HYDROMETER. An instrument for meastuing the specific gravi ties of liquids. Its commonest form is a long graduated tube loaded at the bottom, and allowed to float in the liquid to be tested, the depth to which it sinks being indicated by the scale. The principle of the instrument consists in the fact, that a floating body displaces a quantity of liquid exactly equal in weight to itself, The hydrometer affords, therefore, a measure of the volume of the liquid necessary to counterbalance in weight the weight of the instrument ; and by comparing these measures of volume when different liquids are tested, the specific gravities of the respective liquids may be compared and determined.
The most convenient form of hydrometer on this principle is that of Zanetti, manufactured at Manchester. It is sold in sets of six, and the S. G. is got by adding a cipher to the number of de grees indicated ; the assumed temperature being 60° Faht.
The hydrometers of Twaddle, and Beaume, are constructed on the same principle, but graduated differently. There are two forms of
Beaume's hydrometer ; one for measuring the S. G. of liquids heavier than water, and called a "pise-oxide," or "pase-sirop ;" the other, for liquids lighter than water, and called a " pgse-esprit." These instruments are much used both in France and England, and are sometimes called " Areometers." An entirely different principle of construction has been adopted in the hydrometers of Nicholson, and Fahrenheit. In these instruments the line of flotation in water At 60° is marked, and when immersed in the liquid to be tested the instrument is loaded until it sinks to the same level,—the weight required to be added or removed deter mining the S. G. of the liquid.
The hydrometer does not afford an exact test of the strength of an old nitrate bath, be,cause the bath acquires by use nitrate of potass and other contaminations, which increase its specific gravity.