HYDROMETER

is an instrument for determining the density of liquids and solids. It is an application of the principle that when a body floats in fluid under the action of gravity the weight of the body is equal to that of the fluid dispiaced. Boyle was the first to employ h’s. A h. consists of a gla bulb with a long stem sealed and graduated. The bulb is weighted with mercury or small shot, a sufficient vuantity being used to immerse the whole of the bulb in the heaviest liquid in which it is to be used. The height and diameter of the stem must be such that the h. will float in the lightest liquid in which it is to be used. The point to which the h. sinks in the lightest liquid, and that to which it rises in the heaviest liquid, are then marked on the stem with the densities of these liquids, and the distance between graduated. The density of any liquid not greater or le than those marked on the stem can be obtained by placing the h. in a tall cylindrical ve el full of the liquid and reading the mark to which it sinks. Care must be taken that no air-bubbles adhere to its surface and that it does not touch the sides of the ve el.—Nicholson’s Hydrometer, made of metal, has a pan above and below the bulb and only one mark on the stem. From these pans the weights in air and water of a solid are obtained, and the formula weight in air divided by weight in water is the required density.