te for the _Revue musicale_ a series of articles entitled
_Acoustique fondamentale_ (1831), containing an ingenious, but now
exploded, theory of the vibration of the air. His other works are: _Systeme
universel_ (8 vols., 1812); _Du Sort de l'homme_ (3 vols., 1820); _Cours de
philosophie_ (8 vols., 1824), reproduced as _Explication universelle_ (3
vols., 1826-1828); _Jeunesse, maturite, religion, philosophie_ (1837), _De
la phrenologie, du magnetisme, et de la folie_ (1843).

AZALEA, a genus of popular hardy or greenhouse plants, belonging to the
heath order (Ericaceae), and scarcely separable botanically from
_Rhododendron_. The beautiful varieties now in cultivation have been bred
from a few originals, natives of the hilly regions of China and Japan, Asia
Minor, and the United States. They are perhaps unequalled as indoor
decorative plants. They are usually increased by grafting the half-ripened
shoots on the stronger-growing kinds, the shoots of the stock and the
grafts being in a similarly half-ripened condition, and the plants being
placed in a moist heat of 65deg. Large plants of inferior kinds, if
healthy, may be grafted all over with the choicer sorts, so as to obtain a
large specimen in a short time. They require a rich and fibrous peat soil,
with a mixture of sand to prevent its getting water-logged. The best time
to pot azaleas is three or four weeks after the blooming is over. The soil
should be made quite solid to prevent its retaining too much water. To
produce handsome plants, they must while young be stopped as required.
Specimens that have got leggy may be cut back just before growth commences.
The lowest temperature for them during the winter is about 35deg, and
during their season of growth from 55deg to 65deg at night, and 75deg by
day, the atmosphere being at the same time well charged with moisture. They
are liable to the attacks of thrips and red spider, which do great mischief
if not promptly destroyed.

The following are some well-known species:--_A. arborescens_
(Pennsylvania), a deciduous shrub 10-20 ft. high; _A. calendulacea_
(Carolina to Pennsylvania), a beautiful deciduous shrub 2-6 ft. high, with
yellow, red, orange and copper-coloured flowers; _A. hispida_, a North
American shrub, 10-15 ft. high, flowers white edged with red; _A. indica_
(China), the so-called Indian azalea, a shrub 3-6 ft. or more high, the
original of numerous single and double varieties, many of the more vigorous
of w