The following tables from Kohlrausch, Lcit faden der praktisehen Physik (Leipzig. 1900), afford a comparison of these scales with the true values of specific gravity: The specific gravity of liquids changes with the temperature so that it is of the utmost impor tance to have the liquid at the temperature for which the instrument is made, or to use a ther mometer, and then apply suitable corrections. For this purpose many of the finer hydrometers contain within their steins a thermometer tube whose bulb is placed in the lower portion of the instrument.
In the United States Internal Revenue Service the hydrometers furnished to the inspectors are so graduated as to indicate the number of parts by volume of proof spirit equivalent to the vol ume of the liquor at the standard temperature. which is G0° Fahrenheit. They are constructed so as to read 100 for proof spirit, and 200 for absolute alcohol. Proof 'spirit in the United States is defined by law to be that mixture of alcohol and water which contains one-half of its volume of alcohol, the alcohol when at a tem perature of GO' Fahrenheit being of specific grav ity .7939 referred to water at its maximum density. Proof spirit has at G0° Fahrenheit a spe cific gravity of .93353, 100 parts by volume of the same consist ing of 50 parts of absolute alcohol, and 53-71 parts of water.
A hydrometer of a dif ferent type from those de scribed above is the weight hydrometer.•here the sub merged volume remains constant, but as the speci fic gravity of the liquids changes, the weight of the instrument must be varied in order to immerse it to a given point. The Nichol son hydrometer is repre sentative of this class, and consists of a brass tube with conical ends, which floats upright and carries above a thin stem the carrying pan in which may be placed a substance whose specific gravity is to be found. When used to determine the specific gravity of a liquid the weight of the apparatus is first ascertained, and then it is placed in water and weights added until a marked point on the stem is at the surface of the water. if the in strument is placed in a liquid of greater specific gravity, then additional weights must be placed on the pan in order to sink the stein to the mark, while if the liquid is less dense the number of the weights must he diminished. The weight of the instrument increased by the amount of the weights added when the instrument was placed in water, divided by the weight of the in strument and the weights added when the instru ment was in the liquid under test. will give the
specific gravity. This hydrometer can also he used to determine the specific gravity of a solid. in which ease the latter is first placed in the upper pan. while the instrument is in water. and the number of weights which must be removed is noted. The substance is then placed in the lower pan, and the amount of weight which must be removed to restore it to its former position ascertained. The difference iu the two amounts represents the weight of the water displaced by the substance, and if divided into the sun] of the weights removed when the body was placed in the upper pan, will give the speeilie gravity. The hydrometer of Fahrenheit is based on a similar principle, but is made of glass instead of metal, and has a bulb filled with mercury at its lower end instead of a weighted cup. It can only be used for liquids. These instruments are not as reliable as the ordinary hy drometers, and are not as widely used. In the most accurate deter minations of specific gravity a chem ical balance is employed, and equal volumes of the liquid actually weighed, or if the substanco is it solid it is weighed both in the air and in distilled water, suitable cor rection• being applied for tempera ture. and other disturbing influences, The hydrometer is without doubt one of the earliest pieces of physical apparatus, its invention being gen erally ascribed to Archimedes. to whom is due the principle on which it is based. The instrument is men tioned by Priscian, who died about A.D. 500, and it is also described by Synesills of Ptolemais in a letter to Ilypatia of Alexandria, under the name of hydroscopium, as follows: "It is a cylindrical tube the size of a reed or pipe, a line drawn along it lengthwise 'which is intersected by others, and these point out the weight of water. At the end of the tube is a cone the base of which is joined to that of the tube so that both have hut one base. This part of the instrument. is called baryllion. if it be placed in water it remains in perpendicular di rection so that one can readily discover the weight of the The date of this letter can be approximately used by the fact that llypatia was murdered in A.D. 115. The use of the hydrometer was known to the Saracens of the tenth and eleventh centuries, and one of their writers. Al Klifizini. attributes its invention to a Greek phi losopher named Popping, a contemporary of Theodosius the (treat. In this connection this same writer refers to the fundamental discovery of Archimedes.