216 
Book IV. 
site. 
Magnesia. 
Alumina. 
VEGETABLE SUBSTANCES. 
table kingdom. Indeed it is a very uncommon thing 
to find a plant entirely destitute of lime : salsola soda is 
almost the only one in which we know for certain that 
this earth does not exist *. 
2. Silica exists also in many plants, particularly in 
grasses and equisetums. Mr Davy has ascertained that 
it forms a part of the epidermis, or outermost bark of 
these plants ; and that in some of them almost the whole 
epidermis is silica. 
Parts Silica. 
100 parts of the epid. of bonnet cane yielded 90 
bamboo . . .... 71.4 
(arundo phragm.) common reed . . . 48.1 
stalks of corn . . . . 6.5 
The concretions which are sometimes found in the 
bamboo cane have been ascertained by Mr Macie to be 
composed of pure silica. 
3. Magnesia does nos exist so generally in the vege 
table kingdom as the two preceding earths. It has been 
found, however, in considerable quantities in several 
sea plants, especially fuci f. But the salsola soda con 
tains a greater proportion of magnesia than any plant 
hitherto examined. Mr Vauquelin found that 100 
parts of it contained 17.929 of that earth t. 
4. Alumina has only been found in very small quan 
ties in plants. 
The following TABLe will shew the quantity of these 
four earths which exist in several vegetables. 
* Vauquelin, Ann. de Cbim. xviii. 76. 
t Id. Ibid. 86. and ix. 94. 
t Ibid. 78.