CHLORINE, BROMINE, IODINE, AND FLUORINE. 171 



It occurs rather abundantly in nature, combined with cal- 

 cium chiefly. The beautiful mineral fluor-spar is calcium 

 fluoride. The hydrogen compound of fluorine is of great 

 importance to the chemist and in the arts, owing to its val- 

 uable property of dissolving silica and attacking glass. 

 This hydrofluoric acid, as it is called, HF1, is a colorless, 

 acid gas, soluble in water. You can make a pretty experi- 

 ment with it, but be careful not to breathe the fumes. 

 Take a small leaden dish, and put into it some powdered 

 fluor-spar, calcium fluoride. Next take a watch-glass, warm 

 it gently, and make beeswax to flow evenly over the convex 

 surface. Now write a word, or scratch any thing you please 

 with a pin on this wax-covered glass, removing the wax 

 only where you wish lines to be eaten into the glass. Pour 

 some strong sulphuric acid into the leaden dish containing 

 the calcium fluoride, heat gently, and, as soon as you see 

 white fumes, cover the dish with the wax-covered glass. 

 The gaseous hydrofluoric acid will eat away the glass 

 where not covered by the wax. Remove the glass after 

 some minutes, scrape off the wax, wash the rest off with 

 benzol, and you will have an etched surface exposed. Par- 

 affin may be used instead of wax. 



QUESTIONS. 



208. What four bodies form a natural group ? 209. Where and how 

 does chlorine occur in nature? Is it abundant? 210. Describe and ex- 

 plain a method of obtaining chlorine from common salt. 211. From hy- 

 drochloric acid. 212. What is said about breathing chlorine ? 213. What 

 about chlorine water? 214. State what action chlorine has on some met- 

 als. 215. Show its attraction for hydrogen by describing the experiment 

 named. 216. How is advantage taken of this attraction in bleaching? 

 What really does the work? 217. What happens if the chlorine be in too 

 great excess? 218. What advantages has this method over grass-bleach- 

 ing? Who discovered chlorine? 219. Explain the difference between 

 chlorine bleaching and sulphur bleaching. 220. How is chlorine used as a 



