CHAP. III. FUNCTION OF NUTRITION. 225 



form of a carbonate, but also in union with other mineral 

 and vegetable acids. Carbonate of lime is largely deposit- 

 ed in the stems of some of the Charge, which it completely 

 incrusts with stony matter. Silica is the earth which 

 next to lime occurs in the greatest abundance, especially 

 among some of the monocotyledonous tribes. The glossy 

 surfaces of canes, reeds, and other grasses, are com- 

 posed of a very large percentage of it ; and if two canes 

 be rubbed together in the dark, they emit a flash of light 

 similar to that which is obtained by the friction of two 

 quartz pebbles. When a stack of corn or hay has been 

 rapidly consumed, the ashes are fused into a semi-vitri- 

 fied mass : the straw abounding both with silica and an 

 alkali, the two chief ingredients necessary to the form- 

 ation of such a compound. In the hollow portions 

 of the stem between the joints of the bamboo, a sub- 

 stance named tabasheer is deposited in lumps which 

 very much resemble fragments of opaque and semitrans- 

 parent opal. This remarkable deposit contains 70 per 

 cent, of pure silica, and possesses very peculiar and 

 curious optical properties. Silica is also deposited in 

 little semi-crystalline lumps along the angles of the 

 stems of some species of Equiseta, especially the Equi- 

 setum hyemale or Dutch reed, which from this circum- 

 stance is serviceable to watchmakers and others in 

 polishing their work. 



(221.) Salts. The salts of potash are particularly 

 abundant in most plants, but the salts of soda are more 

 especially confined to such as grow near the sea. It is 

 however remarkable, that plants which abound in the 

 salts of soda whilst growing in these latter situations, 

 secrete the salts of potash when they are no longer 

 within the influence of the sea. In such plants, it is 

 difficult not to believe that the presence of one or other 

 of these alkalis is in some way beneficial to their health, 

 even though it may not form any essential part of their 

 structure. The common soda of commerce is a carbon- 

 ate obtained from the incineration of several maritime 



