114? PORTLAND STONE. MARL. 
The warm baths of Hungary are often so thickly 
coated at the sides and bottom' with tufa, that, during 
certain intervals, it actually fills up the tubes and canals 
through which they are supplied. The fur in tea- 
kettles is a somewhat similar deposit from water in 
boiling. 
18?. PORTLAND STONE, BATH STONE, KET- 
TON STONE, are different kinds of limestone; and of a 
texture so hard and compact as to be used in building. 
They have their names from the places where they are re- 
spectively found, in Portland Island, near Bath, and at 
Ketton, in the county of Rutland. 
Of Ketton stone several of the colleges in Cambridge 
are built. Its grain has a singular resemblance to the 
petrified roe of a fish, whence also it is sometimes 
called roestone. The bridges, St. Paul's Cathedral, the 
Monument, and nearly all the buildings of late date in 
London, are constructed of Portland stone. 
Some of these kinds of stone, when first dug out of 
the quarry, are so soft that they are readily worked into 
any form which use or ornament may require. This is 
owing to the moisture with which they are naturally 
impregnated ; but when they once become hardened, by 
exposure to the sun and air, they are extremely firm 
and solid. On the contrary, other kinds of limestone 
that are used for buildings imbibe and retain the mois- 
ture of the atmosphere, in consequence of which they 
burst or are crumbled by frost. 
We are informed that Portland stone was first used 
in London in the reign of James the First, that mo- 
narch, by the advice of his architects, having employed 
it in the construction of the banquetting house at 
Whitehall. After the great fire in London, it was 
brought into general use by Sir Christopher Wren. 
188. MARL is a combination of clay, silex (76), and lime: 
and is denominated calcareous, argillaceous, or siliceous, as 
the lime, clay, or silex, is most abundant. 
The calcareous part of marl is frequently composed 
