§ 68. HYDIIOFLUOEIC ACID. § 69. OXALIC ACID. 99 



color ; a drop of this solution on starch paper colors it 

 blue ; the latter reaction is very delicate. 



HYDROFLUORIC ACID. IIF. 



68. Calcic and magnesic fluorides are difficulty soluble 

 in water and acids. 



Reactions* — When a fluoride in powder is moistened 

 with concentrated sulphuric acid, in a leaden or platinum 

 cup, and gently heated, hydrofluoric acid is evolved ; if 

 the cup is covered with a piece of Bohemian glass that is 

 protected with a coating of wax, except along a few lines 

 where the wax has been removed with a sharp point, the 

 glass will be corroded on these lines, in a few hours at 

 the most. If but a small quantity of hydrofluoric acid is 

 present, the marks may not be seen until all the wax is 

 carefully cleaned off and the glass is breathed upon. 



To be sure that these faint marks are produced by 

 traces of hydrofluoric acid in the substance, wipe the 

 glass off carefully with water, and see that they can be 

 developed again by the breath — and be sure, also, that the 

 sulphuric acid used does not contain traces of hydroflu- 

 oric acid, as it sometimes does. 



If a silicate is present, this reaction may not take place ; 

 in this case mix the substance with strong sulphuric acid 

 in a watch-glass, heat until the mass is dry, and wash the 

 residue off with water. If fluorine was present, the glass 

 will be found to be corroded where it came in contact 

 with the substance. 



OXALIC ACID. H2C2O4. 90. 



69. Oxalates of barium, calcium, magnesium, iron, 

 manganese, zinc, lead, and copper, are sparingly soluble, 

 or insoluble, in water, but soluble in dilute acid. 



Reactions. — Oxalates are decomposed, but not black- 

 ened when heated. 



