42 
THE HALL. 
The contraction suffered during desiccation has produced fissures? 
or cracks, which have subsequently been filled by deposition of 
carbonate of lime. These veins of calcite being disposed with 
tolerable regularity through a darker base, produce on section a 
peculiar pattern which is well seen in the polished Slab No. 157. 
Portland cement , which has practically superseded the use 
of Roman cement, is made by calcining a mixture of chalk, 
and finely divided argillaceous matter, such as the mud dredged 
from the Thames and the Medway. It is also prepared from 
Lias limestone and the associated Lias shales ; and from 
Carboniferous limestone mixed with local clays. A large series 
illustrating the cement industry is now in course of arrangement 
(1895). 
With the cements will be found some specimens of artificial 
stone, prepared by Mr. Fred. Ransome, of Ipswich. This gentleman 
has at different times patented several processes for this purpose, 
but the general principle consists in using water-glass (silicate 
of soda) as a cement by which sand is formed into a compact 
stone, remarkable for its high cohesive power. By the side of 
this artificial stone are some specimens of flints which have been 
acted upon by heated soda, in the preparation of the sodium 
silicate. Ordinary flints are subjected to the action of a strong 
solution of caustic soda in digesters under steam pressure of 
from 60 to 80 lbs. to the square inch, and are rapidly dissolved 
with production of solution of silicate of soda. In 1870 
Mr. Ransome effected a great improvement in the manufacture of 
his artificial stone by mixing the Farnham stone, or soluble 
silica, with silicate of soda or of potash, lime, sand, alumina,, 
chalk, or other convenient materials ; the alkaline silicate is then 
decomposed, the silica combining with the lime to form an 
insoluble silicate of lime, and also forming with some of the 
materials a silicate of alumina, whilst the caustic alkali set free 
by the decompostion seizes upon the soluble silica of the Farnham 
stone and forms a fresh silicate, which in turn is decomposed by 
more lime. This material called Apoenite , is exhibited in this 
Case, and its application is further illustrated by the Yase No. 105. 
Basalt. — Columns from the Giants’ Causeway. No. 90. 
These specimens well illustrate the peculiar columnar structure 
which basalt very frequently assumes, and on which depends 
the characteristic scenery of the Giants’ Causeway, the Isle of 
Staffa, and other well-known basaltic districts. The basalt of 
which these are composed is a dark-coloured, fine-grained, 
igneous rock, composed of an intimate mixture of felspar, 
usually labradorite, and augite, associated with olivine, magnetic 
iron, &c. The columns, as seen by the specimens exhibited, are 
large polygonal pillars, often six sided, each prism being usually 
terminated by a convex face at one end, and a corresponding 
convexity at the other ; and by a kind of ball-and-socket joint 
thus formed, the individual columns are articulated one to another. 
