ALT 
ALTERNATION is used for the differ- 
ent ways which any number of quantities 
may be changed, or combined. See Com- 
bination. 
ALTHAEA, marsh-mallow, in botany, a 
genus of plants, with a double calyx, the 
exterior one being divided into nine seg- 
ments ; the fruit consists of numerous cap- 
sules, each containing a single seed. It be- 
longs to the Monadelphia Polyandria class 
and order. There are nine species. The 
A. officinalis is perennial, and flowers from 
July to September. It grows plentifully in 
salt marshes, and on the banks of rivers and 
ditches, in many parts of England, Holland, 
France, Italy, Siberia, &c. It has been in 
great request in every country where medi- 
cine has been regularly cultivated. All its 
parts abound with a glutinous juice with 
scarcely any smell or peculiar taste. 
ALTIMETRY, denotes the art of mea- 
suring altitudes or heights. See Mensu- 
ration. 
ALTITUDE, in geometry, one of the 
three dimensions of body ; being the same 
with what is otherwise called height. 
Altitude of a figure is the distance of its 
vertex from its base, or the length of a per- 
pendicular let fall from the vertex to the 
base. 
Thales is supposed to have been the first 
person who applied the principles of geo- 
metry to the mensuration of altitude : by 
means of the staff he measured the height 
of the pyramids of Egypt, making the alti- 
tude of the staff and pyramid proportional 
to the length of the shadows. 
Aptitude, in optics, is the height of 
an object above a line, drawn parallel to 
the horizon from the eye of the observer. 
Altitude of the eye, in perspective, 
is its perpendicular height above the geo- 
metrical plane. 
Altitude of a star, Sfc. in astronomy, 
is an arch of a vertical circle, intercepted 
between the stars and the horizon. 
This altitude is either true or apparent, 
according as it is reckoned from the rational 
or sensible horizon, and the difference be- 
tween these is what is called by astrono- 
mers the parallax of altitude. Near the 
horizon, this altitude is always increased by 
means of refraction. 
Altitude of the mercury, in the baro- 
rometer and thermometer, is marked by de- 
grees, or equal divisions, placed by the side 
of the tube of those instruments. The alti- 
tude of the mercury in the barometer in 
and about the metropolis is usually com- 
ALU 
prised between 28.4 and SO. 6 inches : in the 
course of the last seven years it has not va- 
ried from these limits more than twice. 
During the same period, the thermometer 
in the shade has been rarely higher than 82° 
or 83°, and this seldom more than three or 
four times in a whole summer, nor often 
lower than about 8° or 10° below the freezing 
point. This degree of cold is not common, 
ALTO-RELIEVO. See Relievo. 
ALUM, in chemistry and the arts, is 
denominated the sulphate of alumina, though 
it is not merely a combination of alumina 
with the sulphuric acid. It possesses the 
following characters: 1. It has a sweetish 
astringent taste. 2. It is soluble in warm 
water, and the solution reddens vegetable 
colours, which proves the acid to be in ex- 
cess. 3. When mixed with a solution of car- 
bonate of potash, an effervescence is pro- 
duced by the uncombined acid, which pre- 
vents the first portions of alkali, that are ad- 
ded to the solution of sulphate of alumina, 
from occasioning any precipitate. 4. When 
sulphate of alumina is heated, it swells up, 
loses its regular form, and becomes a dry, 
spongy mass ; but according to the experi- 
ments of Vauquelin, the whole of its acid 
cannot be thus expelled. 5. The combi-' 
nation of sulphuric acid with alumina is in- 
capable of crystallizing without an addition 
of potash, which makes a constituent part 
of all the alum of commerce. 6. It is de- 
composed by charcoal, which combines with 
the oxygen of the acid, and leaves the sul- 
phur attached to the alumina. 
Dr, Thomson says there are four varieties 
of alum, all of which are triple salts ; two 
neutral, and two he calls super-salts. These 
are thus denominated : 
1. Sulphate of alumina and potash. 
2. Sulphate of alumina and ammonia. 
3. Super-sulphate of alumina and potash. 
4. Super-sulphate of alumina and am- 
monia. 
The discovery of alum was made in Asia, 
from whence it continued to be imported 
till the end of the fifteenth century, when 
a number of alum works were established 
in Italy. In the sixteenth century it was ma- 
nufactured in Germany and Spain j and dur- 
ing Queen Elizabeth’s reign an alum manu- 
factory was established in England. The alum 
of commerce is usually obtained from native 
mixtures of pyrites and clay, -or sulphuric 
acid and clay. Bergman has published a 
very complete dissertation on the process 
usually followed. The earth from which it 
is procured is usually called aluminous shis- 
