DEFINITION OF TERMS. 



103 



ed in our own times. Silex exists nearly 

 pure, in large masses, forming entire 

 rocks, as quartz rock, and constituting 

 the chief ingredient in all granite 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 lare 

 minerals, but, in a mixed state, is well 

 known as forming clays and a large fami- 

 ly of rocks, usually called argillaceous. 



Lime, an earth well known from its 

 important uses in society, occurs combin- 

 ed 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 sulphuric 

 acid, when it bears the name of gyp- 

 su?n. 



Magnesia 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 combi- 

 nation with sulphuric acid, is often met 

 with in metallic veins.) are only known 

 to the chemist as occurring in the com- 

 position of certain minerals, which, for 

 the most part, are exceedingly rare. 



The earths are very similar to the al- 

 kalies, (q. V.) forming with the acids, pe- 

 culiar salts, and resembling the alkalies 

 likewise in their composition. 



They consist of peculiar metals in com- 

 bination with oxj'gen, and compose the 

 greatest part of the solid contents of the 

 globe. They differ from the alkalies prin- 

 cipally in the following peculiarities : 

 they are incombustible, and cannot, in 

 their simple state, be volatilized by heat ; 

 with different acids, especially the car- 

 bonic, they form salts insoluble, or solu- 

 ble 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 colois almost 

 in the same way as alkalies, and their 

 affinity for acids is sometimes stronger 

 than that of the alkalies. They combine 

 with sulphur, and form compounds per- 

 fectly similar to the sulphuretted 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, calca- 

 reous 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 great- 

 est heat of our furnaces, and, by being 

 exposed to heat, in a greater or less de- 

 gree, they lose their property of easy 

 solubility in acids. Some of them are in- 

 capable 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 cokim- 

 bium, &c. ; 4, zirconia is found less fre- 

 quently than the preceding, in the zircon 

 and hyacinth ; 5, silex. The earths were 

 regarded as simple bodies until the bril- 

 liant researches of Sir Humphrey Davy 

 proved them to be compounds of oxygen 

 with peculiar bases, somewhat similar to 

 those of the alkalies, potassium and so- 

 dium. 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 reduc- 

 tion, had proved unsuccessful. After as- 

 ceilaining 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 affini- 

 ty between the principles of the earths, 

 as well as to their being less perfect elec- 

 trical conductors. By submitting them 

 to galvanic action, in mixture with potash, 

 or with metallic oxides, more successful 

 results were obtained ; and a method em- 

 ployed by Berzeliusand Pontin, of plac- 

 ing them in the galvanic circuit with 

 quicksilver, terminated very perfectly in 

 affording the bases of barytes and lime, 



