BICARBURETTED HYDROGEN. 70. COAL GAS is a familiar illustration of this compound; it differs from carburetted hydrogen in containing two volumes of the vapour of carbon with two of hydrogen: on this account it burns with a brighter flame. It is generally obtained by dry distillation, or carbonization in iron retorts, of vegetable and animal substances, or coal which is of vegetable origin. Those in use are coal, especially the cannel coal, oil, resin, (improperly called rosin,) and the liquid resin tar. We will now endeavour to point out the changes that take place during the distillation of coal: it has been proved by experiments that the coal should not be introduced into the retorts until they are heated to a cherry red; at this temperature a portion of the tar is converted into gas instead of passing over in the state of vapour; this operation is generally continued for five or eight hours. The residuum in the retort is termed COKE, upon an average 1lb. good coal yields 4 cubic feet gas. 15 cubic feet gas. 1lb. tar yields 12 cubic feet gas. as coal is the cheapest it is now generally used. 71. As the distillation of coal yields tar, ammonia, sulphuretted hydrogen and carbonic acid with the illuminating gas, bicarburetted hydrogen, it is necessary to purify it. Without entering into a minute description of the process, the following statement may be deemed intelligible. The heated retorts, which are set in brickwork with their mouths presented to the workmen, are half filled with coal, the doors are then carefully closed by fire-clay; in a short time the coal yields up its volatile compounds, which pass into pipes that first ascend and then descend by bending into a large cast iron tube half filled with water, the bent tubes are styled dip pipes, (No. 1.) the water cylinder is named the hydraulic main, (No. 2.) The ends of the dip pipes are immersed in the water, which condenses a portion of the tar and ammoniawhile the gas passes on by a pipe to the condenser, (No. 3,) which consists of a number of vertical pipes surrounded by cold water; these are very palpable in the gas works of this city. The tar cistern (No. 4,) is connected with these pipes, in which the tar and ammonia are condensed after coursing through the vertical pipes-the carburetted hydrogen, carbonic acid, and sulphuretted hydrogen then pass to the purifier, (No. 5,) which contains a thick mixture of lime and water called cream of lime, with a revolving spindle set with vanes- -the lime seizes the carbonic acid, (vide exper. 1) and abstracts the sulphur from the hydrogen, which unites with the illuminating gas, now purified, that passes through a most ingenious instrument, the gas meter, (No. 6,) which registers the quantity as it takes its course towards the gasholder or gasometer, (No. 7.) From this vesssel the citizens are supplied. It is deserving of notice that the iron retorts are destroyed by the vapour of carbon uniting with the iron, forming artificial plumbago or black lead, which is extensively used in galvanic experiments. Experiment 58.-Fill the bowl of a tobacco-pipe with pounded coal, cover up the mouth with a lute of fire-clay, insert the bowl into the fire, and in a short time the gas can be ignited when issuing from the small tube. 72. Coal gas was first obtained by the Rev. Dr. Clayton, in 1739. Mr. Murdock, engineer to Messrs. Bolton and Watt, was the first person who publicly used it as a substitute for oil, in 1802. In that year there were illuminations for the peace of Amiens: Mr. Murdock on that occasion, in the Soho factory, displayed, for the first time, gas-light. Dublin was not illuminated with gas till 5th October, 1825. QUESTIONS. 274. How does bicarburetted hydrogen differ from carburetted? 275. How is it generally obtained? 276. Why is the retort made red hot before the coal is introduced? 277. Which yields most gas-coal, oil, tar, or resin? 278. Why is it necessary to purify coal gas? 279. Can you describe the changes that take place as the gas goes to the gasometer? 280. What is formed on the inside of the retort? 281..Who first used coal gas? 282. In what year was Dublin illuminated? END OF FIRST PART. AT REDUCED PRICES. LIFE and REMAINS of the late Rev.PETER ROE, of Kilkenny. By the Rev. S. Madden, large 8vo. cloth, published at 14s. reduced to 3s. LIFE and REMAINS of the late B. W. MATHIAS, of the Bethesda, Dublin, 12mo. cloth, published at 5s. reduced to 1s. 6d. MEMOIRS and SELECT REMAINS of the Rev. T. RAWSON TAYLOR, with an Introduction by James Montgomery. 12mo. cloth, published at 5s. reduced to 2s. EPISCOPACY, TRADITION, and the SACRAMENTS; being a Reply to the Oxford Tracts. By the Rev. William Fitzgerald, Professor of Moral Philosophy in T.C.D. 8vo. pub. at. 1s. 6d. reduced to 6d. 8vo. 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