Effervescence.

Effervescence. Nearly all gases are more or less soluble in water, the amount of solubility depending on various conditions of pressure and temperature. As a rule, the lower the temperature and the greater the pressure, the greater is the solubility of a gas, so that when the temperature of such a solution is raised, or the pressure lowered, the gas escapes in small bubbles, giving rise to the phenomenon of effervescence. The most familiar instance of effervescence is when a bottle of soda-water is uncorked, the excess of carbonic acid gas over what can remain in solution escaping with effervescence. Again, when a seidlitz powder is mixed with water, effervescence occurs, owing to the inability of the water to retain the gas in solution. Many slight circumstances affect effervescence. Most people know that by stirring a glass of soda- water, or by dropping into it a fragment of cork or a crumb of bread, greater effervescence occurs. Sometimes the liquid is rather viscid, and a persistent froth is produced, as when a siphon of lemonade is discharged into a tumbler. In such a case, the addition of a few drops of milk or of a little alcohol causes more rapid effervescence and settling of the froth. In the former of these cases, the bread crumb or the stirring acts by making it more easy for the gas to form bubbles and escape. In the latter case, the effect of milk may have a twofold cause, one similar to that of the crumb, the milk globules acting as nuclei; the other similar to the calming influence of oil on sea foam, the natural oil, butter, here coming into play. The alcohol merely acts by thinning the liquid.