Irish moss by John Spottiswood.

This purple seaweed can be cooked as a nutritious vegetable, but cooks are often more interested in the liquid it cooks in, for it thickens like gelatin when it cools. It's widely used in the British Isles to make puddings and molded gelatin desserts, or to thicken soups. A carrageen extract, called carrageenan, is used commercially to make ice cream, jelly, and other things. To use the dried seaweed as a thickener, first rinse it carefully, then soak it in water until it swells. Next add the carrageen to the liquid you wish to set, boil the liquid for about 10 minutes, then strain out and discard the carrageen. Look for dried carrageen in health food stores.