AERATED WATERS.

into use in large cities. In ordinary bread-making, upper division. The powders, consisting of bicarwheat-flour is moistened with water and worked bonate of soda (NaO,HO,2CO,) and tartaric acid (T), into dough, to which common salt and yeast are are then placed in the upper globe by means of the added. The latter causes the flour to ferment or small funnel D, and care is taken, by plugging up decompose, when carbonic acid is given off at every the tube communicating with the lower part by the part; and when the fermented dough is placed in stopper F, that no powder passes into the larger an oven, the bubbles of carbonic acid gas expand, globe. The long tube, C, is then inserted into the and cause the formation of the spongy mass char- globes, and screwed well in. The apparatus is acteristic of good loaf-bread. The process of prepar- inclined till water from the lower globe enters and ing A. B. consists in placing the flour in a strong fills the upper globe about one-third; then it is enclosed iron box, and moistening it with carbonic placed erect, and allowed to be at rest for two hours, acid water, prepared as stated under A. WATERS. when, if the screw stop-cock at the upper part be The dough is then worked up by machinery for opened, the carbonated water will flow out readily ten minutes or so inside the box, from which it is into any vessel placed to receive it. The explanadropped into moulds, which form it into loaves. It tion of the action which goes on in the vessel is, is then placed in an oven, when the carbonic acid, that tartaric acid and bicarbonate of soda have previously introduced with the water within the no action on each other so long as they are dry; dough, expands, and forms a light palatable bread. but whenever water is admitted, the tartaric acid The advantages which this method of working combines with the soda and water to form tartrate bread possesses are-1. There is a saving of the of soda and water (NaO,HO,T), and at the same whole of the waste caused by fermentation, which time, carbonic acid (CO) is given off, and descendadmits of more bread being made out of a sack of ing the tube into the lower globe, dissolves in the flour than by the old process. 2. The process, water contained therein. Occasionally, bisulphate instead of occupying eight or ten hours, is com- of potash is used instead of the tartaric acid, to save pleted in half an hour. 3. The cost of machinery the greater expense of the latter.-The gazogènes can and gas is less than that of yeast used in the old likewise be used in the preparation of true sodaprocess. 4. The dough requires no handling to water, or Eau de Vichy, by adding a little carknead it, and form it into loaves. 5. The bread bonate of soda to the water in the lower globe before is absolutely pure-it is simply flour, water, and charging with carbonic acid. A. wine may be salt. Finally, should the whole of the bread in obtained by placing white wine with a little sugarthe kingdom be thus made, a very considerable candy in the lower globe instead of water. Sparksaving would be effected in the consumption of flour. See UNFERMENTED BREAD.

ling lemonade is procured when the carbonic acid water is run into a tumbler containing a little sirup of sugar; and A. fruit-beverages, when the water charged with carbonic acid is received in a glass containing about a table-spoonful of either of the fruit-sirups.

AERATED WATERS are employed largely as refreshing, refrigerant beverages to allay thirst during warm weather, and during feverish conditions of the animal frame. The most common A. beverage is Carbonic acid water, generally spoken of as SodaThe less common A. W., prepared on the large water, though it seldom contains any soda. It is pre-scale, are-1. A. soda-water (true soda-water), obtained pared on the large scale by placing whiting, chalk, or carbonate of lime (CaO,CO,) in a lead vessel

with water and sulphuric acid (SO,), when the sulphuric acid combines with the lime to form stucco or sulphate of lime (CaO,SO,), and carbonic acid (CO2) is evolved as gas. The latter is received in a reservoir, and is thereafter forced into water, so that the latter dissolves about five times its own volume of the gas. The water then constitutes a brisk sparkling liquid, with a pungent, but pleasant acidulous taste. On the small scale, and for family use, carbonic acid water may be conveniently prepared in the apparatus known as the gazogène or seltzogène. The complete apparatus is seen at A, and dissected at B and C. In proceeding to use the vessel, the lower globe at B is filled with water by means of the long funnel E, taking care that no water runs into the smaller and

by adding 15 grains of crystallised carbonate of soda to each bottle before it is charged with the carbonic acid water; 2. A. potash-water, by employing in a similar way 20 grains of bicarbonate of potash; 3. A. Selters-water, when carbonate of soda and chloride of sodium (common salt) are dissolved in carbonic acid water; 4. A. Carrara-water, when finely divided Carrara marble is dissolved in the acid-charged water; 5. A. lime-water, when other forms of lime than the Carrara marble are used; 6. A. magnesian-water, when magnesia, or the carbonate of magnesia, is used; and A. chalybeate-water, when a compound of iron is dissolved in the carbonic acid water. The latter beverage has lately been employed in medicine, as an easy means of introducing iron into the blood, and with good effect. A. Carrara and lime waters are now administered in cases where the bony structure requires to be strengthened; and A. magnesia-water is a very agreeable mode of giving a patient a dose of magnesia. The well-known effervescing draughts called soda-powders, and seidlitz-powders, are two other kinds of A. drinks. In the former, bicarbonate of soda and tartaric acid are added to water in a tumbler, and a refreshing draught instantaneously prepared. Seidlitz-powders contain tartrate of soda and bicarbonate of soda in one paper, and tartaric acid in the other; and when both are added to water, effervescence ensues, and the liquid is then partaken of.

A. W. likewise occur naturally. Water, as it is drawn from a spring, tastes differently from the same water after being boiled and cooled; and this is due to the unboiled water containing the gases oxygen, nitrogen, and carbonic acid-especially the latter-dissolved in it. Spring-water is therefore a natural A. beverage. Rain-water has a mawkish,

AERIAL POISONS-AEROLITES.

insipid taste, mainly because of the minute quantity phers as instances of popular credulity and superof gas therein dissolved; but when that rain-water stition. It was not till the beginning of the 19th c. trickles down the mountain-side, and is dashed that the fact was established beyond a doubt.from ledge to ledge of rock, it absorbs and dis- According to Livy, a shower of stones fell on the solves the gases from the air, and is thus naturally Alban Mount, not far from Rome, about 654 B.C. aërated. Many waters are aërated in a natural but The fall of a great stone at Egospotami, on the peculiar way, which confers upon them important Hellespont, about 467 B. C., is recorded in the Parian medicinal properties; and these will come before us Chronicle (q. v.), and mentioned by Plutarch and under their more popular title of Mineral Springs. Pliny. It was still shewn in the days of Pliny AERIAL POISONS. See MIASMA. (d. 79 A. D.), who describes it as of the size of a AERODYNAMICS is that branch of science A. D., a ponderous stone, weighing 260 lbs., fell from wagon, and of a burned colour. In the year 1492 which treats of air and other gases in motion. It the sky near the village of Ensisheim, in Alsace; examines first the phenomena of air issuing from a vessel, which correspond in many respects with those part of it is still to be seen in the village church. of water. See HYDRODYNAMICS. Much depends, as in Normandy, on the 26th April 1803. An extraordinary shower of stones fell near L'Aigle, The celein the case of water, upon the nature of the orifice, brated French philosopher, M. Biot, was deputed by whether a mere hole in the side of the vessel, or a tube or adjutage. Another subject of A. is the motion authentic facts; and since the date of his report, the government to repair to the spot and collect the of air in long tubes, where the resistance of friction, reality of such occurrences has no longer been ques&c., has to be ascertained. That resistance is found tioned. Nearly all the inhabitants of a large district to be nearly in proportion to the square of the velo- had seen the cloud, heard the noises, and observed city, to the length of the tube, and inversely to its the stones fall. Within an elliptical area of seven width. A. examines also the velocity of air rushing miles by three, the number of stones that had fallen into a vacuum, of wind, &c. The instrument used could not be less than two or three thousand; the for the latter purpose is called an anemometer. See largest were 17 lbs. in weight. These are only a few WINDS. Air is found to rush into a void space at out of hundreds of instances on record. the rate of from 1300 to 1400 feet per second. One of the most important inquiries in A. is the resistance offered to a body moving in air, or which is the same thing-the pressure exerted by air in motion upon a body at rest. The law may be stated, with sufficient accuracy for practical purposes, as follows: The resistance or pressure is proportional to the square of the velocity. We might conclude from reason, without experiment, that such would be the case; for if one body is moving through the air four times faster than another of the same size, not only will it encounter four times as many particles of air, but it will give each of them four times as great an impulse or shock, and thus encounter 4 x 4, or sixteen times as much resistance.

This resistance is greatly increased by another circumstance, especially with great velocities. The air in front of the moving body becomes accumulated or condensed, and a partial or even entire vacuum is formed behind it. With a velocity of 1700 feet per second, for instance, the resistance is found to be about three times as great as the simple law of the square of the velocity would give. By the operation of these laws of resistance, a heavy body let fall with a parachute attached to it, comes, after a certain time, to move with a velocity approaching more and more nearly to a uniform motion.

A'ËROLITES (Gr. aer, air, and lithos, stone), or METEORIC STONES, FIREBALLS, and SHOOTING-STARS, are now classed together as being merely varieties of the same phenomenon. Aërolites that fall during the day, are observed to be projected from a small dark cloud, accompanied by a noise like thunder, or the firing of cannon; at night, they proceed from a fireball, which splits into fragments with a similar sound. It is believed that the dark cloud that accompanies the fall of aerolites by day, would be luminous at night; and smoking, exploding fireballs have sometimes been seen luminous even in the brightness of tropical daylight. The connection between aërolites and fireballs is thus established. Fireballs, again, cannot be separated from shootingstars, the two phenomena being sometimes blended, and also being found to merge into one another, both with respect to the size of their disks, the emanation of sparks, and the velocities of their motion.

There are numerous records and stories in all ages and countries of the fall of stones from the sky; but until recent times, they were treated by philoso

As was 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, have all the same history.

The existence of such bodies once admitted, led to assigning a meteoric character to strange ferruginous masses found in different countries, and which had no history, or were only adverted to in vague tradition. Of this kind is the immense mass seen by Pallas in Siberia, now in the Imperial Museum at St Petersburg. The largest known is one in Brazil, estimated at 14,000 lbs.

One constant characteristic of meteoric stones is the fused black crust, like varnish, with which the surface is coated. From the circumstance of this coat being very thin, and separated from the inner mass by a sharply defined line, it is thought to indicate some rapid action of heat, which has not had time to penetrate into the substance of the stone. This view is favoured by the fact that the stones are found in a strongly heated, but not incandescent state, when they fall. Their specific gravity ranges from two to seven or even eight times that of water.As to their chemical composition, the predominating element is iron, in a native or metallic state, generally combined with a small proportion of nickel. According to Humboldt, the aerolites that fell in the neighbourhood of Agram, in Croatia, in 1751, the Siberian stone, and specimens brought by that philosopher from Mexico, contain 96 per cent. of iron; while in those of Sienna the iron scarcely amounts to 2 per cent., and, in some rare instances, metallic iron is altogether wanting. A writer in the Quarterly Review, No. CLXXXIII., thus sums up the result of all the chemical analyses hitherto made:

We find the actual number of recognised elements discovered in aërolites to be nineteen or twentythat is, about one-third of the whole number of elementary substances (or what we are yet forced to regard as such) discovered on the earth. Further, all these aërolitic elements actually exist in the earth, though never similarly combined there. No new substance has hitherto come to us from without; and the most abundant of our terrestrial metals, iron, is that which is largely predominant in aërolites, forming frequently, as in some of the instances just

AEROLITES.

aërolites. A few there are, especially examined by Berzelius and Rose, containing olivine, augite, hornblende, and other earthy minerals; and closely resembling certain crystalline compounds which we find on the surface of the earth.'

mentioned, upwards of 90 parts in 100 of the mass. Seven other metals-copper, tin, nickel, cobalt, chrome, manganese, and molybdena-enter variously into the composition of these stones. Cobalt and nickel are the most invariably present; but the proportion of all is trifling compared with that of Besides those solid masses of considerable size, iron. Further, there have been found in different numerous instances are on record of showers of dust aerolites, six alkalies and earths-namely, soda, over large tracts of land; and it is remarkable that potash, magnesia, lime, silica, and alumina; and, in such dust has generally been found to contain small addition to these, carbon, sulphur, phosphorus, and hard angular grains resembling augite. Stories of hydrogen. Finally, oxygen must also be named as the fall of gelatinous masses from the sky are a constituent of many aërolites, entering into the ranked by Humboldt among the mythical fables of composition of several of the substances just men- meteorology. It has been supposed that such fables tioned. As respects the manner of conjunction of may have originated in the very rapid growth of these elements, it is exceedingly various in different gelatinous algæ, as Nostoc (q. v.).

Fireballs and Shooting-stars. From the height | swarms are periodic, or recur on the same days of and apparent diameter, the actual diameter of the the year. Attention was first directed to this fact largest fireballs is estimated by Humboldt to vary on occasion of the prodigious swarm which appeared from 500 to 2800 feet; others allow a diameter of in North America between the 12th and 13th of about a mile. Shooting-stars are much smaller, their November 1833, described by Professor Olmsted of weight varying from 30 grains to 7 lbs. In most Newhaven. The stars fell on this occasion like cases of luminous meteors, a train of light many flakes of snow, to the number, as was estimated, of miles in length is left behind. One or two instances 240,000 in the space of nine hours, and varying in are on record where the train of the fireball con- size from a moving point or phosphorescent line to tinued shining for an hour after the body disap- globes of the moon's diameter. The most important peared. The heights of shooting-stars are found to observation made was, that they all appeared to range from 15 to 150 miles, at the points at which proceed from the same quarter of the heavens-the they begin and cease to be visible. Their velocities vicinity, namely, of the star y, in the constellation vary from 18 to 36 miles in a second. When it is Leo; and although that star had changed greatly remembered that the velocity of Mercury in its orbit its height and azimuth during the time that the is 26.4 miles in a second, of Venus 19-2, and of the phenomenon lasted, they continued to issue from the Earth 164, we have in this fact a strong confirmation same point. It was afterwards computed by Encke, of the planetary nature of meteorites. that this point was the very direction in which the earth was moving in her orbit at the time. Attention being directed to recorded appearances of the same kind, it was observed with surprise that several of the most remarkable had occurred on the same day of November, especially that seen by Humboldt at Cumana in 1799, and by other observers over a

One of the most remarkable facts connected with shooting-stars is, that certain appearances of them are periodic. On most occasions they are sporadic that is, they appear singly, and traverse the sky in all directions. At other times, they appear in swarms of thousands, moving parallel; and these

AERONAUTICS—AEROSTATIC PRESS.

ESCHINES-ESOP.