ASHES OF PLANTS. 215 



ganic matter. Besides the lignin, gum, sugar, starch, 

 gluten, and other similar substances which consist 

 of carbon, hydrogen, oxygen, and nitrogen, and 

 which burn in the air when heated, there are always 

 found substances which cannot burn, but which are 

 of an earthy nature; these substances remain as 

 ashes, when plants are burnt. 



645. The inorganic substances found in plants are 

 by no means numerous ; the most common are salts 

 of lime, potash, soda, magnesia, oxide of iron, and 

 silica. These substances are found in the ashes of 

 plants, combined with carbonic, sulphuric, phosphoric, 

 and some other acids. 



546. The ashes of plants very frequently contain 

 a considerable quantity of carbonate of potash (494); 

 this salt does not exist in the growing plants, but is 

 formed during their combustion. Potash is found in 

 a very great number of plants, sometimes in combina- 

 tion with sulphuric acid, or silica, but more commonly 

 united to tartaric, oxalic, or some other organic acid, 

 constituting a tartrate, oxalate, or other salt of potash. 

 When plants are burned, all the organic acids are 

 burned, as well as the lignin and similar substances 

 which th^y contain ; consequently, although in living 

 plants the potash is combined with organic acids to 

 form neutral or even acid salts, yet, as these acids 

 are destroyed when plants are burned, caustic potash 

 is left (199), which, of course, combines with carbonic 

 acid to form carbonate of potash. 



547. In the same way, carbonate of soda is fre- 



