28 HIK PLANT. 



to escape, while sulphate of lime (a compound of 

 sulphuric acid and lime) produces an opposite effect, 

 and prevents their escape. 



The substances coming under the signification of 

 neutrals, are less affected by the laws of combina- 

 tion, still they do combine with other substances, and 

 some of the resultant compounds are of great impor- 

 tance to agriculture. 



ALKALIES. 



The alkalies which are found in the ashes of 

 plants are four in number ; they are potash, soda, 

 e, and tnagnes'xi. 



POTASH. 



When we pour water over wood ashes it dissolves 

 the potash which they contain, and carries it away 

 in solution. This solution is called ley, and if it 

 be boiled to dryness it leaves a solid substance 

 which is chiefly pure potash. Potash left exposed 

 to the air absorbs carbonic acid and becomes car- 

 bonate of potash or pearlash / if another atom of car- 

 bonic 'acid be added, it becomes super-carbonate of 

 potash, or salmratus. Potash has many uses in agri- 

 culture. 



1. It forms a constituent of nearly all plants. 



2. It unites with silicic acid and forms a compound 

 which water can dissolve and carry into the roots of 

 plants ; thus supplying them with an ingredient 

 which gives them much of their strength. 



