210 



pagated by its runners, and yet it produces seed ; the Jerusalem arti- 

 choke is propagated by its tubers ; from these facts we may infer that 

 the failure of the potato is not caused by the manner of its propaga- 

 tion. The potato blight is caused by electricity, which produces the 

 formation of nitric acid in the atmosphere; this being condensed by the 

 cold, mingles with the dew, rain or moisture on the leaves of plants, 

 and bm-ns small patches or spots on the leaves ; if dry weather sets in, 

 these spots wear out ; if the weather be rainy or moist, these gangre- 

 nous spots become enlarged, and continue to spread until the leaves and 

 stem are completely destroyed. The rot is a necessary consequence of 

 the blight; for when the leaves of a plant are destroyed by poison of 

 any kind, a partial or total destruction of the root follows. The blight 

 is not confined to the potato; many trees, plants and flowers are annually 

 injured by it. 



COMPOSITION OF THE POTATO. 



From experiments which have been made, it has been found that the 

 apples of the potato contain nearly as much alcohol as grapes. Starch, 

 flour, and sugar have been extracted from the tubers ; M. Sprengel found 

 by chemical analysis, that one hundred thousand pounds of potatoes con- 

 tained the following ingredients: 



Potash, 390 lbs. 



Soda, 234 « 



Lime, 33 " 



Magnesia, -. 32 '• 



Oxide of Iron, 2f " 



Silicia, 8^ « 



Sulphuric Acid, 54 " 



Phosphoric Acid, 40 " 



Chlorine, 15^ " 



Alumina, _ 5 " 



814 lbs. 



The ashes of plants are in weight very small, when compared with 

 the plants. Dried wheat straw in 1000 parts, contains but 70 of ash; 

 potatoes give but 45 parts in 1000. By carefully observuig the compo- 

 sition of the ashes of plants, we can ascertain the food, or manure 

 which they require. Liebig says, " it is certain that the mineral sub- 



