METALS                                                           1212                                            METRIC SYSTEM

in the ores goes into the lead. The greater part of the sil v er produced in the world is extracted by lead in this way. If the ores contain sulphides, they are roasted before they are smelted in the blast-furnace- Coke is the usual fuel, and limestone and iron-ore are generally used as fluxes. The silver which lead contains is usually extracted by dissolving small quantities of zinc in the hot, molten metal. As the metal cools, the zinc becomes solid; then it rises to the surface, bringing the silver with it, and is skimmed off.

Zinc cannot be obtained by ordinary smelting processes, because it boils and is vaporized at the temperature at which it is reduced to the metallic state. This metal is therefore obtained by first roasting the ore, if it is the usual sulphide, and then heating it with coal in retorts made of fire-clay. The zinc distills and is condensed and collected.

A distillation process is also used for obtaining the volatile metal mercury from its ores.

Aluminium is not reduced to the metallic state from its compounds by the ordinary smelting processes. The principal method of producing it consists in passing a powerful electric current through melted cryolite in which aluminium oxide is dissolved.

The processes used for obtaining several other metals are similar to those that have been mentioned.

Metallurgy is a very ancient art that has been gradually developed and improved during historical times, but the greatest improvements were made during the 19th century, and they have been largely due to the assistance afforded by advancing knowledge of engineering and of chemical and physical sciences.                                  Horace L. Wells.

Met'als. See Gold, Silver, Le ad, Iron etc.

Metamorphosis (mĕt'-mr'f-ss), change of form in the life of an animal following the embryo-stage. It may be complete or incomplete; in the former there is change of form and habit, as with toads and frogs; in the latter the newly-hatched young closely resemble the parent, as with grasshoppers, the young differing from adults only in absence of wings. Owing to metamorphosis species are protected. While one form may suffer from certain causes, another form survives and carries on the race. See Larva, Nymph and Pupa.

Meteorology (mě't-ēr-ŏl'o-jŷ), the science which deals with the phenomena of the earth's atmosphere. These phenomena may be grouped under three different heads: aerial phenomena, including winds, cyclones etc. ; aqueous phenomena, as rain, fogs, clouds etc. ; and luminous phenomena, as lightning, the aurora borealis etc. This science, which is of enormous importance to our race, is universally recognized as yet in an embryonic state. For subjects ordinarily grouped under this head the student is referred to such individual articles as Fog, Cloud, Cyclone, Lightning.

Me'teors are small bodies traveling in large numbers and in many directions through space. They are known as aerolites, fireballs and shooting stars, and ma}' be seen every clear night, sometimes few only, but at other times in showers. The whole number which the earth meets in one day's travel is estimated at 7,500,000, but as this large number weighs in all only 100 tons, many of the meteors must be very small. The air acts as a shield, and offers so much frictional resistance that the meteor generally burns. The aerolites are the large masses which actually fall to the earth. Some of them are of iron, some of stone, some of stone and iron. When their fall is noted, there always are a noise, as of an explosion, and a cloud or smoke and a melting of the mass, at least on the surface, showing the action of heat. The iron is combined with nickel, cobalt, copper etc. in a way different from any combination found on the earth, though no new element has been discovered. The falls of aerolites have been more numerous than might be supposed, the British Museum having over 300 specimens of them. The fireballs are brightly-shining bodies seen crossing the sky, and are considered to be aerolites before their explosion and fall. Many hundreds have been observed, Arago giving a list of over 800. They are of all sizes, and travel about 26 miles a second. Shooting-stars may be seen on almost any evening, and 'if carefully watched will seem to come from the same point in the sky. These points are called radiants, and are named for the constellation in which they are found — as the Leonids, a group whose radiant is in the constellation Leo. When there is a meteoric shower, the earth is passing through a group or swarm of these meteors, which are also moving, as the earth does, each in an orbit of its own. The Leonids, which are seen in November, are calculated to move round the sun once in 33 J years, the earth crossing their track every year, but only meeting the main swarm when this reaches the point of crossing at the same time as the earth. When this happens, there is a meteoric shower, such as took place on Nov. 13,1833, when the stars fell like snowflakes and fireballs darted back and forth, making the most wonderful display of the kind ever seen. Astronomers predicted another shower in i860, and it came within a few hours of the time agreed upon. The latest investigations point to a common origin for these meteors and the comets, or rather indicate that meteoric swarms are composed of disintegrated comets. Besides the great November group, other groups are active in August, April, September and October. See Young's General Astronomy.

Me'ter. See Metric System. Met'ric System, The, is an international system of measurement of lengths, surfaces, weights and volumes which was gradually