Pozzuolana is a form of cementing material which has been somewhat in use since very ancient times. Apparently it was first made from the lava from the volcano Vesuvius, the lava being picked up at Pozzuoli, a village near the base of the volcano. It consists of a combination of silica and alumina, which is mixed with common lime. Its chemical composition is therefore not very unlike that of hydraulic lime. It also possesses the ability to harden under water. Its use is very limited, and its strength and hardness comparatively small, compared with that of Portland cement. It should never be used where it will be exposed for a long time to dry air, even after it has thoroughly set. It appears to withstand the action of sea water somewhat better than Portland cement; and hence it is sometimes used instead of Portland cement as the cementing material for large masses of masonry or concrete which are to be deposited in sea water, when the strength of the cement is a comparatively minor consideration. Artificial pozzuolana is sometimes made by grinding up blast-furnace slag which has been found by chemical analysis to have the correct chemical composition.