216 SILEX IN PLANTS. 



quently formed by burning plants which contain salts 

 of soda united to various organic acids. Before the 

 mode of obtaining carbonate of soda from common 

 salt (220) was discovered, the chief source of that 

 salt was the ashes of sea-weed, and other marine 

 plants. 



548. The ashes of many plants contain silica (266), 

 generally in combination with either potash or soda. 

 Canes, and almost all plants allied to the grasses, are 

 completely coated over with a thin film or varnish of 

 silica. Sometimes even, as in the case of the bamboo, 

 solid lumps or concretions of silica, called tabasheer, 

 are found in the joints of the plant. 



549. When a stack of hay or straw has been burnt, 

 so that all the volatile or combustible matters have 

 been dissipated, there is found a large quantity of a 

 dark-colored glassy-looking substance, which consists 

 principally of silicate of potash previously existing in 

 the plants, and which, being unaltered by the heat 

 given out during their burning, is merely melted, to- 

 gether with other substances contained in the plants, 

 into a glass. This may also very easily be observed, 

 by burning a piece of straw in the flame of a candle : 

 abundant white ashes w^ill be left, which continue to 

 retain nearly the form of the straw, and which, if 

 kept in the flame of a candle, melt into an imper- 

 fect sort of glass; this is principally silicate of potash. 



650. The quantity of this substance which exists 

 in the stems of different plants, varies much ; some, 

 such as the plants used for polishing, commonly called 



