AGRICULTURAL EARTHS. 19 



a state of purity, or uncompounded ; and in this state of purity (as 

 rock-crystal, or pure quartz-rock), it can have no action whatever 

 as an agricultural earth. Two other chemical earths, alumina and 

 lime, are only found combined with other bodies; and, as thus 

 combined, exhibiting very different properties from the pure earths, 

 which can be produced only by chemical decomposition. A fourth 

 earth, magnesia, likewise is never found uncoinbined, and rarely 

 in other than very minute proportions, and always intermixed 

 with other earths, so as to be imperceptible by the senses. The 

 other chemical earths (barytes, strontian, zircon, &c.) are so rarely 

 found, and still more rarely in soil, and most of them only in such 

 minute quantities that, as to any influence on agriculture, they may 

 be deemed as non-existent.* 



These few preliminary remarks will serve to expose something 

 of the difficulty of distinguishing and clearly defining the earths of 

 agriculture. That the attempt which will here be made will but 

 imperfectly reach the desired object, will not be more evident to 

 other persons than to the writer. 



The agricultural earths will here be understood as bodies natu- 

 rally existing, and, when separate, as pure as ever presented by 

 nature ; and of which, each one, except humus, is composed princi- 

 pally of some one chemical earth. They are five in number silicious, 

 aluminous, calcareous, magnesian, ancl vegetable or humus. These 

 agricultural earths, variously intermixed, serve to compose the 

 superficial layer of the globe. This layer, more or less productive 

 of vegetable growth, is %>il; and however varying in different 

 places, all soils, for almost their entire bulk, are composed of one 

 or more of the three principal agricultural earths the silicious, 

 aluminous, and calcareous, with more or less of humus, or vegetable 

 mould. It is convenient, though still a farther departure from 

 scientific strictness of definition, to include humus among the earths 

 of agriculture. 



1. Silicious earth is presented in the cleanest, most crystalline, 



* The cliemic.il earths are combinations of different metals (which are 

 known only in these combinations) with oxygen. Before Davy's splendid 

 discovery of these metals, and their combinations with oxygen, the earths 

 were supposed to be simple bodies, or incapable of being decomposed. A 

 single combination of one of these very rare chemical earths, the sulphate 

 of barytes, has been recently found to be a very effective manure, acting 

 on clover with the remarkable power of sulphate of lime (gypsum). Pro- 

 fessor Armstrong, of Washington College, has fully tested it by the practi- 

 cal use of the earth as manure. He also informed me that the sulphate 

 of barytes was found in some parts of that mountain region in sufficient 

 quantity to be used for manuring, in the small proportions required for its 

 effects. These interesting facts do not contradict the remarks in the text 

 above, which referred to barytes and the other scarcer earths only as con- 

 stituents of soils. 



