40. Baumé Hydrometer. — A Baumé hydrometer is shown in Fig. 3. It consists of a glass tube, near the bottom of which are two bulbs. The lower and smaller bulb is loaded with mercury or shot, so as to cause the instrument to remain in a vertical position when placed in the liquid in the vessel a. The upper bulb b is filled with air, and its volume is such that the whole instrument is lighter than an equal volume of water.
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The point to which the hydrometer sinks when placed in water is usually marked, the tube being graduated above and below in such a manner that the specific gravity of the liquid can be read directly. It is customary to have two instruments: one with the zero point near the top of the stem, for use in liquids heavier than water; and the other with the zero point near the bulb, for use in liquids lighter than water.
41. Refining: the Distillates. — Refining is the final process of putting the distillates in shape for general use. As they come from the still, they are more or less colored and have a strong disagreeable odor, which becomes worse when the oils are burned. To make the oils clear and to deprive them of their offensive odor, they must be refined. This is done in large vats by means of various chemicals. After being treated the oil is of a whitish-yellow color, without disagreeable odor. In order to clarify it, the oil is run into settling tanks, where the water mixed with it settles to the bottom and the oil gradually clears. In case time is an object, the oil is filtered instead; but this latter process is not so satisfactory as the settling-tank method. The oil is now ready for barreling or shipping in tank cars to the dealers.
42. Rating Oil and Gasoline. — In selecting gasoline for any use, it is usually sufficient to know its density by
