Page:The American Cyclopædia (1879) Volume IV.djvu/769

 COAL PRODUCTS 753 nature, care is taken that the retorts are heat- ed very gradually, and do not acquire more than a low red heat. The tarry matters thus escape decomposition, and by repeated distil- lations afford crude naphtha and its secondary products. The foregoing table represents in the clearest manner the great variety of substances besides coke which are obtained by means of this process, and their immediate sources. Coal tar was for a long time a troublesome product of the gas works, no useful applica- tion of it to any great extent being known. It was employed as a covering to protect iron work exposed to the weather ; and the pitch obtained by distilling it was found when mixed with earthy matters to be a good substitute for the natural product asphaltum, used for arti- ficial pavement, water-tight covering for roofs, &c. Finally the tar came to be an object of purchase by tar distillers, who learned to ex- tract from it the crude naphtha and also the light oily fluids. The pitch, too, by repeated distillations was made to yield more oily mat- ters, which are useful for lubricating machinery and other purposes. The crude naphtha is now purified by mixing it with -fa its bulk of con- centrated sulphuric acid, adding when cold 5 per cent, of peroxide of manganese and distil- ling off the upper portion. A rectified naphtha is thus obtained, which readily dissolves caou- tchouc, and mixed with wood naphtha produces a powerful solvent of various resinous substan- ces, useful in making varnishes. Still further purified, the liquid benzole is obtained, which has been applied to many useful purposes, especially that of an illuminating agent. The light essential oils, as also the heavier qualities which come over after these, are found to pos- sess antiseptic properties, which render them of value for preserving wood from decay. From the essential oils the tar creosote or car- bolic acid is obtained, which possesses extra- ordinary antiseptic properties, and is used in the preparation of a valuable dyestuff called carbazotic acid. The heavy oil yields a sub- stance called aniline, which gives with bleach- ing powder and other agents a magnificent blue color, and is employed in dyeing. Naph- thaline also, which is a solid white substance obtained in large quantities in the distillation of the tar, yields two coloring matters, one call- ed naphthalic acid, and the other chloronaph- thalic acid ; the latter of which is nearly iden- tical with the coloring principle of madder, and gives with alkalies a beautiful red color. In- stead of naphthaline, by conducting the distil- lation at a lower temperature, may be obtained the waxy substance called paraffine, which is now used for the manufacture of candles and the paraffine oils. The most important of these products will be found more particularly no- ticed under their separate heads, and the gen- eral subject will receive further attention in treating of gas light. By oxidizing aniline with bichromate of potash a bronze-colored sub- stance is produced, dissolving in alcohol with a beautiful purple color. In concentrated sul- phuric acid its solution is green. On adding water and precipitating with an alcoholic solu- tion of potash, the coloring matter is precip- itated unchanged. It is of intense hue, and considered as good if not better than archil ; it is very stable, not being decomposed at a tem- perature of 482 F. One pound of the solid sub- stance will dye 200 Ibs. of cotton a moderately dark lilac, the color standing well the action of light and heat, acids and alkalies. The dis- tillation of coal for obtaining parafBne and the 011 accompanying has been conducted in the following manner : The retorts used are like those employed in making gas. A worm pass- es from them through a refrigerator kept at a temperature of about 55 F. The oil condenses in the worm, and is collected in a receiver. It deposits some paraffine when cooled to a low degree. Some illuminating gas is generated in the process and escapes. Coke also remains in the retort, as in the gas-making process. The oil is purified by heating a cistern of it to about 150 F. Water and insoluble impurities subside on standing, and the oil may be drawn off. It is redistilled to dryness, and a carbon- aceous residuum is left in the iron still. From the cold condenser it is run into leaden vessels, and fa its bulk of oil of vitriol is added and the mixture is well stirred for an hour. In 12 hours the sulphuric acid has settled, together with the impurities it has taken up. The oil is then drawn off into an iron vessel, and 4 gallons of a solution of caustic soda, of sp. gr. 1-3, are added to every 100 gallons of oil. This is well stirred for an hour, to take up all the remain- ing acid, and it is then left for 6 or 8 hours for the soda and impurities to subside. The oil is again drawn off and distilled. Paraffine oil thus obtained contains a more volatile oil, that may be mostly separated from it. Half its bulk of water is added to it, and the mixture is dis- tilled as before, the boiling being continued for 12 hours by renewing the loss in the still with more water. The steam carries the more vola- tile fluid over with it. They condense in the worm, and separate by standing in the receiv- ers. This fluid is suitable for burning in lamps and for other purposes. The oil left in the still is to be drawn away from the water left with it, purified by distilling from -fa its bulk of sulphuric acid, and then from chalk added as the caustic soda was used. The oil is kept for a week in a warm place, and being then drawn off from the sediment is fit for lubricating or illuminating purposes. Cooled to 30 or 40 F., the paraffine crystallizes, and may be col- lected by pressing out the oil through cloths. An improvement made upon this process consists, in introducing highly heated steam into the retorts with the bituminous matters. It rapidly unites with and carries off the vola- tile matters arising from the coal, checking their conversion into gas, and greatly increas- ing the amount of oily or condensable product. The condenser is kept at a temperature of