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EARTHQUAKE ; a shaking of certain parts of the earth's surface, produced by causes not perceivable by our senses. This motion occurs in very different ways, and in various degrees of violence. Sometimes it is perpendicular, throwing portions of the ground into the air, and making others sink. Sometimes it is a horizontal, undulating motion, and sometimes it appears to be of a whirling nature. Sometimes it is quickly over; sometimes continues long, or recurs at intervals of weeks, days or months. At one time, it is confined within a small circle ; at another, it extends for many miles. At one time, it is hardly perceptible ; at another, it is so violent, that it not only demolishes the works of human art, but changes the appearance of the ground itself. Sometimes the surface of the ground remains unbroken; sometimes it bursts open into clefts and chasms; and then occasionally appears the phenomenon of the eruption of gases, and also of flames, with the ejection of water, mud and stones, as in volcanic eruptions. The eruptions of proper and permanent volcanoes are preceded by, and proportionate to, the agitations of the earth in their neighborhood. These observations furnish grounds for the conclusion, that earthquakes cannot proceed from external causes, but arise from certain powers operating within the circumference or crust of the earth. Moreover, all the phenomena of earthquakes bear so much affinity to those of volcanoes, that there can hardly be a doubt, that both proceed from the same causes, acting differently, according to the difference of situation, or different nature of the surface on which they operate. A volcano differs from an earthquake, principally, by having a permanent crater, and by the reappearance of the eruptions in the same place, or in its immediate vicinity. All the other phenomena of a volcano, such as tViti cnKtorranonn tlinnrlorlilro nniet>a tVia same chemical process its seat at a great deptl ent surface of the < remarkable earthquake are those which destro and Lisbon, in 1755; persons were killed. Greenland to Africa similar fate befell Call province of Caracas, in 1812, and Aleppo, Several earthquakes '. quite lately, in South ticularly dreadful at L Guatemala, also, was n the spring of 1830, by < continued five days sue EARTHS. The term common life, to denote ous, diy, uninflammabli substance, which is din of a moderate specific of the earths are found in nature; but their occurrence is in intima other, and with various oxides. Under these c constitute by far the g strata, gravel and soil, up the mountains, vali our globe. Their numi names are silex, alumi barytes, strontites, zircon, thorina. The four fin known to mankind; tl been discovered in our exists nearly pure, in lar entire rocks, as quartz i ing the chief ingredi rocks and sandstones, s be asserted to form moj the crust of the earth, pure in two or three minerals, but, in a mi known as forming clay; ly of rocks, usually \ jngly rare. The earths are very similar to the alkalies (q. v.), forming, with the acids, peculiar salts, and resembling the alkalies likewise in their composition. They consist of peculiar metals in combination with oxygen, and compose the greatest part of the solid contents of the globe. They differ from the alkalies principally in the following peculiarities : they are incombustible, and cannot, in their simple state, be volatilized by heat; with different acids, especially the carbonic, they form salts, insoluble, or soluble only with much difficulty, and with fat oils, soaps insoluble in water. They are divided into two classes, the alkaline and proper earths. The former have a greater similarity to the alkalies. In their active state, they are soluble in water, and these solutions may be crystallized. They change the vegetable colors almost in the same way as alkalies, and their affinity for acids is sometimes weaker and sometimes stronger than that of the alkalies. They combine with sulphur, and form compounds perfectly similar to the sulphureted alkalies. With carbonic acid, they form insoluble salts, which, however, become soluble in water by an excess of carbonic acid. The alkaline earths are as follows: 1. barytes, or heavy earth, so called from its great weight; 2. strontites \q, v.); both these earths are counted among the alkalies, by many chemists, on account of their easy solubility in water;3. calcareous earth, or lime, forms one of the most abundant ingredients of our globe;4. magnesia is a constituent of several minerals. The proper earths are wholly insoluble in water, infusible at the greatest heat of our furnaces, and, by being exposed to heat, in a greater or less degree, they lose their property of easy solubility in acids. Some of them are incapable of combining with carbonic acid, and the remainder form with it insoluble compounds. They are the following: 1. alumine; 2. glucine, which is found only in the beryl and emerald, and a few other minerals; 3. yttria is found in the gadolinite, in the yttrious oxide of columbium, &c.; 4. zirconia is found less frequently than the preceding, in the zircon and hyacinth ; 5. silex. The earths were regarded as simple bodies until the brilliant researches of sir H. Davy proved them to be compounds of oxygen with peculiar bases, somewhat similar to those of the alkalies, potassium and sodium." Some of the heavier of the earths had often been imagined to be analogous to the metallic oxides; but every attempt to effect their decomposition or reduction had proved unsuccessful. After ascertaining the compound nature of the alkalies, Davy submitted the earths to the same mode of analysis by which he had effected that fine discoveiy. The results obtained in his first experiments were less complete than those afforded with the alkalies, owing to the superior affinity between the principles of the earths, as well as to their being less perfect electrical conductors. By submitting them to galvanic action, in mixture with potash, or with metallic oxides, more successful results were obtained ; and a method employed by Berzelius and Pontin, of placing them in the galvanic circuit with quicksilver, terminated very perfectly in affording the bases of barytes and lime, in combination with this metal. By the same method, sir II. Davy decomposed strontites and magnesia; and, by submitting silex, alumine, zircon and glucine to the action of the galvanic battery, in fusion with potash or soda, or in contact with iron, or by fusing them with .potassium and iron, appearances were ^obtained sufficiently indicative of their decomposition, and of the production of bases of a metallic nature. Thorina, the last discovered earth, was decomposed by heating the chloride of thorium with potassium. The metallic bases of the earths approach more nearly than those of the alkalies to the common metals, and the earths themselves have a stricter resemblance than the alkalies to metallic oxides. Viewing them as forming part of a natural arrangement, they furnish the link which unites the alkalies to the metals. Accordingly, many of the more recent systems of chemistry treat of all these bodies as form ing a single group under the name of the metallic class. Still (as doctor lire justly remarks), whatever may be the revolutions of chemical nomenclature, mankind will never cease to consider as earths those solid bodies composing the mineral strata, which are incombustible, colorless, not convertible into metals by all the ordinary methods of reduction, or, when reduced by scientific refinements, possessing but an evanescent metallic existence. (For a more particular account of the properties of the earths, and of their bases, consult the articles relating to them, respectively, in this work.)