The diameter of a soda straw is apparently determined by the specific gravity and surface tension of water, for they are all nearly the same diameter, about one-quarter inch. In a cave in western Australia one soda straw has reached a length of 20 feet, 6 inches, yet is still only one-quarter inch in diameter. If the drip rate decreases, the tip of the soda straw may sometimes seal itself closed. [Illustration: Calcite crystals in layer of flowstone, 8 inches high.] [Illustration: ] Some speleothems apparently defy gravity. Now and then internal hydrostatic pressure causes secondary formations to project out from others in unusual directions. These are _helictites_ (see illustration on page 21). A related form is _popcorn_ (see illustration on page 22), the mat of small nodules which coat the “beehive” and other objects in the cave. Also called “cave coral,” popcorn can form under water or along wet walls in response to air currents. [Illustration: Helictite formed on soda straw] Soft, fibrous mats of calcium carbonate deposited near the surface at Oregon Caves are termed _moonmilk_. In time moonmilk may harden into popcorn mats. Its manner of formation is not fully understood. [Illustration: Popcorn in Adam’s Tomb] [Illustration: Rimstone] At the “Devil’s Washboard,” and at the foot of Paradise Lost, we find still another type of speleothem—_rimstone_—which forms in pools of water. Agitated by dripping or flowing water, some of the carbon dioxide in the pool escapes. The resultant deposition of mineral takes place on irregularities in the bottom of the pool, or creates stone wavelets where the pool spills over. Ridges and dams subsequently build up, often constricting the pool surface (see illustration on page 22). Another type of rimstone, or _cave ice_, develops when flowstone builds up around the edge of a pool and gradually closes across it. The pool may be completely sealed over, just like a pond in winter, except that “cave ice” can never melt. Cave students are often confused by another deposition found in the form of thin _blades_ jutting out of the wall (see illustration below). They have a woven, crystalline texture. Prior to removal by solution, some of the marble cracks were filled with calcium carbonate. Being less soluble than marble, the sheets of calcite crystals remain for a time after the surrounding rock is dissolved away. [Illustration: Blade formation]