SILICA. 95 



(Podostemaceae). 1 It maybe recognized in the fol- 

 lowing way : A very thin section of the tissue to be 

 tested is heated on platinum-foil, by which means 

 all of its organic constituents are destroyed, leav- 

 ing the inorganic elements, which consist of silicic 

 acid and salts of lime, in a somewhat distorted 

 skeleton. The calcareous portion is then removed 

 by adding a drop of hydrochloric acid, the silica 

 remaining as a network, which is soluble in hydro- 

 fluoric acid. 



It may happen that the silica and lime salts fuse 

 together in the process of heating, which inter- 

 feres with the reaction. It is, therefore, desirable 

 to remove these salts by the Schultze maceration 

 before incineration. When the section has been 

 boiled in this it is washed in hot distilled water. 

 After the final addition of hydrochloric acid the 

 silica alone remains. The operation needs to be 

 skilfully performed. The epidermis of Equisetum 

 is to be recommended for experiment. 



Sachs has proposed a somewhat modified method. 

 Larger tissue masses are moistened with concen- 

 trated sulphuric acid on platinum-foil, and heated 

 over a Bunsen burner. The acid is at once 

 blackened, while there is a violent evolution of gas. 

 The heating is continued only until the pure white 

 ashes remain, a result which is reached somewhat 

 more rapidly by this method than by the other. 



1 Carlo: Bot. Zeitung, 1881, p. 31. Warming: Naturh. Forenings 

 vidsk. Meddel., Copenhagen, 1881, p. 89. 



