37 
acid. It fuses readily before the blow-pipe, gives off 
water, and at last also sulphuric acid, leaving only pure 
magnesia. It occurs in Andalusia, Siberia, and North 
America, and more sparingly in Germany at Jena, Zeller- 
field, etc., and in Britain at Epsom in Surrey, and Hurlet 
in Renfrewshire. The bitter mineral springs of Saidschiitz 
and Pilna contain epsomite in large quantity in solution, 
and it can be prepared from these waters. Most of the 
Epsom salt of commerce and medicine is prepared from 
the magnesian limestone of Yorkshire. 
XII. SALTS OF AMMONIA. 
Fig. 25.— Salts of Ammonia, Sal Ammoniac, 
Muriate of Ammonia. 
This salt crystallises in regular octahedrons, cubes, and 
leucitoids; it frequently occurs also in twins with wedge- 
shaped distortion, globular, drusy, etc. Colourless or yellow, 
vitreous lustre, hardness 1*5 to 2*0, specific gravity 1*52. 
Volatilises in the matrass, and sublimes ; burns quite away 
before the blow-pipe. That which is colourless is pure 
chloride of ammonium = NH 4 Cl; the yellow variety con¬ 
tains chloride of iron, and is therefore a so-called iron sal- 
ammoniac. It was formerly found very fine in many 
lavas of Vesuvius and Etna, and especially at the burning 
mountain at Duttweiler, as a product of the burning of 
stone-coal. Great masses of sal-ammoniac have also of 
late years occurred at some eruptions of Vesuvius, so that 
the gathering of it was profitable, and the inhabitants were 
in some measure compensated for their losses. When rubbed 
up with hot lime, it gives off ammonia; it is readily soluble 
in water, to which it imparts a sharp saline, somewhat 
piercing, taste. In the preparation of sal-ammoniac, cubes 
are sometimes obtained in the form of twins, as in Fig. 25. 
They are generally rendered impure by animal oil, and 
coloured brown, and in a section they present a beautiful 
six-rayed star. Most of the sal-ammoniac is now artifi¬ 
cially prepared, and obtained as a secondary product in 
the manufacture of prussiate of potass from animal sub¬ 
stances. 
XIII. COMBUSTIBLE SUBSTANCES. 
These are more or less easily' set fire to, and are 
totally or partially consumed, while they take up the 
oxygen of the atmosphere and form liquid acids. Some 
of them are actual minerals, others, such as amber, and 
all kinds of coal, are derived from the vegetable king¬ 
dom. Others like the honey-stone, are compounds 
of an organic acid with an inorganic base; and with 
regard to others, such as the anthracites, it is doubtful 
whether they belong to the vegetable or to the animal 
world. 
PLATE XII. 
Figs. 1—3.—Sulphur. 
Sulphur is a simple element, which principally occurs 
in volcanic districts, associated with gypsum ; frequently 
also combined with metals which it mineralizes, not only in 
the igneous but also in the stratified rocks. 
The natural or native sulphur has a rhombic octahe¬ 
dron for its primary form, and may be split in the direction 
of its planes, as well as in those of the rhombic prism. 
Besides the primary form, we find truncations of the verti¬ 
cal angles with bevelling of the newly-formed basal edges 
(Fig. 1) ; further truncations of the acute pyramidal, and 
of the basal edges combined with the two foregoing pyra¬ 
mids (Fig. 2), double and triple truncations of the .same 
kind, as Fig. 3 ; also rhombic prisms and twins; single 
pairs of planes are frequently extended at the expense of 
the secondary planes, so that oblique forms arise ; there are 
also compact, granular, earthy, and fibrous varieties. Sul¬ 
phur is present in many fresh-water limestones in the 
South of France as a medium of petrifaction, where it fills 
the shells of plcmorbes and paludines , and has probably 
been deposited by sulphureous springs. Crystallised sul¬ 
phur is citron yellow passing into reddish and grey, ada¬ 
mantine to resinous in lustre, transparent to translucent, 
of conchoidal uneven fracture, and strongly lustrous in the 
planes of fracture. The compact variety is yellow, grey, 
and even brown, frequently presents but little lustre, and 
is generally rendered impure by bitumen. The crystals 
are always harder (= 2*5) than the compact (= 1*5), the 
specific gravity is 2*0. If crystallised sulphur is heated in 
the hand, it often crepitates and makes numerous leaps, 
and takes on polar electricity. The chemical constitution 
is pure sulphur = S ; it is sometimes, however, made impure 
by the pressure of bitumen, selenium, and arsenic. In the 
heated retort it melts readily at 226° Fahr., and crystallises 
on cooling in oblique rhombic prisms, which afterwards 
become opaque. When raised to a higher temperature, it 
becomes agitated and volatilises, and condenses on the cold 
part of the tube as a loose yellow powder (flour of sulphur). 
When heated for a long time and plunged into water, it 
forms a tough mass, which is plastic for a considerable 
period. It is insoluble in water and alcohols; it is soluble 
in bi-sulphate of carbon, and crystallises from the solution 
in rhombic octahedrons. Heated with nitric acid it forms 
sulphuric acid. In hot fatty oils it is also soluble, and in 
potash and soda ley. It is very inflammable in the atmo¬ 
sphere, and burns with a bluish flame, and gives off a suffo¬ 
cating vapour of sulphureous acid (S). If this gas is collected 
L 
