Carbonic Acid
This acid is composed of oxygen 72, carbon 28; specific gr. 1.529, air being 1.000. All forms of carbon when burned in the air unite with oxygen to form carbonic acid. It constitutes 44 per cent of limestone. A cubic inch of marble yields four gallons of the gas. Under a pressure of 36 atmospheres at 32 deg. carbonic acid shrinks into a colorless liquid lighter than water. It does not resume the gaseous state when the pressure is removed, but evaporates with great rapidity, one portion absorbing heat from another, and thus freezing it into a white substance like snow. Unlike other acids, it does not unite with water to form a definite hydrate. It is anhydrous in all the three states--gaseous, liquid, and solid. It is incombustible, and a non-supporter of combustion. See GASES; LIQUID; SOLID.