286 EARTHS. 



Alumina enters more or less into the composition of most soils, 

 and it generally forms strata in valleys and low grounds and plains, 

 where it arrests the water which has filtered down from the hills, and 

 causes it to issue from the ground, in springs and rivulets. On ac- 

 count of its impermeability to water, clay is employed in the con- 

 struction of tanner's vats, of artificial mill ponds, &c. where it is wish- 

 ed to retain the water. 



In soils, this earth is of the first importance ; perhaps it is not too 

 much to say, that there cannot be a good soil without it. Its pe- 

 culiar office appears to be, to retain moisture, and to prevent the 

 waste of the soluble parts of animal and vegetable manures, which 

 so rapidly filter through siliceous sand and gravel. Still, a soil may 

 contain too much alumina ; it will then be stiff, cold, and difficultly 

 penetrated by the roots of plants ; but if it is mixed with a good pro- 

 portion of siliceous sand and gravel, it will be warm, still retentive of 

 moisture, and sufficiently mellow. 



Lime is an excellent ingredient in soils, as will be mentioned more 

 particularly under the carbonate of that earth. 



Alumina exists abundantly in rocks, especially in felspar, which is 

 a constituent of granite and gneiss ; in clayslate, steatite, asbestus, 

 and serpentines, and in a great variety of minerals. It is nearly pure 

 in the sapphire, and all the most precious oriental gems ; it forms 

 nearly the whole of corundum ; it exists in a vast proportion of min- 

 erals, and forms a large part of the crust of the globe. 



PORCELAIN AND POTTERY. 



In all the manufactures which go under the general name of pot- 

 tery, from the coarsest tile or water pot, to the most beautiful porce- 

 lain in chemical lutes, in fuller's earth, and bricks, silica and alu- 

 mina, in certain proportions, are the essential ingredients. 



History. Known from the remotest antiquity ; the most barbarous 

 nations fabricate rude vessels of baked earth, as well as by hollowing 

 out soft stones ; bricks were employed in the tower of Babel,* two 

 thousand years before the Christian era, and they are found in the an- 

 cient Roman structures in Britainf and elsewhere. Earthen lach- 

 rymatories are discovered in the tombs of the ancient Greeks and 



* In Yale College, are some Babylonish bricks brought out by the late Mr. E. 

 Lewis, of N. Haven ; they were never baked ; they contain straw and bitumen, and 

 some of them have " inscriptions in the arrow headed character ;" the dimensions 

 of the largest are twelve and three fourths inches square by three and a half thick. 



t In the Roman wall at York, the bricks are seventeen inches long, eleven broad, 

 and two and a half thick ; and there is in Yale College, a piece of brick and mortar, 

 trom Roman baths at Paris, presented by Mr. Joel Root, who obtained it from the 

 ruins. 



