HYDKOMETEK. 379 HYDEOMETEE. above and below tliis mark on the basis that the amount of the stem bet^en any two scale divi- sions should equal -j-^j of the volume of the part immersed in water. With an instrument graduated in this manner the specific gravity is obtained by dividing the reading into 100. In practice, it is more convenient to use a hydrom- eter whose scale is graduated to read specific gravity direct, but in this event the scale is ir- regular, and great care must be exercised in the construction and testing of the instrument. The accuracy of a hydrometer is greatly increased by making its stem as slender as po.ssible, and con- sequently increasing the space between the divi- sions, so it is customary to construct a number of instruments, each having a limited range, and designed for liquids of different densities. The hydrometer when used for a special purpose or substance, frequently undergoes modifications. The alcoholmeter, for example, is so graduated as to give at once the percentage of pure alcohol in a mi.xture of alcohol and water. The urinom- eter, lactometer, sacharimeter, and other instru- ments are all hydrometers used for special pur- poses. Hydrometers with arbitrary scales also play an important part in scientific work, though the present tendency is toward the exclusive use of specific gravity values. In the Baunie scale, which is encountered frequently in chemical proc- esses, a second fixed point on the hydrometer stem is determined in addition to that given by water. This is obtained by dissolving one part by weight of common salt in nine parts of water, and then the space between these two points is divided into ten equal parts, which are called degrees. The water-point is marked 10 on the scale, and the division is carried beyond for 40 degrees. For liquids heavier than water the sec- ond fixed point is determined by immersion in a solution of 15 parts of salt in 8.5 parts of water, and the space between it and the water-point, which is marked 0, is divided into 1.5 equal divi- sions. The scales of Cartier and Beck are also vsed. though less frequently than that of Baumf. The following tables from Kohlrausch, Leit- faden der praktischen Phtisik (Leipzig, 1900), afTord a comparison of these scales with the true values of specific gravity: Liquids Lighter than Water 8P. GR. Baum^ Beck Cartier .T.? .80 68.*° 46. S 35.6 26.1 17.7 10. 56.7° 42.5 30. 18.9 8.9 0. 43.'° .85 33.6 .90 25.2 .95 17.7 l.OO . 11. I.iQriDs Heavier than Water SPKCiriC GHATITT Banni^ Beck 1.0 0.0° 13.2 24.3 .33.7 41.8 48.8 54.9 60. 65. 69. 73. 0.0° 1.1 15.4 1.2 28.3 1.3 39 2 1.4 48.6 1.5 66.7 1.6 63.7 1.7 70. 1.8 76. 1.9 81. 2.0 85. 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 Vol. X.— 25. flflf^^i^ 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 stems 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 60° 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 .79.39 referred to water at its maximum density. Proof spirit has at 00° Fahrenheit a spe- cific gravity of .93353, 100 parts by volume of the same consisting 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, where 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 stem to the mark, while if the liquid is less dense the number of the weights must be 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 be used to determine the specific gravity of a solid, in which case 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 in the two amounts SET OF TMTKn 6TATE8 STANDAKL) B VDRO.METEliS A9 rsED BY THE INTER- NAL BEVENCE OFFICE.
