The delicacy of the A. depends on the distance of the divisions on the scale, or on the thinness of the stem compared with the bulbs. An instrument possessing this ad vantage cannot be made to serve boils for liquids heavier and lighter than water, for the stem would be-of an inconvenient length;?iihd it is usual to construct two ammeters one marked with the water-point at the top, and the scale descending to 50, for fluids heavier than water; and the other, with the water-point at the bottom, and the scale ascending to 150 for fluids lighter than water. The scale is generally marked on a slip of paper, which is fixed inside the stem. Gay-Lussac's A. is also known under the name of " volumometer." Although it cannot be surpassed either for accuracy or simplicity, it is much less used than other instruments of a similar nature furnished with arbitrary scales requiring the aid of tables to interpret the readings. The best known of these is Twaddle's A., used in England; and Beaume's A., extensively adopted ou the conti nent. The A. with an equally-divided scale is a very ancient instrument; it was known, amono. the Greeks under the name of " baryllion.' Ou some areometers the divisions! arc not at equal distances, but are so drawn as to give at once, without table or calcula tion, the specific gravity of the fluid in which they are placed. Although very desira ble, iu practice they do not possess the accuracy of the A. with equally-divided scales, because the graduation of them is attended with considerable difficulty.
No form of A. can be made to determine specific gravities with perfect accuracy, and such instruments are only useful where a ready and good approximation is all that is needed. They are, in consequence, employed chiefly to ascertain the specific gravity of the various liquors and solutions which occur in the arts and manufactures, and very frequently they are graduated with reference to special liquids, as spirits, wine, milk, brine, etc. The alcoholometer or hydrometer of Sikes is an instrument of this latter description, and is in general use in the excise for estimating the strength of spirits. It is represented in Fig. 2. BC is a hollow brass ball, surmounted by a fiat stem AB, and loaded below by a short conical stem CD, terminated by the pear-shaped bulb D. It is accompanied by eight weights, by which the weight of the instrument niay be increased, and the range of the scale extended to fluids heavier as well as lighter than water. One of these weights, W, is shown in the figure; it is furnish eel with a slit., so as to allow of it being slipped on to the narrowest part C, of the lower stem. The stein, AB, is graduated into 11 equal parts, and these again into halves; and the instrument is so adjusted that its indications give the volumes of water that must he added to or taken from 100 volumes of the mixture under examination to reduce it to proof-spirit (see ALcouoi.), which is a mixture of nearly equal parts of water and alcohol. Thus, if the A. indiaates 11 over-proof, 11 volumes of water must be added in order to bring the liquid down to proof-strength; and 100 gallons of such strength would be reckoned as 111; 100 gallons, at 11 under-proof, would in the same way be charged as 89. Very carefully-constructed tables accompany the instrument, in which the specific gravity and percentage of alcohol of different mixtures, at different temper atures, are marked, corresponding to each degree of the A. Since the specific gravity of alcohol is known, it might be thought that if that of a mixture of it with water were known, the relative proportions of each would also be known. Such, however, is not
the case, for alcohol and water possess a chemical affinity for each other, which causes the combined volumes of the two to measure less than the two volumes separately. Thus, 50 volumes of alcohol mixed with 50 volumes of water does not make 100 volumes of the mixture, but only 96, and thereby the specific gravity of the mixture is higher than it would have been if no contraction had taken place. As the law of this con traction is very complicated, the relative proportions of the two in a combination of given specific gravity are only to be estimated from tables founded upon experimental data.
The peculiar feature of areometers with weights is, that instead of a scale, they have only one mark on the stem, to which the A. is in all cases sunk. One of the best-known instruments of this kind is the A. of Nicholson. It consists of a brass tube, BC (fig. 3), about 1 in. in diameter, closed above and below by conical ends, to the upper of which a wire is fixed, carrying on the top of it a cup A, capable of containing the weights; and to the lower a hook is attached, from which hangs the cup D. The lower part of the cup, D, is also provided with a hook, and the whole instrument is kept vertical, partly by the weight of the cup, and partly by the weight of the ball, E, suspended from it. On the wire, a notch, W, is made, to serve as the mark or fixed point to which the A. is sunk. The specific gravities of liquids are determined by Nicholson's A. in the following way: The weight of the A. itself is first ascertained—let it be in a given case 2000 grains—it is then put into water at the of 60° F., and weights (say 500 grains) put in, till it is sunk to W. It is now removed to the liquid under examination; and if the weight required to sink the instrument now to the standard-point be only 100 grains, we have the specific gravity of the liquid equal to Mg, or sc. In both fluids the same volume has been displaced, and that is in each case equal to the weight of the A.; but the weight of the A. in the second case was 2000 + 100, and in the former, 2000+500; hence the above result. Nicholson's A. is seldom used for finding the specific gravity of fluids; its use is almost entirely restricted to ascertaining that of small solid substances, as gems and small pieces of min erals. The following example will show how this is done: If in the cup of the A.. already mentioned, when placed hi water, the be put, and only 440 grains be then necessary to bring the instrument to W, 60 grains is manifestly the weight of the gem, because 500 grains were needed without it to do the same thing. The gem is next placed in the lower cup, D, and if 460 grains are now needed to sink to the standard point, the gem has thus lost 20 grains of its weight by being immersed in the water. According to the principle of Archimedes (q.v.), these 20 grains are also the weight of a volume of water equal to that of the gem; so the specific gravity of the gem is t-, or 3. By reversing the cup D, which is furnished with perforations to allow free passage to the air, and attaching the weight, E, to the handle of it, the specific gravity of substances lighter than water may also be determined by this instrument. The other forms of weight-areometers are those of Fahrenheit, Tralles, and Charles. For the more accurate determination of the specific gravities of liquids and solids, see SPECIFIC GRAVITY.