MICROCHEMIS TRY. 55 



presence of silicic acid can only be recognized in the ash by 

 the sodium silico-fluoride reaction (cf. 81). 

 80. To test the skeletons obtained by either of these 

 methods for the presence of silicic acid, hydrofluoric acid 

 may be used, in which pure silica-skeletons should dissolve 

 completely. 



Karner (I, 262) recommends for this purpose a dilute 

 aqueous solution of hydrofluoric acid, which, as it attacks 

 glass, must be kept in a bottle of rubber or lead, and must 

 be placed upon the objects to be tested with a platinum 

 wire or a rubber rod. The slide which supports the object 

 must, of course, be protected from the action of the acid, 

 and for this purpose covering with Canada balsam, glue, 

 vaseline, or glycerine has been recommended. But, accord- 

 ing to Karner (I, 262), it is most convenient to cover the 

 slide with a piece of transparent sheet-wax, which must be 

 first somewhat warmed and smoothed by rubbing between 

 the hands. Instead of a cover-glass this author recommends 

 tha use of gelatine paper. He also fastens a bit of the same 

 paper to the objective with Canada balsam, to protect it 

 from the vapor of hydrofluoric acid. 



When Karner (I, 266) allowed hydrofluoric acid to act 

 upon membranes not previously treated with some acid 01 

 the like, he usually found only a partial solution of the 

 silicic acid. Whether this was due to the physical action of 

 other constituents of the membrane or to a chemical union, 

 perhaps of silicium with cellulose, is not yet certain. 



8l. Besides its solubility in hydrofluoric acid, one may 

 use for the recognition of silicic acid the formation of crys- 

 tals of sodium silico-fluoride, which are with great difficulty 

 soluble in water. To obtain these crystals hydrofluoric acid 

 and some sodium chloride are added to the ash and allowed 

 to slowly evaporate. The crystals of sodium silico-fluoride 

 which then form if silicium is present belong to the hex- 

 agonal system and represent chiefly combinations of prisms 

 and pyramids, or of these with six-sided plates also. In 

 stronger solutions six-rayed stars and rosettes are also ob- 

 served as skeleton forms (Haushofer, I, 98). 



