104 REVIEW — AN EASY INTRODUCTION TO CHEMISTRY. 
“ Although silica is the most abundant ingredient in clay, yet 
the peculiar plasticity of that substance is due to the large admix- 
ture of another earth, viz. ALUMINA. This, when pure, is infusible 
in any furnace; but, when mixed with silica, either artificially or 
(as in clay) naturally, it undergoes an imperfect fusion, and forms 
porcelain or pottery, differing in quality according to the purity of 
the ingredients employed. Common clays generally contain iron ; 
and from the oxide of this metal bricks derive their red colour. For 
the best kind of porcelain no sufficiently pure clay can be found 
among, what are termed by geologists, the secondary strata ; and 
recourse must always be had to the primitive rocks, which furnish 
a perfectly white material, derived from the spontaneous decom- 
position of felspar. Clay fit for such uses should form with water 
a tenacious mass, soft enough to receive the minutest impressions, 
and so devoid of elasticity as to retain them unaltered in drying, 
and even up to any degree of heat. The glazing used for earthen- 
ware is of various kinds : the best is finely-powdered felspar ; but 
coarse articles are generally glazed with a mixture of silica and 
oxide of lead. Porcelain jasper is a natural production, which owes 
its form to the action of subterranean fires. The ruby and sapphire 
consist chiefly of alumina. 
. “ Lime, which is another of the earths, may be obtained in a 
tolerable state of purity by burning either chalk, limestone, or 
marble. Its usefulness as an ingredient in mortar, and also as a 
dressing for land, is too well known to need more than a passing 
allusion. When ignited it shines with a very bright light, which 
may be seen by directing the flame of a blow-pipe upon a fragment 
of chalk. Lime is slightly soluble in water, but the solution, though 
clear at first, is gradually decomposed by the air, and the lime falls 
down in the form of chalk. The addition of sugar materially in- 
creases the solubility of lime. Steel goods, dipped into lime-water 
and dried, are said to become less liable to rust. Marls are clays 
which contain chalk, and may be recognised by the effervescence 
which ensues when vinegar is poured over them. When a little 
water is added to quick-lime they unite with great heat, the water 
is solidified, and the lime slacked. Compounds of this nature, with 
which water enters as a constituent part, are termed HYDRATES : 
thus, slacked lime is a hydrate of lime. 
“ MAGNESIA is found native in white masses, and, in combination 
with the sulphuric and muriatic acids, exists abundantly in sea 
water. Sometimes it is contained in limestone; in which case the 
latter is unfit for manure ; because magnesia, instead of slacking 
as lime does, remains caustic and injures vegetation. Magnesia is 
insoluble in water. 
“ Of BARYTA and Strontia nothing more need be said than 
