2. (Science: chemistry) One of a class of caustic bases, such as soda, potash, ammoma, and lithia, whose distinguishing peculiarities are solubility in alcohol and water, uniting with oils and fats to form soap, neutralising and forming salts with acids, turning to brown several vegetable yellows, and changing reddened litmus to blue. Fixed alkalies, potash and soda. Vegetable alkalies. Same as alkaloids. Volatile alkali, ammonia, so called in distinction from the fixed alkalies.