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.

SELENIUM.

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?

285. How are the amorphous and crystalline forms of selenium prepared?

286. Compare the properties of selenium with those of sulphur.

287. What occurs when selenium is heated in air or oxygen? Give the formula and properties of the substance formed.

288. What is the effect of light on the conducting power (for electricity) of selenium ?

289. How may a solution of selenium be obtained?

290. What is peculiar about the vapour density of selenium?

TELLURIUM.

291. Name some naturally occurring compounds containing tellurium ?

292. From what compound is tellurium usually prepared, and by what methods?

293. Compare the properties of tellurium with those of sulphur and selenium, and write the formulæ of several analogous compounds of each element.

294. What body is formed when tellurium is burned in oxygen? How may it be distinguished from the bodies produced by burning selenium and sulphur in oxygen?

AIR.

295. Who first determined the composition of the air? Describe fully the method he employed.

296. What are the two chief constituents of the air? By whom and when were they each discovered?

297. Name the bodies which are always present in the air in small quantities, and those which are occasionally present.

298. Describe a method for recognising with certainty each constituent of the atmosphere.

299. Is air a mechanical mixture or a chemical compound? Give reasons in support of your answer.

300. How do you account for the presence of carbon dioxide in the air? Why does the quantity remain nearly uniform?

301. What changes are produced in air by the breathing of animals and during the growth of plants?

302. A candle is burned in a closed bottle of air till it goes out, and a mouse is kept in a closed bottle of air

till it dies. Why does the candle go out and the mouse die, and what changes are produced in the air in each case?

303. Describe experiments to prove (a) that plants give out oxygen in daylight, (b) that animals breathe out carbon dioxide.

304. How do you account for the presence of each substance found in air only in small quantity?

305. How is the remarkable uniformity in the composition of the air explained?

306. Compare the action of air on plants and animals, (a) while they are alive, (b) after they are dead.

307. Describe several constantly occurring processes (a) which remove oxygen from the air, (b) which add carbon dioxide to it.

308. In what quantities are nitrogen, oxygen, carbon dioxide, and ammonia present in air?

309. How may phosphorus be used to demonstrate approximately the proportions of oxygen and nitrogen present in the air?

310. Describe fully how Dumas and Boussingault determined the composition of air by weight, and sketch the apparatus required for this purpose.