White phosphorus is both a smoke producer and a particularly nasty incendiary agent, known as WP. Its white smoke has the highest total obscuring power (TOP) of any smoke. It was widely used in World War I in grenades and trench mortar rounds to screen troop movements. Most military smokes are now of other types, often colored with dyes. The 4.2-in. "Chemical" mortar of World War II was developed to throw white phosphorus shells, as well as whatever other chemical or biological agents might be required, but was later also found valuable as a general heavy mortar. This was a simple, light, portable weapon of great power, equivalent to a 105 mm howitzer, but of lesser range. It consisted of a tube about 5 ft long, a steel baseplate, and a bipod support with screws for elevation and traverse. The cylindrical round was simply dropped down the tube, and it sailed away on a high trajectory. The phosphorus sticks to whatever it hits, burns, and if what it has hit is combustible, sets it on fire. White phosphorus burns quickly and cooly and so is not a very effective incendiary agent. It is generally mixed with rubber or polystyrene to slow down the burning. Water will put out white phosphorus temporarily, but as soon as the phosphorus has access to air, it will start burning again. White phosphorus wounds are very unpleasant, since the phosphorus must be thoroughly washed out with a nonpolar solvent that is also noninflammable, for obvious reasons, before the burn can be treated. Carbon tetrachloride would be suitable, but it is dangerous because of the cancer hazard.