40 
THE HALL. 
siliceous rock give it immense advantages, expensive though 
those stones are, in consequence of the necessity of carefully 
piecing them together. 
Some of the lavas from the extinct volcanoes of the Lower 
Eifel furnish millstones which have long been justly celebrated. 
They were well known to the Romans, and are still extensively 
quarried at Niedermendig, near Andernach, whence they are 
sent down the Rhine to Holland, and exported to most parts of 
the globe. Under the name of “ Dutch Blue Stones ” they were 
formerly much used in this country. 
Plaster of Paris. Cements. — Cases V, VI., VII. Cast of Apollo 
Belvedere, 159; of Dying Gladiator, 86; of Greek Vase, 
104. Pavement in Keene’s Cement, 11. 
The well-known mineral Gypsum is a hydrous sulphate of 
lime occurring abundantly in the New Red or Keuper Marls, 
often associated with rock salt. When transparent and crystal- 
lized, it is known as selenite, and, when fibrous, as satin spar ; 
specimens of both varieties will be found in Case V. The fine 
semi-crystalline form of gypsum termed Alabaster has been 
already noticed as an ornamental stone (p. 28). All these sub- 
stances are natural hydrates ; but the mineral called Anhydrite 
is a sulphate of lime, destitute of any essential water, its 
composition being lime, 41T8 ; sulphuric acid, 51*82. 
When gypsum is calcined at a moderate temperature, it parts 
with the whole of its water, and has then a composition re- 
sembling that of anhydrite. Calcined gypsum, when reduced to 
powder, forms the well-known Plaster of Paris, so called from the 
circumstance that the mineral from which the plaster is obtained 
is found in abundance in the tertiary deposits of the Paris basin, 
especially at Montmartre. Mixed with sufficient water to convert 
it into a paste, the plaster eagerly absorbs the liquid, and, re- 
turning to its original hydrated condition, rapidly solidifies or 
sets. If, however, the gypsum be overburnt this setting is 
prevented, and the material is as useless as powdered anhydrite 
would be. To ensure rapid consolidation it is desirable to so 
perform the calcination that about 5 per cent, of water is left in 
the plaster. Good plaster of Paris is far from being an 
anhydrous sulphate. 
The facility with which, by means of plaster of Paris, copies 
of any objects can be obtained renders it of great value in 
multiplying the finest works of ancient and modern art. Some 
applications are shown in this case. 
Fictile ivory, of which there are several examples, is plaster 
of Paris, which, after drying, has been made to absorb melted 
spermaceti; or it maybe prepared according to Mr. Franchi’s 
process as follows : Plaster and colouring matter are employed 
in the proportions of a pound of superfine plaster of Paris to half 
an ounce of Italian yellow ochre. They are intimatelymiixed by 
