108 
THE PRINCIPAL FLOOR. 
other calcium borates. The applications of the borates are well 
illustrated by a collection in the Gallery of Model Room B. 
Of the salts of potassium only a small number occur native. 
The chloride was an extremely rare species until found some 
years ago in the salt deposits of Stassfurt in Prussia. Some fine 
crystals of this mineral, known as Sylvine, are here exhibited. 
The nitrate, called Nitre or saltpetre, is of considerable value, 
being extensively employed in the manufacture of gunpowder. 
Alum is placed here as a salt containing, in its typical form, 
potash and alumina in combination with sulphuric acid and 
water. Such at least is the composition of ordinary alum, but 
in other varieties the potash may be replaced by soda or 
ammonia; indeed of late years the manufacture of ammonia- 
alum has largely superseded that of the potash salt. Alum was 
long manufactured on a large scale from the alum slate or shale 
of the upper lias of Whitby. The shale in various stages of 
decomposition, and some fine alum crystals are here shown. 
Wehsterite, a sulphate of aluminium, discovered by Thomas 
Webster at Newhaven, in Sussex, is represented by various 
specimens which have gained admission here by their rather 
remote relationship to the alum group. 
An interesting series of minerals from the salt-deposits of 
Stassfurt is placed in this Case. Borings for rock-salt in the 
neighbourhood of Stassfurt, near Magdeburg, in Prussia, revealed 
the existence., some years ago, of vast deposits of various salts of 
potassium overlying the sodium salts. This represents precisely 
the order in which they would crystallise from a solution 
containing both kinds of salts ; the salts of sodium being less 
soluble than those of potassium and magnesium would separate 
first, leaving the more soluble compounds in the mother-liquor. 
In this section are also placed a few minerals containing 
magnesium, such as Brucite and Epsomite. By the action of 
hydrochloric acid on carbonate of magnesium there is obtained 
a chloride, from which the metal is reduced by the action of 
sodium. Examples of the metal will be found in Case 
No. 35 (p. 57). 
Case G. — Salts of Barium and Strontium. 
The very widely-diffused mineral Barytes, or sulphate of 
barium, is frequently employed as a pigment, either alone or 
associated with white lead ; and for this purpose the mineral is 
raised in Shropshire and in Ireland. From its density barites is 
commonly known as heavy spar ; while the massive earthy 
varieties often pass under the name of cawk. 
Witherite, a carbonate of barium named after Dr. Withering, 
occurs in remarkably fine crystals near Hexham, in Northum- 
berland, where it is worked commercially. 
The two species of haryto-calcite and alstonite are double 
salts, being carbonates of barium and calcium. 
