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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?
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 formule 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.
267. Describe fully the changes which occur when sulphur is heated from the ordinary temperature to its boiling-point.
268. Describe the three modifications of sulphur, and state exactly how each is obtained.
269. Which forms of sulphur are obtained (a) by evaporating a solution of sulphur in carbon disulphide, (b) by adding an acid to calcium pentasulphide ?
270. Describe an experiment in proof of the statement that sulphur sublimes at the ordinary temperature.
271. In what form is sulphur obtained when sublimed at a temperature of 100° in a current of air?
272. How may a sulphur be converted into ẞ sulphur, and vice versâ?
273. What occurs when copper is dropped into boiling sulphur?
274. What gases are produced when sulphur burns (a) in air, (b) in oxygen, (c) in nitrous oxide?
275. How may sulphur be directly united (a) to carbon, (b) to chlorine?
276. What occurs in each case when sulphur is boiled (a) in nitric acid, (b) in sulphuric acid, (c) in caustic soda? Give equations.
277. Represent by equations the reactions which occur when iron pyrites is strongly heated (a) in presence of air, (b) in absence of air.
278. What weight of sulphur is contained in a litre of sulphuretted hydrogen, and in a litre of sulphur dioxide?
279. In what respect is the vapour density of sulphur abnormal? At what temperature is its vapour 32 times as heavy as hydrogen?
280. How could you obtain free sulphur from calcium sulphate ?
281. Give other names for the substances called brimstone, galena, monoclinic sulphur, zinc blende, plastic sulphur, gypsum, and rhombic sulphur.
282. Write formulæ for several substances which show the analogy of oxygen and sulphur compounds.
283. By whom was the element selenium discovered ? From what substance was it first prepared? What is the origin of the name?
284. From what substance and how is it generally prepared?