A chemical with a characteristic pungent, irritating, odour. Has the chemical formula NH3. It is a gas at normal ambient temperatures and pressures, but can be liquefied by pressure. The change instate on conversion from liquid to gas with a change in pressure is accompanied by a decrease in temperature and makes ammonia a useful refrigerant. Ammonia is not present in living fish, or only in trace amounts, but is formed in muscle tissue by biochemical processes in the tissues soon after death of the animal, and as result of bacterial degradation of nitrogen-containing compounds during spoilage of fish. This is particularly evident in elasmobranchs because of the degradation of urea prevalent in the muscle tissue of this class of fish.