SOUKCES OF POTASSIUM AND SODIUM. 303 



Potassium. Potassium, the oxide of which is potassa, or 

 the true vegetable alkali, is spread throughout the mineral 

 kingdom. The ashes of all plants growing elsewhere than on 

 the sea-shore afford by lixiviation an impure carbonate of 

 potassa. The proportion, however, varies much ; but those 

 plants which are rich in this alkali refuse to grow if the soil is 

 not rich in potassa salts. All fertile soils contain a smaller or 

 larger proportion of potassium in the form of potassa. It 

 passes into these soils from the several sorts of clay in which 

 it exists to the extent of three or four per cent. The clays, 

 again, derive the potassa from the disintegration of felspar, or 

 from some kinds of mica. In felspar potassa is present in the 

 proportion of ten or twelve per cent, and in the micas referred 

 to in the proportion of five or six per cent. In the animal 

 kingdom it is not in general so abundant as in the vegetable 

 kingdom : it exists, however, in the form of salts of several 

 acids, in the milk, the blood, and the urine. In the urine 

 of herbivorous animals it is particularly plentiful, since the 

 excess of potass contained in their food is thrown off by the 

 kidney. 



Sodium. In the ashes of seaweeds, and of plants growing 

 on the sea-shore within reach of sea- water, the carbonate of 

 soda exists. Kelp is the name applied to the impure carbonate 

 of soda, which is obtained by the incineration of algae or sea- 

 weeds. Barilla is the name of the substance, richer in soda 

 than kelp, which is extracted from the ashes of plants growing 

 on the turfy beach of seas where the tides are not strong. Soda 

 was formerly called the mineral alkali, owing probably to the 

 mineral-like character both of kelp and barilla. But as certain 

 salts of soda, particularly the chloride of sodium or common 

 salt and the nitrate of soda, exist in immense masses in the 

 mineral kingdom, it turns out that soda is really entitled to the 

 name of mineral alkali. Sodium is also found, like potassium, 



