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179. Name the most important compounds of boron which are found native.
180. How was boron first isolated by Gay-Lussac and Thenard, and by Davy ?
181. How may crystallized boron be obtained ? By whom was it first prepared ?
182. Compare the properties of the allotropic modifications of boron.
183. Express by equations the reactions which occur when potassium is heated (a) with boron trioxide, (6) with potassium borofluoride.
184. Amorphous boron is heated in (a) air, (b) oxygen, (c) nitrogen, (d) chlorine. What occurs in each case ? Give the names and formulæ of the products.
185. What occurs when adamantine boron is strongly heated in air or oxygen ? What occurs when it is fused with soda or potash ?
186. One hundred and thirty-seven grammes of boric acid are heated until all the water is expelled. What weight of trioxide remains, and with how much potassium must it be heated in order to prepare boron ?
187. Under what conditions is boron soluble in water?
188. How does carbon occur in nature, (a) free, (6) combined ?
189. How are the three allotropic forms of carbon proved to consist of the same substance ? 190. The Koh-i-noor diamond weighs 106 carats (I
3'17 grains). What volume of oxygen at N.T.P. would be necessary to burn it, and what volume of carbon dioxide would result?
191. Give the physical properties of the three varieties of carbon, and mention the chief uses of each.
192. How may the presence of carbon in organic matter be shown ?
193. By what other names is graphite known? Give the names of the different kinds of amorphous carbon, and state how each is prepared.
194. 2'62 grammes of a sample of coke yielded when
burned in oxygen 8.77 grammes carbon dioxide. Find the percentage of carbon contained in it.
195. How may graphite be obtained artificially?
196. Give an account of the different methods em ployed for preparing charcoal from wood. How would you demonstrate the preparation of charcoal on the small scale ?
197. Although the specific gravity of charcoal is about I'5, yet it floats on water. How is this explained ? How could a piece of charcoal be made to sink in water ?
198. What happens when a piece of perfectly dry charcoal is placed in a jar of ammonia gas standing over mercury? What is the cause of the change?
199. How does charcoal act as a disinfectant?
200. What happens when charcoal is heated with solution of indigo or logwood ? Which kind of charcoal acts most readily on these bodies, and to what useful purpose is it applied ?
201. What is coke? Give the different ways employed for preparing it.
202. How is animal charcoal prepared, and what are its properties?
203. How is the substance called gas carbon obtained, and what is it used for ? How is lamp-black obtained ?
204. What weight of carbon dioxide will result from burning 112 lbs. of charcoal containing 85.73 per cent. of carbon ?
205. What relation has been noticed by Angus Smith to exist with regard to the volumes and weights of gases absorbed by charcoal ?
206. Name the most abundant rninerals in which silicon occurs.
Name other substances of which it is an essential constituent.,
207. How are the different modifications of silicon prepared ? How was it originally prepared, and by whom? Give equations for all the reactions..
208. A specimen of granite contains 61 per cent. quartz, 30 per cent. potash felspar, and 9 per cent. mica. Find the percentage of silicon in the specimen.
209. Give the chief properties of amorphous and crystalline silicon, and state the action of heat, of acids, and of alkalies on each variety.