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phosphate (a) using charcoal only, (b) using charcoal and sand.

228. The skeleton of a full-grown man weighs 24 pounds, and the bones contain 58 per cent. of calcium phosphate. Calculate the weight of phosphorus in the skeleton.

229. Why are preparations of bone-ash frequently employed as manures?

230. A piece of bone is heated (a) in a covered crucible, (b) in an open fire. Describe the substances obtained in each case, and state which would contain the larger proportion of phosphorus.

231. How is crude phosphorus purified? How is it cast into sticks?

232. How is amorphous phosphorus obtained on the large scale? How is it purified from traces of common phosphorus ?

233. How may amorphous phosphorus be converted into common phosphorus, and vice versâ? How may these changes be shown on the small scale?

234. Compare the properties of the two varieties of phosphorus, and point out the differences between them.

235. How could you prove, in spite of the great differences in properties, that common and red phosphorus are modifications of the same element?

236. Two hundred and seven grammes of phosphorus pentoxide were obtained in each case by burning (a) phosphorus, (b) phosphine, in air. What weight of substance was used in each case?

237. What is the weight at N.T.P. of 1,234 litres of phosphorus vapour?

238. To what property of the element does phosphorus owe its name? Under what conditions does it lose this property?

239. Describe a process for the detection of phosphorus in cases of poisoning.

240. A manufacturer prepares phosphorus from 169 tons of bones containing 537 per cent. of calcium phosphate. How much phosphorus should he obtain, and how much sulphuric acid and carbon should be required for the process?

241. How has amorphous phosphorus been obtained in crystals, and what are the properties of this modification? 242. How may phosphorus be directly combined with oxygen, chlorine, hydrogen, and sulphur?

243. Which modification of phosphorus dissolves in carbon disulphide? What occurs when the solution is poured over a sheet of filter paper?

244. How are lucifer matches made, (a) using common phosphorus, (b) using amorphous phosphorus ?

245. Give a short history of various means proposed for obtaining light, prior to the introduction of phosphorus matches.

246. Why is the end of an ordinary wooden match tipped with sulphur or stearine as well as phosphorus paste?

247. How may the so-called "safety matches" be kindled without bringing them in contact with amorphous phosphorus ?


248. How does arsenic occur in nature? Name some of its ores, and state how the element is obtained from them?

249. How is the occasional presence of arsenic in hydrochloric acid explained? How may its presence be ascertained?

250. To what two substances is the name arsenic applied? Give the formula for each.

251. How has the density of arsenic vapour been ascertained, and what is peculiar about it?

252. What occurs when arsenic is heated (a) in oxygen, (b) in nitric acid, (c) in sulphuric acid?

253. How is arsenic prepared on the large scale from arsenical pyrites, and how is it purified?

254. How is arsenic obtained in the crystalline and amorphous modifications?

255. Discuss the various reasons which lead some chemists to class arsenic with the metals, and others with the non-metals.

256. Compare the properties of nitrogen, phosphorus, and arsenic, and give reasons for placing them in the same chemical group. Give the formulæ for the chlorides, oxides, sulphides, and hydrides of each.

257. Name poisonous and non-poisonous compounds of arsenic. Is the element itself poisonous ?

258. 1173 grains of arsenic trisulphide were obtained by analysis in a case of poisoning. To how much arsenic trioxide does this quantity correspond?

259. How much arsenic is contained on the walls of a room which is covered with 1,869 square feet of paper, each containing o'78 gramme of Scheele's green ?



260. In what condition and in what localities does free sulphur occur in nature? Name the most important natural sulphides and sulphates.

261. What gases containing sulphur are sometimes found (a) in air, (b) dissolved in water?

262. With what minerals is native sulphur usually associated? How may this association be explained?

263. How is native sulphur extracted in Sicily? Give both the old and the improved process.

264. How is crude Sicilian sulphur purified in this country? Sketch the apparatus used.

265. How are flowers of sulphur and roll sulphur obtained? What is milk of sulphur, and how is it prepared?

266. Describe other methods employed on the large scale for obtaining sulphur.

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