COMPOSITIONS FOR GLASS. 
159 
brated Geyser, where some hot springs contain so much as to form 
siliceous petrifactions. 
Theſe are the different forms and states in which we find the 
hard ſtony bodies. We are now to take notice of their chemical 
properties. 
The pureſt kinds, or thoſe which contain the moſt of the silica, 
when they are exposed to a strong heat, generally become opaque, 
white, and brittle. But they suſtain the moſt violent heat without 
melting, or being softened by it. We can melt them, however, 
easily and perfectly, with fixed alkaline salts, with which they unite 
in the fire to form a perfect and workable glaſs. By workable glaſs, 
I mean a glaſs, which, while it is allowed to cool from its melted 
state, continues soft and ductile a considerable time, and passes 
through all the degrees of foftness before it become hard and rigid. 
During these states of softness it has great ductility, and can be 
wrought or moulded to any form. Glass that is workable in this 
manner is one of the most valuable productions of chemistry. 
All good glaſs contains siliceous matter and a fixed alkali as its on 
ly or principal ingredients. And sand, when it is composed of pure, 
or nearly pure siliceous matter, is preferred to the other forms of the 
hard earths, on account of its being easily mixed with the alkali and 
other materials, without requiring the expensive operations that would 
be necessary for reducing to powder, or to ſmall grains, the hard masses 
of other siliceous matter. 
In the composition of some kinds of glass, other materials are added 
to the sand and alkali; but the effect of these other materials is 
chiefly to increaſe tranſparency, or to affect the colour, or to diminiſh 
expence. 
The siliceous matter and alkali must bear certain proportions to 
one another to make good glaſs. If the alkali be deficient in the due 
quantity or goodneſs, the siliceous matter is not completely dissolved, 
and the glass not sufficiently transparent. If the alkali is redundant,