REVIEW— AN EASY INTRODUCTION TO CHEMISTRY. 103 
The Chapter (Y) treating “ Of Earths,” being but a short one, 
we shall extract it entire, and thus enable our readers to judge for 
themselves of the merits of the work : — 
“ The crust of this globe, diversified as it appears, is formed of 
a very limited number of earths. These, when pure, are always 
white ; and the varied colours which we observe in minerals are 
owing to the presence of metallic oxides. 
“ The earths of more usual occurrence, and possessing the greatest 
interest, are silica, alumina, lime, magnesia, baryta, and strontia. 
“ Flint is SILICA nearly pure; quartz, still purer; sand, the same 
substance in another form. The amethyst, topaz, and chrysoprase, 
are but coloured varieties. United with either of the fixed alka- 
lis, silica forms glass, which you may make with the blow-pipe 
by fusing together a few grains of sand and a little carbonate of 
soda. In this operation you will observe the soda sink into the 
charcoal. Enough, however, will be left on the surface to vitrefy 
the sand. 
“ In order to render glass more fusible, several substances are 
added in the manufacture, the principal of which is oxide of lead. 
This makes the glass more brilliant, but at the same time more 
soft and liable to scratch. Tubes of this quality are much the 
cheapest, and answer well enough for all ordinary experiments ; 
but we can neither expose them to a high heat, nor attempt in 
them any metallic reduction. For such purposes it is necessary to 
employ hard German glass, which is made with potass, and per- 
fectly free from lead. The use of manganese in glass-making is 
to convert any iron that may happen to be in the materials from 
protoxide to peroxide, and thus to get rid of the dirty green colour 
produced by the former. Hence, manganese is sometimes termed 
glass-soap. Too much of it, as already remarked, produces a 
purplish colour. 
“ When a great excess of alkali has been used, the glass is 
soluble in water ; and from a solution of this kind, left undisturbed 
for several years, small crystals of silica, resembling rock crys- 
tal, have gradually been deposited. Even common glass is slowly 
decomposed by water, more especially at a boiling heat ; and some 
kinds, if finely powdered and placed upon turmeric paper, with a 
few drops of water, will part with enough soda to change its colour. 
In stables and other places, where the air is loaded with ammonia- 
cal vapours, glass is rapidly decomposed. 
“ Glass was known to the ancients. Among the ruins of Rome 
fragments are constantly met with of all colours; and some of them 
have a flower or other pattern running throughout their substance. 
In Egypt also imitations of the amethyst, emerald, and topaz, have 
been discovered. 
