CHEMISTRY OF SOIL. 49 



the bases (40). combination can take place only in the 

 equivalent proportions. It is thus evident, that soil, con- 

 sisting of silicates, metalloid compounds, and salts, is a tixed, 

 unvarying, chemical combination of these substances, though 

 mixed in proportions, somewhat varied by local causes, yet 

 presenting, in the mass, a great similarity of composition. 

 When the subject of the composition of the vegetable por- 

 tion of soil is discussed, the value of a slight knowledge of 

 chemical notation and of combining proportions, will be 

 manifest. It is not to be neglected, however unconnected it 

 may seem with practical farming. The doctrine of chemical 

 equivalents is important to the farmer, even if he pursues it 

 no farther than to understand and remember the combining 

 proportions of a few substances, known to him only by 

 name ; such are the common acids, oil of vitriol, aquafortis, 

 spirits of salt, or sulphuric, nitric, and muriatic acids ; the 

 usual alkalies, ammonia, potash, soda, lime ; acids and bases, 

 which combine only in their equivalents. It is sometimes 

 remarked, in agricultural experiments with different salts, 

 that equal quantities, if correct comparative trials are to be 

 made, should be used. The doctrine of equivalents teaches 

 not an equal, but an equivalent portion — that is, 28 of lime 

 are equal to 48 of pure potash- It may assist the memory 

 here, and furnish a good " rule of thumb," to recollect, that 

 the three alkalies, ammonia, soda, potash, are to each other, 

 as 17 : 32 : 48, or as 1 : 2 : 3, nearly. When the subject 

 of manures is considered, the doctrine of equivalents will 

 be found important, in determining their relative value. 

 Though the numbers here used are those of some chemists 

 of high authority, they are not all universally admitted. 

 They have the convenience of being small whole numbers. 

 They are readily retained in the memory, and simplify the 

 Jubject, by freeing the calculation from multiplication and 

 3 



