AM A 
ALU 
distance between which at one extremity is 
0.5 inch, and the other extremity 0.3 inch; and 
the rides are exactly 24.0 indies in length, and 
divided into 240 equal parts, called degrees. 
These degrees commence at the widest end 
of the scale. The first of them indicates 
a red heat, or 947° Fahrenheit. The clay- 
pieces are small cylinders, baked in a red 
heat, and made so as to fit 1° of the scale. 
They are. not composed of pure alumina, 
but of a fine white clay. Alumina is scarcely 
soluble in water; but may be diffused 
through that liquid with great facility. Its 
affinity for water, however, is very consider- 
able. In its usual state it is combined with 
more than its own weight of water, and we 
have seen with what obstinacy it retains it. 
Even this combination of alumina and water 
is capable, in its usual state of dryness, of 
absorbing 2A times its weight of water, with- 
out suffering any to drop out. It retains this 
water more obstinately than any of the 
earths hitherto described. In a freezing 
cold it contracts more, and parts with more 
of its water, than any other earth ; a cir- 
cumstance which is of some importance in 
agriculture. Alumina has no effect upon 
vegetable' blues. It cannot be crystallized 
artificially; but it is found native in beautiful 
transparent crystals, exceedingly hard, and 
having a specific gravity of 4. It is distin- 
guished in this state by the name of sapphyr. 
It does not combine with metals ; but it has 
a strong affinity for metallic oxydes, espe- 
cially for those oxydes which contain a maxi- 
mum of oxygen. Some of these compounds 
are found native. Thus, the combination 
of alumina and red oxyde of iron often oc- 
curs in the form of a yellow powder, which 
is employed as a paint, and distinguished by 
the name of ochre. There is a strong affi- 
nity between the fixed alkalies and alumina. 
When heated together, they combine, and 
form a loose mass, without any transparency. 
Liquid fixed alkali dissolves alumina by the 
assistance of heat, and retains it in solution. 
The alumina is precipitated again unaltered, 
by dropping an acid into the solution. This 
is a method employed by chemists to pro- 
cure alumina in a state of complete purity ; 
for alumina, unless it be dissolved in alkali, 
almost always retains a little oxyde of iron 
and some acid, which disguise its properties. 
Liquid ammonia is also capable of dissolving 
a very minute proportion of newly preci- 
pitated alumina. Barytes and strontian also 
combine with alumina, both when heated 
with it in a crucible, and when boiled with 
it in water. The result, in the first ease, is 
a greenish or bluish-colouredmass, cohering 
but imperfectly : in the second, two com- 
pounds are formed ; the first, containing an 
excess of alumina, remains in the state of an 
insoluble powder; the other, containing an 
excess of barytes or strontian, is held in 
solution by the water. Alumina has a strong 
affinity for lime, and readily enters with it 
into fusion. None of the earths is of more 
importance to mankind than alumina; it 
forms the basis of china and stone-ware of 
all kinds, and of the crucibles and pots era- 
• ployed in all those manufactures which re- 
quire a strong heat. It is absolutely neces- 
sary to the dyer and calico-printer, and is 
employed too with the greatest advantage 
by the fuller and cleaner of cloth. 
ALURNUS, in natural history, a genus 
of insects of the order Coleoptera. Essen, 
character : antennas filiform, short • feelers 
four to six, very short : jaw liorney, arched. 
There are three species — A. grossus, an in- 
habitant of South America and India : A. 
femoratus, found in India : and A. dentipes, 
found at the Cape of Good Hope. 
ALYSSO, or Ai.yssum, mad-worl, in 
botany, a genus of the Tetradynamia Esscu- 
losa class of plants ; the flower is of the cru- 
ciform kind, and consists of four leaves : the 
fruit is a small roundish capsule, divided in- 
to two cells, in which are contained a num- 
ber of small roundish seeds. 
The alyssum is arranged in three divi- 
sions, viz. into A . in which the stem is 
somewhat shrubby : B. stems herbaceous: 
C. silules inflated, or calyx oblong, closed. 
There are 33 species; but according to 
Martyn only 17. All the species may be 
propagated by seed, and most of them by 
slips and cuttings. In rich ground they 
seldom live through the winter in England ; 
but in dry, poor, rubbishy soil, or on old walls, 
they will abide the cold, andlastmuchlonger. 
AMALGAM, in the arts. The metals in 
general unite very readily with one another, 
and form compounds; thus pewter is a com- 
pound of lead and tin, brass is a compound 
of copper and zinc, &c. These are all called 
alloys, except when one of the combining 
metals is mercury ; in that case the' com- 
pound is called an amalgam: thus mercury 
and gold form a compound called the amal- 
gam of gold. 
The amalgam of gold is formed very 
readily, because there is a very strong affi- 
nity between the two metals. If a bit of 
gold be dipped into mercury, its surface, by 
combining with mercury, becomes as white 
as silver. The easiest way of forming this 
