206 



SMITH'S INTERMEDIATE CHEMISTRY 



thyroids (thyroxin) is administered with remarkable success in 

 cases of degeneration caused by abnormally small natural de- 

 velopment of this gland (cretinism). Potassium iodide is also 

 used in medicine, to cause absorption of blood-clots and effusions 

 of blood, for example in the eye. Silver iodide is contained in 

 the coating on photographic plates and films. 



FLUORINE F 2 



Compounds of fluorine are found in large quantities as minerals, 

 but the compounds are so stable that the element is very difficult 

 to liberate. The natural compounds, however, have many im- 

 portant uses. 



Occurrence. Calcium fluoride CaF 2 (fluorite) occurs in 

 nature in beautiful cubical crystals. Cryolite AlF 3 ,3NaF is used 

 in the modern manufacture of aluminium 

 (p. 466). Apatite Ca 5 (P0 4 )3F is a common 

 constituent of rocks and soils. When slowly 

 decomposed, by weathering, it furnishes soluble 

 phosphates. These are absorbed by plants, for 

 which they are a necessary food. 



FIG. 62 



Preparation of Fluorine. The element 

 is obtained by electrolysis of potassium-hydro- 

 gen fluoride KHF 2 dissolved in liquefied hydro- 

 gen fluoride. The electrodes are made of an 

 alloy of platinum and iridium, with which 

 fluorine has little tendency to combine. The 

 vessel is a U-tube of copper (Fig. 62) and, to 

 prevent vaporization of the hydrogen fluoride (b.-p. 19.4), is kept 

 at 23 to 40 during the operation. Hydrogen is liberated 

 at one pole and fluorine at the other. 



Properties. Fluorine is a yellow gas, with a density greater 

 than that of air (G.M.V. weighs 38 g.). It is the most difficult 

 of the halogens to liquefy (b.-p. 187). 



