538 Nicholson explains how the instrument may be employed as a thermometer, since, fluids generally expanding more than the solids of which the instrument is constructed, the instrument will sink as the temperature rises. To determine the density of solids heavier than water with this instrument, let the solid be placed in the upper scale pan, and let the weight now required to cause the instrument to sink in distilled water at standard temperature to the mark B be denoted by w, while W denotes the weight required when the solid is not present. Then W-w> is the weight of the solid. Now let the solid be placed in the lower pan, care being taken that no bubbles of air remain attached to it, and let u be the weight now required in the scale pan. This weight will exceed w in consequence of the water displaced by the solid, and the weight of the water thus dis placed will be W Y - iv, which is therefore the weight of a volume of water equal to that of the solid. Hence, since the weight of the y _ iff solid itself is "W" - w, its density must be . w^-w The above example illustrates how Nicholson s or Fahrenheit s hydrometer may be employed as a weighing machine for small weights. In all hydrometers in which a part only of the instrument is immersed, there is a liability to error in consequence of the surface tension, or capillary action, as it is frequently called, along the line of contact of the instrument and the surface of the liquid (see CAPILLARY ACTION). This error diminishes as the diameter of the stem in reduced, but is sensible in the case of the thinnest stem which can be employed, and is the chief source of error in the employ ment of Nicholson s hydrometer, which otherwise would be an instrument of extreme delicacy and precision. The following is Nicholson s statement on this point: " One of the greatest difficulties which attends hydrostatical experiments arises from the attraction or repulsion that obtains at the surface of the water. After trying many experiments to obviate the irregularities arising from this cause, I find reason to prefer the simple one of carefully wiping the whole instrument, and especially the stem, with a clean cloth. The weights in the dish must not be esteemed accurate while there is either a cumulus or a cavity in the water round the stem. " It is possible by applying a little oil to the upper part of the bulb of a common or of a Sikes s hydrometer, and care fully placing it in pure water, to cause it to float with the upper part of the bulb and the whole of the stem emerging as indicated in fig. 4, when it ought properly to sink almost to the top of the stem, the surface tension of the water around the circumference of the circle of contact, AA , providing the additional support required. The universal hydrometer of Mr G. Atkins, described in the Phil. Mag. for 1808, vol. xxxi. p. 254, is merely Nicholson s hydrometer with the screw at C projecting through the collar into which it is screwed, and terminating in a sharp point above the cup G. To this point soft bodies lighter than water (which would float if placed in the cup) could be attached, and thus com pletely immersed. Atkins s instrument was constructed so as to weigh 700 grains, and when immersed to the mark on the stem in distilled water at 60 F., it carried 300 grains in the upper dish. The hydrometer therefore displaced 1000 grains of distilled water at 60 F., and hence the specific gravity of any other liquid was at once indicated by adding 700 to the number of grains in the pan required to make the instrument sink to the mark on the stem. The small divisions on the scale corresponded to differences of -^th. of a grain in the weight of the instrument. The " Gravimeter," constructed by Citizen Guyton and described in Nicholsons Journal, 4to, vol. i. p. 110, dilfers from Nicholson s instrument in being constructed of glass, and having a cylindrical bulb about 21 centimetres in length and 22 millimetres in diameter. Its weight is so adjusted that an additional weight of 5 grammes must be placed in the upper pan to cause the instrument to sink to the mark on the stem in distilled water at the standard tempera ture. The instrument is provided with an additional piece, or "plongeur," whose weight exceeds 5 grammes by the weight of water which it displaces ; that is to say, it is so constructed as to weigh 5 grammes in water, and consists of a glass envelope filled with mercury. It is clear that the effect of this " plongeur," when placed in the lower pan, is exactly the same as that of the 5 gramme weight in the upper pan. Without the extra 5 grammes the instru ment weighs about 20 grammes, and therefore Moats in a liquid of pecific gravity 8. Thus deprived of its additional weight it may be used for spirits. To use the instrument for liquids of much greater density than water additional weights must be placed in the upper pan, and the " plongeur" is then placed in the lower pan for the purpose of giving to the instrument the requisite stability. Charles s balance areometer is similar to Nicholson s hydrometer, except that the lower basin admits of inversion, thus enabling the instrument to be employed for solids lighter than water, the inverted basin serving the same purpose as the pointed screw in Atkins s modification of the instrument. Adie s sliding hydrometer is of the ordinary form, but can be adjusted for liquids of widely differing specific gravities by drawing out a sliding tube, thus changing the volume of the hydrometer while its weight remains constant. Adie s statical hydrometer is really a specific gravity balance, one of the arms of which is 2J inches in length, and the other 8 inches. A brass ball, whose volume is 01 gallon, is suspended from the shorter arm, and immersed in the liquid whose density is to be determined. The ball is balanced by means of a weight which slides along the beam, and a smaller weight which also slides along the beam serves to make the necessary correction for temperature. The hydrometer of Beaume, which has been extensively used in France, consists of a common hydrometer graduated in the following manner. Certain fixed points were first determined upon the stem of the instrument. The first of these was found by immersing the hydrometer in pure water, and marking the stem at the level of the surface. This formed the zero of the scale. Fifteen standard solutions of pure common salt in water were then prepared, contain ing respectively 1, 2, 3, .... 15 per cent, (by weight) of dry salt. The hydrometer was plunged in these solutions in order, and the stem having been marked at the several surfaces, the degrees so obtained were numbered 1, 2, 3 15. These degrees were, when necessary, repeated along the stem by the employment of a pair of compasses till 80 degrees were marked off. The instru ment thus adapted to the determination of densities exceeding that of water was called the hydrometer for salts. The hydrometer intended for densities less than that of water, or the hydrometer for spirits, is constructed on a similar principle. The instrument is so arranged that it floats in pure water with most of the stem above the surface. A solution containing 10 per cent, of pure salt is used to indicate the zero of the scale, and the point at which the instrument floats when immersed in distilled water at 10 R. (54^ F. ) is numbered 10. Equal divisions are then marked off upwards along the stem as far as the 50th degree. The densities corresponding to the several degrees of Beaume s hydrometer are given by Nicholson (Journal of rhiloso2^hy, vol. i. p. 89) as follows : Beaume s Hydrometer for Spirits. Temperature 10 E. Degrees. Density. Degrees. Density. Degrees. Density. 10 1-000 21 922 31 86] 11 990 22 915 32 856 12 985 23 909 33 852 13 977 24 903 34 847 14 970 25 897 35 842 15 963 26 892 36 837 16 955 27 886 37 832 17 949 28 880 38 827 18 943 29 874 39 822 19 935 30 867 40 817 20 928 Beaume s Hydrometer for Salts. Degrees. Density. Degrees. Density. Degrees. Density.
i-ooo 27 1-230 51 1-547 3 1-020 30 1 261 54 1-594 6 1-040 33 1-295 57 1-659 9 1-064 36 1-333 60 1717 12 1-089 39 1-373 63 1-779 15 1-114 42 1-414 66 1-848 18 1-140 45 1-455 69 1-920 21 1-170 48 1-500 72 2-000 24 1-200
Cartier s hydrometer was very similar to that of Beaume, Cartier
