AGRICULTURAL TEXT-BOOK. 



327 



Prof. Way has lately ascertained that all clay soils contain a double 

 silicate of alumina and one of the alkalies or alkaline earths, as : 



Soda, 



Potabh, Magnesia, 



Ammonia. 



which is slowly soluble, but sufficiently so to supply what plants re 

 quire ; and that these silicates, without exception, are capable of ab 

 sorbing and retaining ammonia; and, what is still more important, 

 some of them have the faculty of abstracting ammonia from the air. 

 &quot; Whenever a salt of ammonia or potash reaches the soil, and gets dis 

 tributed through it, a change occurs a double silicate of alumina and 

 ammonia is formed, and the salt which was added no longer exists 

 there. The ammonia or potash henceforth exists in the soil only in 

 the form of silicate, and is presented to the roots of a plant only in 

 that form, or in the form of carbonate derived from it by the action of 

 carbonic acid in the soil. And inasmuch as all average soils possess 

 this property of conversion in more than the degree necessary for the 

 quantity of manure which reaches them, the inference is obvious and 

 incontestable, that nature has given to the soil this power for the spe 

 cific purpose of preparing the food of plants, and we then have the soil 

 occupying a place intermediate between that of mere dead matter, and 

 the living organism of plants.&quot; We can only thus slightly mention 

 this most important and beautiful discovery ; and refer the reader to 

 Prof. Way s own writings in the Jour, of Royal Agricul. Socy. oj Eng 

 land, vol. xiii, itc. 



703. LIME is everywhere an essential of* a fertile soil, and in 

 Great Britain is more extensively used than any other mineral 

 substance. It is applied in a variety of forms, as (a,) carbon 

 ate of lime lime rock (44 Ibs. of carbonic acid and 51 Ibs. 

 of lime ;) (b,) the same lime rock burned, and the carbonic 

 acid driven oft , when it becomes caustic, but again absorbs car 

 bonic acid and moisture rapidly from the air when it again re 

 turns to a carbonate, or is &quot; slaked ;&quot; (&,) Gypsum, Plaster, 

 Sulphate of lime, (d,) Phosphate of lime, as in bones, &c., of 

 which we have already spoken, (e,) Silicate of lime, existing 

 in many rocks and earths ; (f,) Marls ; (ffj Chalk, which is 

 another form of carbonate of lime, chiefly found in England; 

 (k,) Magnesian limestone, the base of which is lime, with a 



