C. T. JACKSON'S ADDRESS. 449 



The acids and bases are here given separately, but in the 

 soil and in plants they are combined with each other, forming 

 various saline compounds, which are generally neutral. We 

 obtain some of them in that state from the ashes of all plants, 

 and when we obtain alkaline matter, it is derived by the de- 

 composition of the organic acid, with which the alkali was 

 originally combined. The saline matters vary in their propor- 

 tions in different plants, and even in the different parts of the 

 same plant. 



I do not wish you to suppose that these ingredients exist in 

 the soil in the same state of combination that they do in the 

 plants. On the contrary, it is evident that decompositions 

 take place in them during their circulation in the vessels of 

 living vegetables, and when we burn a plant, the substances 

 found in the ashes are differently combined from what they 

 originally were in the vegetable tissues. 



In the soil, phosphoric acid occurs in combination with lime, 

 alumina, and oxide of iron, while in the plant, we find a part 

 of it combined with potash, soda, and magnesia, as well as 

 with lime, but never in combination with alumina, which is 

 not an ingredient of the vegetable tissues, and is not found in 

 any plants. 



Sulphuric acid may exist in combination with oxide of iron 

 and alumina, in the soil, as well as with lime and magnesia, and 

 the alkalies; but in the plant only, particular combinations of 

 sulphuric acid and of sulphur are found, and they are not the 

 same as occur in the soil in which the plant grows ; hence the 

 sulphates are not merely accidentally absorbed, but are essential 

 to the growth of the plant. 



Silicic acid in the soil, is combined with potash and soda, and 

 is generally insoluble in water, even when so combined, but by 

 the action of carbonic acid, the insoluble silicates undergo par- 

 tial decomposition, and carbonate of potash is formed, which 

 dissolves a small portion of the silicic acid, and renders it capa- 

 ble of absorption by the plants. Then the silicate of potash 

 is decomposed by organic acids in the plant, and the silicia is 

 set free, and is secreted, and forms a part of the tissues, and 

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