I n southeast England it forms a bed nearly one thousand feet thick. If w e pour a drop of vinegar on a lump of chalk, there is a bubbling up, or effervescence, which is due to carbonic acid gas escaping from the chalk. The chalk, in fact, is nearly pure carbonate of lime, which is a compound of the metal calcium with carbon and oxygen. All limestones can be detected in this way by pouring on them a few drops of some acid. Most limestones are composed of the shells or hard parts of the coral-building animals, but chalk is composed of the shells of minute swimming or floating animals, whose hard parts, after death, sank to the bottom and collected into thick layers. These were afterwards uplifted to the surface and became cliffs or beds of chalk. Chalk is used in connetion wt«h the blackboard in lecture-rooms and schools. Various preparations of it are made for pastel colors. It is also used as a manure.