The first oil obtained is known as gasoline, used in portable gas machines for making illuminating gas. Then, in turn, come naphthas of a greater or less gravity, benzine, high test water white burning oil, such as Pratt's Astral common burning oil or kerosene, and paraffine oils. When the oil has been distilled it is by no means fit for use, having a dirty color and most offensive smell; it is then refined. For this purpose it is pumped into a large vat or agitator, which holds from two hundred and fifty to one thousand barrels. There is then added to the oil about two per cent, of its volume of the strongest sulphuric acid. The whole mixture is then agitated by means of air pumps, which bring as much as possible every particle of oil in contact with the acid. The acid has no affinity for the oil, but it has for the tarry substance in it which discolors it, and, after the agitation, the acid with the tar settles to the bottom of the agitator, and the mixture is drawn off into a lead-lined tank. After the removal of the acid and tar, the clear oil is agitated with either caustic soda or ammonia and water. The alkali neutralizes the acid remaining in the oil, and the water removes the alkali, when the process of refining is finished. A few refiners improve the quality of their refined oil by redistilling it after treating it with acid and alkali. All distillates of petroleum have to be treated with acid and alkali to refine them. There is one thing peculiar about the distillates of petroleum, and that is that the run which follows naphtha, which is called "the middle run oil," is the highest test oil that is made, running as high as 150 and 160 degrees flash, while the common oil which follows, viz., from 45 down to 33 degrees Baume, will range at only about 100 flash, or 115 and 120 degrees burning lest.