BROMINE-IODINE-FL UOttINK 245 



water, is used for etching on glass. This effect is due to the action of 

 the acid upon the silica of the glass, which is converted into either 

 silicon fluoride, SiF 4 ; or into hydrofl uosilicic acid, H 2 SiF 6 . 



Hydrofluoric acid, or strong solutions of it, are powerful antiseptics. In 

 small quantities the acid is used as an admixture to fermenting liquids, as it 

 has been found that it does not act upon the principal ferment of yeast, which 

 causes the decomposition of sugar into alcohol and carbon dioxide, while it 

 readily destroys a number of objectionable ferments. The yield of alcohol is 

 thus considerably increased. 



Experiment 19, Prepare a glass plate by heating it slightly and covering its 

 surface with a thin layer of wax or paraffin ; after cooling, scratch some letters 

 or figures through the wax, thus exposing the glass. Set the plate over a dish 

 (one made of lead or platinum answers best), in which a few grammes of pow- 

 dered fluorspar have been mixed with about an equal weight of sulphuric acid, 

 and set in the open air for a few hours (heating slightly facilitates the action); 

 upon removing the wax or paraffin, the glass will be found to be etched where 

 its surface was exposed to the vapors of the acid. This experiment serves also 

 as the best test for fluorides. (See under Silicon, p. 186.) 



QUESTIONS. How is bromine found in nature? State the physical and 

 chemical properties of bromine. What is hydrobromic acid, and how can it be 

 made ? By what tests may bromine and bromides be recognized ? What is the 

 chief source of iodine? What are the chemical and physical properties of 

 iodine? What is tincture of iodine, what is its color, and how does it stain 

 the skin ? Mention reactions by which iodine and iodides may be recognized. 

 By what element may bromine and iodine be liberated from their compounds? 

 How is hydrofluoric acid made, and what is it used for? 



