20 SILICIOUS AND ALUMINOUS EARTHS. 



whitest, and purest sand, as washed and deposited by rapid streams, 

 or other water in motion. This, the very abundant agricultural 

 or natural earth, often approaches nearly in purity to the chemical 

 earth silica. Silicious earth generally appears as sand ; that is, 

 in separate and loose grains of small size, which are rugged and 

 irregular in shape, usually with sharp angles, rough to the touch, 

 and hard enough to scratch glass. This earth is not soluble in any 

 acid except the fluoric, and cannot be made coherent by any mix- 

 ture with water. The solidity of the particles of sand renders each 

 one impenetrable by water ; and their loose and open arrangement 

 permits water to pass easily through the mass. The same condi- 

 tions of impenetrable grains and loose and open texture cause silicious 

 earth to be incapable of absorbing moisture from the air, or of re- 

 taining, with any force, either moisture or any aerial or gaseous 

 fluid with which it may have been in any manner supplied. Sili- 

 cious earth is also quickly and strongly heated by the sun, which 

 increases the rapidity with which it loses moisture. 



2. Aluminous earth, or argil, or purest clay, as it may also be 

 called for convenience, is composed, for a large part, of the chemical 

 earth alumina, from which this and all other less pure clays derive 

 their peculiar and well-known qualities. Still, this purest of clays, 

 naturally existing (or " pipe clay," as termed by some agricultural 

 chemists), contains no more than 36 to 40 per cent, of alumina, 

 chemically combined with 52 to 60 per cent, of silica, and 3 or 4 

 per cent, of oxide of iron.* Thus even the purest natural clay, or 

 aluminous earth, does not approach the purity of the chemical earth 

 alumina within some 60 to 64 per cent. And all ordinary and less 

 pure clays, of course, have much more of silicious sand, the additional 

 quantity being in the state of mechanical mixture. Aluminous 

 earth and all clays, in proportion to their, purity, when dry, absorb 

 water abundantly ; and when wet, form tough and ductile paste, 

 smooth and soapy to the touch. By burning, the mass becomes 

 brick, hard like stone, and is no longer capable of being softened 

 by water. "When drying from a previous wet and softened condi- 

 tion, aluminous earth and all clays shrink greatly, and, separating 

 by numerous cracks and fissures, the mass is broken into hard 

 lumps. 



3. Calcareous earth, carbonate of limefi or calx, is the next 



* Prof. J. F. W. Johnston's "Lectures on the Applications of Chemistry 

 and Geology to Agriculture," p. 230, et seq. First Am. edition of Wiley 

 and Putnam, New York, 1844. 



j- Carbonate of lime is the chemical name for the substance formed by 

 the combination of carbonic acid with lime. The names of all the thousands 

 of different substances (neutral salts) which are formed by the combination 

 of each of the many acids with each of the various earths, alkalies, and 

 metals, are formed by one uniform rule, which is as simple and easy to be 



