472 COMPOUND PRODUCTS. CHAP. I. 



ticularly in autumn.* This is a phenomenon that 

 seems inexplicable. 



The greater part of the Grasses contain a very 

 considerable proportion of silica, as do also the 

 plants of the genus Equisetum. Sir H. Davy has 

 discovered that it forms a part of the epidermis of 

 these plants, and in some of them the principal part. 

 From 1OO parts of the epidermis of the follow- 

 ing plants the proportions of silica were as follows : 



Bonnet Cane go 



Bamboo 71-4 



Common Reed 48*1 



Stalks of Corn ' 66-5 



Owing to the silica contained in the epidermis, 

 the plants in which it is found are sometimes used 

 to give a polish to the surface of substances where 

 smoothness is required. The Dutch Rush, a plant 

 of this kind, is used to polish even brass. 



3dly, Magnesia. Magnesia does not exist so 

 abundantly in the vegetable kingdom as the two 

 preceding earths. It has been found, however, in 

 several of the marine plants, particularly the Fuel ; 

 but Salsola Soda contains more of magnesia than any 

 other plant yet examined. According to Vauquelin 

 1OO parts of it contain 17*9^9 f magnesia. 



4thly, Alumina. Alumina has been detected in 



* Saussure surla Vegetation. 



