A hydrometer is an instrument for determining the specific gravity of liquids. It depends upon the law that a floating body displaces an amount of liquid equal to its own weight. Its most common form is a graduated glass tube with a large bulb near the lower end, below which is a smaller bulb filled with shot or mercury to make it stand upright. The lighter the liquid, the lower will the instrument sink. The density of the liquid can be read directly from the scale. Special forms for different liquids are often used, as acoholometer, lactometer, etc.
