for purity and lustre are not surpassed by even the best wax candles. Unsized paper, after having been soaked in paraffine, may be kept for weeks in concentrated sulphuric acid without undergoing the slightest alteration. Hence it is an excellent coating for the labels of bottles in which acids are kept. 178. Asphalt.-Asphalt, or pitch, is the residue left after distilling tar. It is used for varnishes, and as a material for making lamp-black. When vege 179. Slow Destructive Distillation. table substances decay with a partial or complete exclusion of air, we have a kind of slow destructive distillation. Hydrocarbon gases, which in some places escape from the earth in large quantities, are one of the products of this process; the well-known coal-oils are another; and coal is a third. The gas obtained by stirring the mud in marshes and at the bottom of stagnant pools is formed in this way, and is made up chiefly of marsh-gas, HC, and carbonic acid, CO,. 180. The Formation of Mineral Coal.-In tropical swamps where vegetation is rank, vast masses of vegetable matter accumulate, and gradually decay under water. In some cases the land at the bottom of these swamps is slowly sinking; and the bed of peat, as it sinks with it, becomes covered with mud and sand, which numerous streams are washing down upon it. This goes on, year after year and century after century, until the bed is buried hundreds of feet beneath the surface. The vegetable matter, thus sunk in the earth and subjected to enormous pressure, gradually undergoes a process of internal combustion similar to that which takes place under water. In many cases the decomposition is hastened by the agency of the internal heat of the earth. It is probable that the vast beds of coal found in various parts of the earth have been thus formed. All this coal is the remains of an ancient vegetation, and it undoubtedly required millions upon millions of years to complete its conversion into coal. 181. Hard and Soft Coals. - The mineral coals may be conveniently divided into hard, or anthracite, and soft, or bituminous coal, and there are several varieties of each. The main differences between the two are these: hard coal is almost pure carbon, while soft coal contains also considerable hydrogen and some oxygen; hard coal still retains the cellular structure of the wood, which is clearly seen under the microscope, while in soft coal this cellular structure is almost entirely wanting; hard coal burns without flame, soft coal with flame. 182. Products of the Distillation of Bituminous Coal. The products obtained by the destructive distillation of coal are still more numerous than those obtained from wood. Wood, containing much oxygen and comparatively little nitrogen, furnishes compounds which contain much acetic acid and little ammonia, and which, therefore, have an acid reaction. Coal, on the other hand, contains much nitrogen and little oxygen, and gives products rich in ammonia, and having consequently an alkaline reaction. When coal is distilled at high temperatures, an abundance of an inflammable gas is obtained, and also a large amount of liquid products, which are then called tars. When coal is distilled at a low temperature, little gas is obtained, and the liquid products are then called oils. 183. The Composition of Coal-tar. Coal-tar has been found to contain three classes of substances: (1) Acid oils. (2) Alkaline oils. (1) The most important and abundant ingredient of the acid oils is carbolic acid, C,H,O. It is analogous to creosote, and has even more powerful antiseptic properties. It is one of the most valuable disinfectants known, and both the acid and the compound which it forms with lime are now much used for this purpose. It is also largely employed in dyeing silk and woollen goods. The dye-stuff is prepared by heating carbolic acid moderately with nitric acid, and is called picric acid. On evaporating this liquid, yellow scaly crystals are obtained. Like all the tar colors, its dyeing qualities, when in solution, are most intense. Silk and woollen goods put into the solution, even when cold, assume a rich yellow color, far surpassing that of other dyes. (2) The alkaline oils constitute but a small fraction of the tar. Their most important ingredients are ammonia and aniline, CH,N. Aniline is an oily substance which, when acted upon by compounds which readily part with oxygen, furnishes a complete series of the most brilliant dyes. The preparation of these rich dyes from aniline is one of the most interesting discoveries of modern times, and has caused almost a revolution in the arts of dyeing and calico-printing. It is still more surprising when we consider that these brilliant colors are obtained from what was until recently a disagreeable waste product of the gas-works. When first prepared, they were worth their weight in gold; now, they can be bought at a comparatively moderate price. Their dyeing qualities are so intense, that a little material goes a great way; so that, notwithstanding their high price, they are more economical than other dyes. (3) The neutral oils are the coal-oils proper. They contain a great variety of compounds, both liquid and solid, the latter being held in solution. Of the liquids, benzole, toluole, and cumole are the most important; and of the solids, paraffine and naphthaline. Benzole is a very 184. Benzole and Nitro-benzole. important compound, as it is the material from which aniline is usually prepared. The symbol for benzole is C.H.. When mixed with concentrated nitric acid, it loses one atom of H, and takes one of NO, in its place, and becomes nitro-benzole, CH,NO2. Nitro-benzole is the artificial oil of bitter almonds, and is much used in the art of perfumery. Its most important use, however, is in the preparation of aniline. When heated with acetic acid (C2HO2) and iron filings, it loses two atoms of oxygen and takes up two of hydrogen, and is converted into aniline, CH,N. 185. Toluole and Cumole.-Toluole, CH, and cumole, C,H12, are the chief ingredients of the wellknown illuminating or lamp oils obtained from coal. 186. Naphthaline.-Naphthaline is a beautiful, pearlywhite solid. It is inflammable, but burns with a smoky flame and a disagreeable odor. Brilliant red and blue colors, rivalling those prepared from aniline, have lately been obtained from this solid. When vegetable matter is distilled at a high temperature, benzole and naphthaline are formed in great abundance, with but small quantities of toluole, cumole, and paraffine. When, on the other hand, the distillation is conducted at a low temperature, toluole, cumole, and paraffine are formed in large quantities, with but little benzole and naphthaline. COAL-OILS. 187. At the beginning of the present century, the means of lighting our dwellings consisted, in the main, of poor tallow candles and dim and dirty oil-lamps. On the continent of Europe, whale-oil and other animal oils were costly, and resort was had to natural tar and bituminous slate, in order to obtain illuminating oils. For more than twenty years past, lamp-oils have there been extensively prepared from wood, rosin, and bituminous matter. In Great Britain and in this country the manufacture of coal-oils is of much more recent growth, because the extensive whale-fisheries supplied all the wants of the market. The manufacture of coal-oil was introduced into this country in 1853, and was at first confined to those districts where bituminous coal could be mined at a cheap rate. Soon after this manufacture was established in this country, and after the value of coal-oils came to be fully recognized, attention was drawn to petroleum, or rockoil, as a ready means of supplying these oils cheaply. On examination, this oil was found to be analogous, in its composition and its properties, to that obtained by the destructive distillation of soft coal and other bituminous substances. - All scientific men 188. The Origin of Petroleum. are agreed that the petroleum found in the earth results from the decomposition of organic matter, and nearly all are agreed that it results mainly from the decomposition of vegetable matter. It is, however, a disputed point, whether it results from the original decomposition of the vegetable substances, or from the action of the internal heat of the earth on the bituminous coal at a subsequent period. It is probable that the petroleum now found in the earth is the product both of the original decomposition and of subsequent distillation. Petroleum is, however, rarely found in contact with bituminous strata of any |