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or Puzzolana

iron, water, black, substance and earth

PUZZOLANA, or POZzOLAN.\. a kind of substance formed of volcanic ashes, more or less compacted together, so called from Pozznolo. as also pelvis Puteolanus, from Puteoli, situ ate near mount Vesuvius, from which these ashes are ejected, and in the vicinity of which it abounds. It occurs of various colours, white, red, or black, reddish, or reddish-brown, gray or grayish-black ; that of Naples is generally gray ; that of Ci vita Vecchia is more generally reddish, or reddish•brown. The red variety is the proper puzzolana ; the black and the white sorts are called, in Italy, lapila, or rapillo. The ashes which overwhelmed Pompeii now form an immense bed of white puzzolana. The surfitee of this substance is rough, uneven, and of a baked appearance. It comes to us in pieces, from the size of a nut to that of an egg. It is wholly destitute of internal lustre and transparency. It is easily frangible, and its fracture is uneven or earthy, and porous ; commonly tilled with particles of pumice, quartz, scoria:, &e. Hardness, 3. Very brittle. Specific gravity from 2.570, which is that of the black, to 2.785, rarely 2.8. Its smell is earthy. It is not diffusible in cold-water ; but in boiling-water it gradually deposits a fine earth. It does not effervesce with acids. Ileated, it assumes a darker colour, and easily melts into a black slag, or, with borax, into a yellowish-green glass. Before it is heated, it is magnetic, but not afterwards. By Mr. Bergman's analysis, it contains from 55 to 60 per cent. of siliceous earth, 19 to 20 of argillaceous earth, 5 or 6 of cal careous earth, and from 15 to 20 of iron. When mixed with a small proportion of lime, it quickly hardens ; and this indu ration takes place even under water. This singular property

proceeds, as Mr. Kirwan supposes, from the magnetic state of the iron it contains ; for this iron, being unoxygenated, sub tillv divided, and dispersed through the whole mass, and thus offering a large surface, quickly decomposes the water with which it is mixed, when made into mortar, and forms a hard substance, analogous to the specular iron-ore, as it does in the iron tubes, in which water is decomposed, in the experiments of M. Lavoisier and Dr. Priestly ; for in these the iron swells and increases in bulk ; and so does puzzolana, when formed into mortar, as we learn from Higgins on Cements. One prin cipal use of lime seems to be to heat the water, as, while it is hot, it cannot pervade the caked argil that invests the ferru ginous particles ; yet, in time, even cold water may pervade it, and produce hardness ; and hence, as M. Dulomieu has observed, lavas become harder when moistened. If the mor tar be long exposed to the atmosphere, fixed air, as well as pure air, will unite to the iron, rust will be produced, and the mortar will not then harden, as Dr. Higgins has noticed. Clay, over which lava has flowed, is frequently converted into puzzolana; but volcanic scorice never afford it ; either because they are much calcined, or retain sulphur, or its acid. The ancients were well acquainted with this substance and its properties : and, among them, its principal use, as it has been also in modern times, was that of mixing it with their cements for buildings sunk into the sea. As it hardens and petrifies in water, it is of particular service in making moles and other buildings in maritime places.