plants they are most abundant, and largest in the green parts, because the gas exchanges in food-making, photosynthesis, exceed those in respiration. In water plants, however, whose opportunity for securing gases from the air is more limited, the aërating system reaches its highest development. (See Hydrophytes.) The tissues may present to the eye a spongy appearance, and in some cases the canals in stems and leafstalks may even be large enough to be easily seen with the naked eye (fig. 2). The internal atmosphere pervading these canals is voluminous enough to permit both considerable exchanges between it and the adjacent tissues and the freer diffusion of gases entering from the restricted area of organs exposed to the air. The composition of the internal atmosphere is always different from that of the outer and varies from time to time. During the day the internal atmosphere contains less carbon dioxide and more oxygen than the external atmosphere, at night the reverse being the case. At all times the internal atmosphere contains a larger amount of water vapor, because the wetness of the cell-wall, which is necessary for gaseous exchanges, permits evaporation. See Transpiration.
A'ËRA'TOR (Literally, "airer," from Lat. aër; air). In dairying, an apparatus for aërating milk to remove the animal and barn odors. The milk is usually caused to run or ripple in a thin layer over an exposed surface, being, in many forms of apparatus, cooled at the same time. In some forms of apparatus the cooling is effected by the use of ice; in others, ice water or cool water from wells or springs is used. The object of cooling milk is to remove the animal heat from it as soon as possible after it is milked, so that the conditions may be less favorable to the growth of the micro-organisms which cause souring and other changes. Cream also is often cooled when it is to be sold for household purposes, especially separator cream. Cooling is, further, a very important step in the process of pasteurizing milk or cream. The practice of aërating and cooling is comparatively new.
AËRENCHYMA, a'er-en'ki-nui (Gk. i?/p,
aci; air + iyxofM, cncht/nia, infusion, in the
sense of a tissue). In plants, a loose, spongy
tissue, which is especially connnon in water
plants (see Hydrophytes), and which is sup-
posed to facilitate aeration — whence the name.
Aerenchyma is typically composed of more or less
radially arranged arms of thin-walled cells in-
closing large air spaces. For illustration, see
Aëration, fig. 2.
AË'RIAL FAU'NA. See Distribution of Animals.
AERIAL PLANTS AND ROOTS. See Epiphytes; Roots.
AËRIAL POI'SONS. See Miasma.
AËRIANS, a-e'ri-anz. A Christian sect founded ill the fourth century by Aërius of Pontus. He opposed prayers for the dead and the keeping of Easter and all set fasts, and asserted the equality between a bishop and a presbyter. John Glas (q.v. ) wrote a scholarly monograph on the so-called heresy of Aërius (Perth, 1745), which so strikingly antedates Reformation doc-
trine.
A'ËROCLI'NOSCOPE (Gk. ἀήρ, aēr, air + «/ia'f/r, l.liiiriii. to incline + ahotiih', s-kopcin, to watch, examine). An instrument invented liy Buys Ballot and used in Holland and elsewhere as a storm signal. It consists of a vertical axis, turning on a pivot, and carrying at the top a horizontal arm whose inclination can be varied. One end of this arm is painted red and the other white, and when weather conditions are normal it rests at a horizontal position. In ease of falling barometer the arm is rotated so that the red end points in the direction of the storm, the amount of inclination indicating the degree of change in the barometer.
A'ËRODYNAM'ICS (Gk. ἀήρ, aēr, air +
ivvaftig, dynamis, power). That branch of sci-
ence which treats of the properties of air and
other gases in motion. It is, therefore, a branch
of pneumatics.
A'ËROLITE (Gk. ἀήρ, aēr, air + λίθος, lithos, stone). Meteoric Stone, Fireball, Uranolith, or Shooting-Star. A solid body reaching the earth from unknown points beyond the earth's atmosphere. When seen at night. aërolites usually consist of a luminous head or fireball, followed by a bright train of incandescent matter. Sometimes there are visible explosions, and even loud detonations are occasionally heard. In the daytime the light of both fireball and train is largely lost against the sky background; it is said, however, that visible clouds at all times replace the luminous train.
There are numerous records and stories in all ages and countries of the fall of stones from the sky, but until comparatively recent times they were treated by scientific men as instances of popular credulity and superstition. It was not till the beginning of the nineteenth century that the fact was established beyond a doubt. According to Livy, a shower of stones fell on the Alban mount, not far from Rome, about 654 B.C. The fall of a great stone at Ægospotami, about 407 B.C., is recorded in the Parian Chronicle (see Arundel Marbles), and by Plutarch and Pliny. It was still shown in the days of Pliny (died 79 A.D.), who describes it as the size of a wagon and of a burned color. In the year 1492 A.D. a ponderous stone weighing 260 pounds fell from the sky near the village of Ensisheim, in Alsace; part of it is still to be seen in the village church. An extraordinary shower of stones fell near L'Aigle, in Normandy, on April 26, 1803. The celebrated French physicist. M. Biot, was deputed by the government to repair to the spot and collect the authentic facts, and since the date of his report the reality of such occurrences has no longer been questioned. Nearly all the inhabitants of a large district had seen the cloud, heard the noises, and observed the stones fall. Within an elliptical area of seven miles by three, the number of stones that had fallen could not be less than two or three thousand; the largest were seventeen pounds in weight. These are only a few out of hundreds of instances on record.
As is natural with objects of such mysterious origin, meteoric stones have always been regarded with religious veneration. At Emesa, in Syria, the sun was worshipped under the form of a black stone, reported to have fallen from heaven. The holy Kaaba of Mecca, and the great stone of the pyramid of Cholula, in Mexico, both have a similar history. The existence of such bodies once admitted led to assigning a meteoric character to strange ferruginous masses found in different countries, and which
