Page:Popular Science Monthly Volume 27.djvu/268

254 slacking the fires, and allowing the distillation to proceed so slowly that the dense portions of the vapors are condensed on the dome of the still, and, falling back upon the surface of the hot oil, are heated above their boiling-points, and decomposed into a lighter oil and a carbonaceous residue. By continuing this process for several hours the oil has passed out of the still, leaving a quantity of residuum, as in the first instance. This cracking process is never complete, as a portion of the oil is cracked too much and another portion too little; but the average gives a burning-oil of the proper density, color, etc., while in other respects it is greatly inferior to the oil that is not cracked. The reader will readily perceive that by mixing these constituent oils of the petroleum suitable for burning-oil, which have been very properly called "normal" burning-oils, with different proportions of the cracked oils, a great variety of products may be obtained; but I propose in this article to speak of only three classes of burning-oils, and to show that these three classes may furnish oils that meet the demands of legal enactments, while at the same time they may be both very dangerous and very bad.

The first class of oils mentioned that are distilled from the petroleum unchanged consists of compounds of hydrogen and carbon combined in such proportions that the percentage of hydrogen is greater than in any other similar substances. In addition, they are very inert to chemical reagents, in this respect resembling paraffine or India-rubber. They may be washed with sulphuric acid or strong solution of caustic soda, and very completely purified; but they are not acted on by either of these powerful reagents, and the product is a pure, colorless oil, with the odor of kerosene, and burning with a dazzling, white flame. The wick is burned but little more rapidly than that of an alcohol-lamp. The flame does not smoke, neither does it emit any unpleasant odor. These oils are safe, healthful, and economical; in fact, they constitute the best and cheapest illuminating agent ever given to man.

When the oils too heavy for illuminating oils are destructively distilled or "cracked," the product is largely contaminated with oils containing less hydrogen in proportion to the carbon, and which are not inert to chemical reagents like those just described. When these oils are treated with sulphuric acid, both the oil and the acid are decomposed. The sulphur and a part of the oxygen of the acid (SO2) take the place of a part of the hydrogen of the oil, while this hydrogen unites with the remaining oxygen of the acid and forms water. This sulphur and oxygen thus become constituents of the oil, and when the oil is burned they escape into the room as sulphurous oxide identical with the fumes of burned brimstone. But this is not all: the sulphur compounds, and the heavy, imperfectly cracked oils, soon impair the capillary attraction of the wick; and, the flow of the oil being impeded, the wick becomes charred and coated with unburned carbon.