14 INTRODUCTION. I. 



arts. The earlier chemists distinguished potash by calling it 

 the "vegetable alkali" in contradistinction to the "mineral 

 alkali," by which they meant soda, and the "volatile alkali" 

 or ammonia. 



Potash compounds are remarkable in the property which 

 they possess of being retained by clay, and especially by the 

 mixture of clay and organic matter found in nearly all fertile 

 soils. In this respect potash differs greatly from soda, for 

 whose compounds soil possesses little or no retentive power. 

 This retention of potash by soil probably explains the fact that 

 in sea-water there is so much more of sodium compounds than 

 of potassium ones, notwithstanding the fact that the primary 

 rocks of the earth's crust contain about equal amounts of these 

 substances. Denudation carries off to the sea large quantities 

 of soluble sodium compounds, but comparatively little potas- 

 sium salts, owing to their retention by the clay, &c., simul- 

 taneously formed by the processes of decay of the felspar, 

 mica, &c. 



Sodium occurs in many silicates, replacing potash. It is 

 extremely widely diffused throughout nature and in the form 

 of common salt plays an important part in animal nutrition. 

 It is asserted by many authorities to be a merely accidental 

 constituent of plants, and in most instances it is found that the 

 exclusion of sodium from a plant's food produces no ill effects ; 

 on the other hand, many marine plants and plants growing 

 near the coast contain large quantities of sodium compounds 

 and a due supply appears essential to their welfare. Although 

 sodium is chemically very like potassium, forming compounds 

 whose properties are very similar to those of that element, its 

 compounds are not retained by the clay or organic matter of a 

 soil, and if applied to the land soon find their way into the 

 drains and thence by streams and rivers to the sea. 



Certain sodium salts are used in agriculture, e.g., sodium 

 nitrate and sodium borate, but in all cases it is the acid con- 

 stituent which is of most value, and rarely that the sodium 

 itself plays any important part, unless it be in rendering more 

 available the potash or other valuable constituents of the soil. 



