AREON'ETER (aratos, thin, and metre4, I measure; Fr. ar&m?tre or p?ee-liquenr; Ger. Araometer or &nkirage). called also hydrometer, an instrument which is allowed to float freely in liquids, to determine their specific gravity or that of solid bodies. By specific gravity (q.v.) is meant the ratio that the weight of any volume of a substance bears to the weight of the same volume of water. 'Thus, a cubic' foot of alcohol weighs 793 ozs., while the same quantity of water weighs 1000 ozs.; the specific gravity of alcohol is set down, therefore, as mp, or .793. A cubic foot of sulphuric acid weighs 1841 ozs., and has, consequently, a specific gravity of 1.841. These relations are not confined to the particular volume, one cubic foot, of these bodies, but bold for any equal volumes of them. Equal volumes of alcohol, water, and sulphuric acid, have always to each other the ratio respectively of 793, 1000, and 1841; and this is only an instance of the general principle, that equal. volumes of different substances have weights bearing to each other the direct ratio of the specific gravities of these substances. i This is the principle on which areometers with weights, or weight-areometers, are structed. If, however, equal weights of any two of these liquids were taken, it would be found that .793 of a cubic foot of water would weigh as much as 1.000 cubic foot of alcohol; 1.000 cubic foot of sulphuric acid as much as 1.841 cubic ft. of water; or .793 of a cubic foot of sulphuric acid as much as 1.841 cubic ft. of alcohol; more generally thus—when equal weights of two different fluids are taken, the volumes of each are inversely as their specific gravities. On this latter principle depends the use of ometers with scales, or scale-areometers. The scale-A. is much more commonly employed than the weight-A., and is, in consequence, a much more important ment. Of the various forms of scale-areometers, that contrived by Gay-Lussac deserves particular notice, from the simplicity of the mode of graduation; and an account of it will give the best idea of the general nature of such instruments. Fig. 1 gives a sentation of it. It consists of a uni A form glass tube, AB, blown into two ' Ng. 1. bulbs C and D, at the bottom. The lower bulb, D, is loaded with mercury, in $. Fig. 2 so that when the instrument floats in A A gi ',..i any liquid, the stem, AB, is main , n"- 1 tanned in a vertical position. We v, shall suppose that the quantity of mercury is so adjusted that when placed in water, the A. sinks to the 1.:'.... •; be called the water-point. iiiiiit.coi,) '.:!
i• ''-. i 8 to the principle of Archimedes, the weight of the volume of water dis- point n' k i :, erif',1 placed by the instrument up to this 1**__ point is equal to the weight of the Li --4,, o instrument. Let us suppose, for the , lit) sake of simplicity, that the water so displaced is a cubic inch, the weight . „,.,-.,---:t. of the A. will be that of a cubic inch of water, or 250 grains (more cor rectly 252.5 grains at 60° F.). If the A 0 be now placed in a fluid heavier than Areometers. water, such as a mixture of sulphuric acid and water, having a specific gravity / or 1.25, it is manifest that if it is sunk again to the water-point, the displaced fluid would weigh t of 250 — 312+ grains, or 62+ grains more than the weight of the instrument. As much, therefore, of the stem of the A. must rise above the liquid as will reduce the weight of the displaced liquid to 250 grains; or reduce the volume to it of what it was before. If the stein in this case rises to B, the volume displaced by the part WB is + of the volume displaced by the instrument at the water-point. If we consider the whole divided into 100 parts, and mark 100 at W, B must be marked 80, as the A. displaces up to that of 100; and if the intervening space on the stem be divided into 20 equal parts, each of them will corre spond with of the water-volume—viz., .01 of a cubic inch, or with T-6 of the weight of the instrument—viz., 2.5 grains. If the same scale be carried above the point W. and the divisions marked as ascending from 100, the A. will be serviceable likewise for fluids less dense than water, and will mark the volumes which it displaces in each of then. The A. thus graduated gives immediately the volumes which it displaces in dif ferent liquids; and from these, seeing that it displaces in every case a weight of liquid equal to its own, -the specific gravities may be calculated according to the principle already stated—viz., that equal weights of two different fluids have volumes inversely as their specific gravities. If, in a mixture of sulphuric acid and water, the A. stands at 90, according to the above principle, 90 volumes of the mixture weigh as much as 100 of water; therefore, its specific gravity is or 1-. If, again, in a mixture of spirits and water, it should stand at 110, 110 volumes of the mixture weigh as much as 100 of water, so that its specific gravity is }c-1, or 44. In all cases, then, 100 is to be divided by the number read on the A., to determine the specific gravity of the liquid in which it floats.