r colour.

_Floating Bricks_ were first mentioned by Strabo, the Greek geographer, and
afterwards by Pliny as being made at Pitane in the Troad. The secret of
their manufacture was lost for many centuries, but was rediscovered in 1791
by Fabroni, an Italian, who made them from the fossil meal (diatomaceous
earth) found in Tuscany. These bricks are very light, fairly strong, and
being poor conductors of heat, have been employed for the construction of
powder-magazines on board ship, &c.

_Mortar Bricks_ belong to the class of unburnt bricks, and are, strictly
speaking, blocks of artificial stone made in brick moulds. These bricks
have been made for many years by moulding a mixture of sand and slaked lime
and allowing the blocks thus made to harden in the air. This hardening is
brought about partly by evaporation of the water, but chiefly by the
conversion of the calcium hydrate, or slaked lime, into calcium carbonate
by the action of the carbonic acid in the atmosphere. A small proportion of
the lime enters into combination with the silica and water present to form
hydrated calcium silicate, and probably a little hydrated basic carbonate
of lime is also formed, both of which substances are in the nature of
cement. This process of natural hardening by exposure to the air was a very
long one, occupying from six to eighteen months, and many improvements were
introduced during the latter half of the 19th century to improve the
strength of the bricks and to hasten the hardening. [v.04 p.0521] Mixtures
of sand, lime and cement (and of certain ground blast-furnace slags and
lime) were introduced; the moulding was done under hydraulic presses and
the bricks afterwards treated with carbon dioxide under pressure, with or
without the application of mild heat. Some of these mixtures and methods
are still in use, but a new type of mortar brick has come into use during
recent years which has practically superseded the old mortar brick.

_Sand-lime Bricks_.--In the early 'eighties of the 19th century, Dr
Michaelis of Berlin patented a new process for hardening blocks made of a
mixture of sand and lime by treating them with high-pressure steam for a
few hours, and the so-called _sand-lime_ bricks are now made on a very
extensive scale in many countries. There are many differences of detail in
the manufacture, but the general method is in all cases the same. Dry sand
is intimately mixed with about one-tenth of its weight of powdered slak