THE PRINCIPAL FLOOR. 
126 
’will be found on the bottom shelf of Cases XXX IV. to XLIV. ; 
they include rude portraits, mostly full length, of Charles II. ; 
'Charles II. and Catherine of Braganza ; James II. ; William III. ; 
Mary, the Queen of William III. ; William and Mary; and 
Queen Anne. 
On the shelves occupying the lower part of Cases XXXIX. to 
XLIV., above the delft dishes but below the Watcombe and 
Boulton ware, are examples of Nottingham brown-glazed ware, 
Newcastle and Sunderland pottery, and the wares of Lowesby, 
in Leicestershire ; Yarmouth, Wroth am, and Cadborough. Some 
of the beautiful porcelain of Belleek, in Co. Fermanagh, is also 
exhibited. 
Glass. 
Gases 52, 53, 66, 68, 71, 72. 
The manufacture of glass, from its intimate relation to certain 
branches of ceramic art, receives appropriate illustration in this 
section of the Museum. 
Glass consists of a fused mixture of various acid silicates, 
usually transparent and insoluble, and always destitute of 
crystalline structure. In a free state silica, or silicon dioxide, is 
highly refractory, but when associated with certain metallic 
oxides, the resulting compounds are often eminently fusible. 
The degree of fusibility enjoyed by these artificial silicates is 
dependent partly on the nature of the base and partly on its 
proportion, those silicates which contain an excess of base being 
most fusible, and therefore most easily worked. In practice, 
however, glass always contains a large preponderance of silica, 
since it happens that the basic silicates, especially those of the 
alkalies, are frequently soluble, and hence to a great extent 
useless for those purposes to which glass is ordinarily applied. 
Excluding those oxides which are introduced simply as 
colouring or decolouring agents, the bases practically employed 
in glass-making are potash, soda, lime, and oxide of lead ; but 
for these bases other oxides may be substituted, as in the zinc 
and manganese glasses exhibited in Case 52 ; whilst, on the 
other hand, the silica may be replaced by boracic acid, as in 
Faraday’s “heavy glass,” a silico-borate of lead, of which a 
specimen will be found in the same case. 
One of the most essential properties of glass is its amorphous 
or non- crystalline character. If, however, the glass be main- 
tained long in a heated condition, its individual silicates have 
opportunity to crystallise, and the glass then becomes, to a 
.greater or less extent, devitrijied. Several specimens of bottle 
“ metal,” and other glasses, are exhibited, in which crystalline 
products are embedded in a vitreous matrix. By subjecting 
glass for a considerable time to a temperature somewhat below 
its melting point, complete devitrification may be effected, and 
an opaque white fibrous product is obtained, known as 
Reaumur’s porcelain ; of which samples are here exhibited. 
