicarbonate of potassa 1 oz. and 160 grains. The acid and either bicarbonate, being separately reduced to fine powder, are divided each into 16 portions. Citric acid is sometimes employed instead of the tartaric acid, in the proportion of 9 drachms to 11 of the soda salt, or l3 of the potash. Seidlitz powders are a mixture of 2 drachms of Rochelle salts (tartrate of potash and soda) and 2 scruples of bicarbonate of soda in one paper, and 35 grains of tartaric acid in the other. The tartaric acid being in excess renders the medicine more pleasant to take, without interfering with its aperient quality. The acid and the carbonate may be prepared in a single powder, in which the chemical action does not take place until it is dissolved in water. Such a preparation should of course be carefully preserved from moisture.

Seidlitz and Rochelle powders are more laxative than the common soda powders.