54 BOTANICAL MICROTECHNIQUE. 



action of acids and heat to obtain completely coherent 

 siliceous membranes, the so-called silica skeletons* From 

 the membranes of the diatoms, which are peculiarly rich in 

 silicic acid, or from the epidermis of the Graminecz or Equi- 

 setacece *, beautiful siliceous skeletons may be obtained by 

 treating them as proposed by Sachs. This method consists 

 in heating the organ or organism on a cover-glass, or on a 

 bit of mica to prevent the residue from adhering, with a 

 drop of concentrated sulphuric acid until the ash remaining 

 after the evaporation of the acid has become quite white. 



In case of objects poorer in silicic acid, satisfactory 

 siliceous skeletons cannot usually be obtained by this simple 

 method. It is then commonly better to remove the soluble 

 inorganic substances from the pieces before burning by 

 treatment with hydrochloric or nitric acid. In this way 

 pure white skeletons may be much more easily obtained 

 and may be freed from foreign admixtures by renewed 

 treatment with hydrochloric acid. 



79. Besides, siliceous skeletons may be very well prepared 

 wholly in the wet way by the method proposed by Mil- 

 iarakis (I). The object is first treated in a beaker with con- 

 centrated sulphuric acid until it is quite black and then a 

 20% aqueous solution of chromic acid is added. In this 

 mixture suberized membranes are also wholly destroyed, 

 and only the siliceous skeletons remain behind. They may 

 then be easily isolated, after the addition of water, by de- 

 canting, and may be completely cleaned by repeated wash- 

 ing with water and alcohol. The siliceous skeletons of 

 diatoms obtained in this way show, especially when exam- 

 ined in air, the finest structural features of their membranes. 



According to Kohl (II, 226) this method is applicable 

 only where considerable quantities of silicic acid are present. 

 This author obtains very delicate siliceous skeletons by 

 burning from parts of plants with a small proportion of 

 silicic acid, which would be completely dissolved by the 

 treatment with chromic sulphuric acid. In other cases the 



*[Our Equisetum hiemale is especially good for this purpose.] 



