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Natural Cement, or Hydraulic Lime.—This is familiarly known to all readers as Portland cement, but it is found of different qualities in many countries, and is also made artificially. Certain mineral substances have the quality when powdered and combined with water of setting hard as stone; hence the name hydraulic. I have seen at Budapest articles of Portland cement made in Hungary which equalled in appearance fine black slate or marble, and, while much less brittle, were indeed in every respect more durable and resistant to exposure. These artificial cements can be largely incorporated with indifferent substances, such as sand; they, however, require intense baking, and may in consequence be regarded as a kind of fictile ware.

Portland cement is very thoroughly treated in Hydraulischer Kalk und Portland Cement (in all their relations), by Dr. H. Zwick.

Tragacanth, though called a gum, is properly nothing of the kind, not being a true adhesive. It is the product of the Astragalus verus, a tree found in Asia. It swells out in water, and softens, but without dissolving. It is more of a glaze than a paste; hence it is used extensively by confectioners, bookbinders, or to stiffen laces. It enters, however, into the composition of several cements.