The variety of soot black known as lamp black is the best; it is used for making copper-plate inks and black coach paints. Its price is much higher than that of ordinary soot black; for the best qualities, twenty times the price of an ordinary black is paid. The materials already mentioned are used for making lamp black, fish oils and rancid vegetable oils, and in recent times mineral oil and tar oils. In regard to vegetable oils it is to be observed that it is advisable to use very rancid oil, which gives a larger yield of soot; experience has shown that a very rancid oil requires a larger quantity of oxygen to burn without a smoky flame than a sweet oil. This indicates that a portion of the carbon in the rancid oil is present in such a form that it requires a higher temperature to burn it than is required for the carbon in sweet oils. The use of rancid oils for making soot blacks has thus two advantages, it is much cheaper and produces a larger yield. The only disadvantage, and not a very important one, attending the use of rancid oil is that the free fatty acids in these oils strongly attack the metallic parts of the lamps. This is especially the case with copper and brass; only such portions of the lamps as is absolutely necessary should be made of these metals; all other parts, and in particular the oil reservoirs, should be made of tin plate. Since the development of the coal-tar industry oils distilled from coal tar have come into commerce at low prices. These oils consist of carbon and hydrogen and are equally volatile with the essential oils, to which class turpentine belongs. There are two kinds of these tar oils, light and heavy; they differ both in specific gravity and in the boiling temperature, which ranges between tolerably wide limits. When the burning qualities of these oils are examined, they show a great difference in the quantities of oxygen necessary for complete combustion with a white flame without smoke. The more oxygen an oil requires to burn with a non-smoky flame, the more suited it is for the manufacture of blacks. Generally speaking, the difficulty of completely burning these oils is in proportion to their specific gravity and the height of their boiling point. By the distillation of rosin, oils are obtained which also consist of carbon and hydrogen and form a useful material for the manufacture of soot blacks. The mineral known as earth-wax or ozokerite, a substance intermediate between asphaltum and petroleum, can also be used, but since it is a solid it must be burnt in troughs.