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Since the introduction of the above, the consumption of nitrate of soda is sometimes lessened by more than half.

Another very recent improvement, the invention of a German chemist named Sprengel, is the substitution of water spray, blown in by steam, for steam jets, in the leaden chambers. By this method a saving of coal to the extent of one third is said to be effected.

In theory, 1 molecule of sulphur requires only 3 molecules of oxygen to convert it into sulphuric acid, viz. 2 to form sulphurous anhydride, and 1 to convert the latter into sulphuric anhydride, which combines with 1 molecule of water to form the acid. Thus, 1 kilogram of sulphur requires 1500 grams or 1055 litres of oxygen, which is equivalent to 5275 litres of air containing 4220 litres of nitrogen; when pyrites is used, a far larger quantity of air is required, for the obvious reason that the

pyrites becomes converted into ferric peroxide. 1 kilogram of pyrites requires for its combustion nearly 6600 litres of air.

In well-regulated works the spent and escaping gases should not contain more than 2 per cent. of oxygen. If from 100 kilograms

of sulphur 306 kilograms of strong acid of sp. gr. 1·84 be obtained, the result is regarded as very satisfactory; more frequently the product from 100 kilograms of sulphur does not exceed 280 or 290 kilograms.

Purif. Commercial sulphuric acid frequently contains nitrous acid and other oxides of nitrogen, arsenic, lead, and saline matter. The nitrous acid may be removed by adding a little sulphate of ammonia, and heating the acid to ebullition for a few minutes. Both nitric and nitrous acid are thus entirely decomposed into water and nitrogen gas. The arsenic may be got rid of by adding a little sulphide of barium to the acid, agitating the mixture well, and, after repose, decanting and distilling it. Lead, which exists as sulphate, may be separated as a white precipitate by simply diluting the acid with water. Saline matter may be removed by simple rectification. A good way of purifying oil of vitriol is to heat it nearly to the boiling point, and pass a current of hydrochloric acid through it; the arsenic is thus carried over as the volatile chloride of arsenic, while the nitrous and nitric acids are expelled almost completely. To obtain a perfectly pure acid, it should be distilled after the removal of the nitrous acid and arsenic by the methods indicated above. “The distillation is most conveniently conducted, on the small scale, in a glass retort, containing a few platinum chips, and heated by a sand bath or gas-flame, rejecting the first 1⁄2 fl. oz. that comes over.” (Ph. E.) In the Ph. D. the first tenth of the distillate is ordered to be rejected, and the process to be stopped when no more than about 1 fl. oz. is left in the retort.

According to Dr Ure, the capacity of the retort should be from 4 to 8 times as great as the volume of the acid, and connected with a large tubular receiver by a loosely fitting glass tube, 4 feet long and 1 to 2 inches in diameter. “The receiver should not be surrounded with cold water.” We find that fragments of glass, or of rock crystals, may be advantageously

substituted for platinum foil, to lessen the explosive violence of the ebullition. Sulphuric acid which has become brown by exposure may be decolorised by heating it gently, the carbon of the organic substances being thus converted into carbonic acid.