enotes the powder of calcined tin, used in polishing and giving the last gloss to works of iron and steel.

TERRA PUZZOLANA, or Pozzolana, is a greyish kind of earth used in Italy for building under water. The best is found about Puteoli, Baiae, and Cumae, in the kingdom of Naples, from the first of which places it derives its name. It is a volcanic product, composed of heterogeneous substances, thrown out from the burning mouths of volcanoes in the form of ashes; sometimes in such large quantities, and with so great violence, that whole provinces have been covered with it at a considerable distance. In the year 79 of the common era, the cities of Herculaneum, Pompeia, and Stabia, although at the distance of many miles from Vesuvius, were, nevertheless, buried under the matters of these dreadful eruptions; as Bergman Puzzolana relates in his Treatise of the Volcanic Products. This volcanic earth is of a grey, brown, or blackish colour, of a loose, granular, or dusty and rough, porous or spongy texture, resembling a clay hardened by fire, and then reduced to a grog powder. It contains various heterogeneous substances mixed with it. Its specific gravity is from 2500 to 2800; and it is, in some degree, magnetic; it scarcely effervesces with acids, though partially soluble in them. It easily melts per se; but its most distinguishing property is, that it hardens very suddenly when mixed with 1/3 of its weight of lime and water; and forms a cement, which is more durable in water than any other.

According to Bergman's Analysis, 100 parts of it contain from 55 to 60 of siliceous earth, 20 of argillaceous, five or six of calcareous, and from 15 to 20 of iron. Its effects, however, in cement may perhaps depend only on the iron which has been reduced into a particular substance by means of subterraneous fires; evident signs of which are observable in the places where it is obtained. If the slate in Henneberg, or Kennekulle in the province of Westergotland, should happen to get fire, the uppermost stratum, which now consists of a mixture of iron and different kinds of rocks, called gruberberg in the account given of them, they might perhaps be changed partly into slag and partly into terra puzzolana.

It is evidently a martial argillaceous marl, that has suffered a moderate heat. Its hardening power arises from the dry state of the half-baked argillaceous particles, which makes them imbibe water very rapidly, and thus accelerates the defecation of the calcareous part; and also from the quantity and semifluidified state of the iron contained in it. It is found not only in Italy but in France, in the provinces of Auvergne and Limoges; and also in England and elsewhere.