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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, (6) 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, (6) after they are dead.

307. Describe several constantly occurring processes (a) which remove oxygen from the air, (6) 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.

311. How may the composition of air be ascertained by means of the eudiometer ?

312. In an analysis by Dumas and Boussingault's method the copper employed gained in weight 1.4904 gramme. Find the number of grains of air used and of nitrogen obtained.

313. Dr. Angus Smith found in 100 volumes of air, at the sea-shore in Scotland, 20-999 volumes of oxygen and in Manchester 20*960 volumes. How much sulphur would be required to unite with all the oxygen in one litre of each specimen ?

314. How much air by volume is required for the complete combustion of J1234 grammes charcoal containing 85-89 per cent. carbon and 2-41 per cent. hydrogen ?

315. Give two distinct physical processes for separating oxygen from air.

316. Lavoisier heated some mercury for several days in contact with 50 cubic inches of air at a temperature of 19° C. and under a pressure of 28 inches of mercury. At the close of the experiment, 42 cubic inches of gas remained unabsorbed, and 45 grains of mercuric oxide were obtained, which when heated yielded 418 grains of

mercury
and 7! cubic inches of oxygen.

How do these results agree with modern determinations ?

317. Find the volume of nitrogen and oxygen present in air from the following results obtained by Bunsen in an analysis of the air at Marburg :

.

Pressure Temp. Vol. at o° C. Vol.

at o °C. C. and i m. pressure. Air employed 882'2

05276 0'9 464.94 + Hydrogen

1053-8

0°6929 0:8 729938
After explosion. 861.8 0:5084 097 43783

Maximum temperature of air = 1.5° C.
Minimum

- 1'25° C.
Barometer = 0*7423.

318. What is the action on air of (a) phosphorus, (6) potassium pyrogallate, at the ordinary temperature? How may the latter substance be used for the analysis of air?

319. Find the volume of air at 13° C. and 755 m.m. pressure required to burn a litre of carbon monoxide at the same temperature and pressure.

320. Find the composition of air from the following results obtained by Bunsen by absorption with potassium pyrogallate:

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