EARTFIS. 



773 



permanent volcanoes are preceded by, and propor- 

 tionate to, the agitations of the earth in their neigh- 

 bourhood. These observations furnish grounds for 

 the conclusion, that earthquakes cannot proceed from 

 external causes, but arise from certain powers operate 

 ing 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 earth- 

 quake, 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 the subterranean 

 thunder-like noises, the shaking, raising, and bursting 

 asunder of the earth, and the emission of elastic 

 fluids, the fire and flames, the ejection, too, of mineral 

 substances, all occur, now and then, more or less, 

 in earthquakes as well as in volcanic eruptions, even 

 when at a distance from active volcanoes ; and the 

 genuine volcanic eruptions are, as has been remark- 

 ed, accompanied or announced by shakings of the 

 earth. All our observations go to prove, that volca- 

 nic eruptions, earthquakes, the heaving of the ground 

 from within, and the disruption of it in the same 

 way, are produced by one and the same cause, by 

 one and the same chemical process, which must have 

 its seat at a great depth beneath the present sin-face 

 of the earth. The most remarkable earthquakes of 

 modern times are those which destroyed Lima, in 

 1746, and Lisbon, hi 1755; in the latter, 20,000 

 persons were killed. It extended from Greenland to 

 Africa and America. A similar fate befell Calabria, 

 in 1783, the province of Caracas, in South America, 

 in 1812, and Aleppo, in Syria, hi 1822. Several 

 earthquakes liave taken place quite lately, in South 

 America, one particularly dreadful at Lima. The 

 city of Guatemala, also, was nearly destroyed in the 

 spring of L830, by earthquakes, which continued five 

 days successively. 



EARTHS. The term earth is applied, in com- 

 mon life, to denote a tasteless, inodorous, dry, unin- 

 flammable, sparingly-soluble substance, which is diffi- 

 cultly fusible, and of a moderate specific gravity. 

 Several < f the earths are found in a state of purity 

 in nature ; but their general mode of occurrence is 

 in intimate union with each other, and with various 

 acids and metallic oxides. Under these circum- 

 stances, they constitute by far the greatest part of 

 the strata, gravel, and soil, which go to make up the 

 mountains, valleys, and plains of our globe. Their 

 number is ten, and then' names are silex, alumina, 

 magnesia, lime, barytes, strontites, zircon, glucine, 

 yttria, and thorina. The four first have long been 

 known to mankind ; the remainder have been dis- 

 covered in our own times. Silex exists nearly pure, 

 in large masses, forming entire rocks, as quartz rock, 

 and constituting the chief ingredient in all granitic 

 rocks and sandstones, so that it may safely be asserted 

 to form more than one half of the crust of the earth. 

 Alumine is found pure in two or three exceedingly 

 rare minerals, but, in a mixed state, is well known 

 as forming clays and a large family of rocks, usually 

 called argillaceous. Lime, an earth well known 

 from its important uses in society, occurs combined 

 with carbonic acid, in which state it forms limestone, 

 marble, chalk, and the shells of snails. It exists 

 also, upon a large scale, in combination with sulphu- 

 ric acid, when it bears the name of gypsum. Mag- 

 nesia is rare in a state of purity, but enters largely 

 into the composition of some of the primary rocks, 

 especially of the limestones. The remaining eight 

 'if we except barytes, which, in combination with 



sulphuric acid, is often met with in metallic veins) 

 are only known to the chemist as occurring hi the 

 composition of certain minerals, which, for the most 

 part, are exceedingly 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 pecu- 

 liar metals hi combination with oxygen, and com- 

 pose the greatest part of the solid contents of the 

 globe. They differ from the alkalies principally ii> 

 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 then: active state, they are soluble 

 in water, and these solutions may be crystallized. 

 They change the vegetable colours almost in the same 

 way as alkalies, and their affinity for acids is some- 

 tunes weaker and sometimes stronger than that of 

 the alkalies. They combine with sulphur, and form 

 compounds perfectly similar to the sulphureted alka- 

 lies. With carbonic acid, they form insoluble salts, 

 which, however, become soluble in water by an ex- 

 cess of carbonic acid. The alkaline eartlis 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 che- 

 mists, 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 hi water, infusible at the 

 greatest heat of our furnaces, and, by being exposed 

 to heat, hi 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 gadoli- 

 nite, hi the yttrious oxide of columbiuni, &c. ; 



4. zirconia is found less frequently than the preced- 

 ing 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 decomposi- 

 tion 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 discovery. 

 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 then- being less perfect 

 electrical conductors. By submitting them to gal- 

 vanic 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 H. Davy decomposed 

 strontites and magnesia; and, by submitting silex, 

 alumine, zircon, and glucine to the action of the gal- 

 vanic battery, in fusion with potash or soda, or hi 

 contact with iron, or by fusing them with potassium 

 and iron, appearances were obtained sufficiently indi- 

 cative of their decomposition, and of the production 



