CALC-TUFF, or CALCAREOUS TUFFA, is the most impure, the most irregular, and the most porous, of all the varieties of limestone. It occurs in beds generally in the vicinity of lakes and rivers, also encrusting rocks, and enveloping animal and vegetable remains in the proximity of calcareous springs. Immense deposits of calc-tuff have taken place at Terni, and on the banks of the river Anio near Tivoli; where some very curious impressions, such as that of a cart-wheel, trunks of trees, &c. are to be met with. The celebrated Grecian temples of Pæstum are formed of this stone, and no doubt owe their existence, at the present period, to the circumstance of its becoming harder the longer it is exposed to the air; for, as the quarries whence it has been procured are in the immediate vicinity, and the stone previous to being exposed is so much softer, modern Vandals have found it easier to go directly to the quarry for what they wanted, than attack the long weather-beaten and now indurated Doric pillars of the temples. From its property of hardening so much on exposure to the atmosphere and to wet, this rock makes a very useful building stone in the formation of bridges. Over the Danube at Ulm a very handsome bridge has lately been constructed of it, which, when brought from the neighbouring moun-
tains, is cut into its required dimensions with the assistance merely of the axe and the saw.
CALCULUS primarily denotes a little stone or pebble, anciently used in making computations, taking suffrages, playing at tables, and the like. In after times, pieces of ivory, and counters struck of silver, gold, and other materials, were used in lieu of the calculi, but still retained the ancient name. Computists were by the lawyers called calculones, when they were either slaves or newly freed men; those of a better condition were named calculatores or numerarii; there was ordinarily one of these in each family of distinction. The Roman judges anciently gave their opinions by calculi, which were white for absolution, and black for condemnation. Hence calculus albus, in ancient writers, denotes a favourable vote, either in the case of a person to be absolved and acquitted of a charge, or elected to some dignity or post; and calculus niger had a contrary signification. This usage is said to have been borrowed from the Thracians, who marked their happy or prosperous days by white, and their unhappy by black pebbles, which were put each night into an urn.
Besides the diversity of colour, there were some calculi also which had figures or characters engraved on them, as those which were in use in taking the suffrages both in the senate and in assemblies of the people. These calculi were made of thin wood, polished and covered over with wax. Their form is still seen in some medals of the Cassian family; and the manner of casting them into the urns is represented in the medals of the Licinian family. The letters marked upon these calculi were U. R. for ut rogas, and A. for antiquo; the first of which expressed an approbation of the law, the latter a rejection of it. Afterwards the judges who sat in capital causes used calculi marked with the letter A. for absolvo; C. for condemno; and N. L. for non liquet, signifying that a more full information was required.
CALCULUS is also used by ancient writers for a kind of weight equal to two grains of cicer. Some make it equivalent to the siliqua, which is equal to three grains of barley. Two calculi composed the ceratium.