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 is reduced, but is sensible in the case of the thinnest stem which can be employed, and is the chief source of error in the employment 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 hydro statical 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, l 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 acavity 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 carefully 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 G. Atkins, described in the Ph-il. Mag. for 1808, xxxi. 254, is merely Nicholson's hydrometer with the screw at C project inf
through the collar into which it is screwe, and terminating in a sharp point above the cup G. To this point soft bodies lighter than water (which would iioat if laced in the cup) could be attached, and tlius completely 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 dis laced 1000 grains of distilled water at 60°PEland hence the specific gravity of any other liquid was at once indicated by adding 700 to the number of grains in the pan re uired to make the instrument sink to the mark on the stem. The small divisions on the scale corresponded to differences of,10th of a grain in the weight of the instrument. The “ Gravimeter, " constructed by Citizen Guyton and described in Nicholson's Journal, 4to, i. 110, differs 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 up r an to cause the instrument to sink to the mark on the stem in distilled water at the standard temperature. The instrument is provided with an additional piece, or “ plongeur, " the weight of which 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 “ plon eur, " 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 instrument weighs about 20 grammes, and therefore floats in a liquid of specific gravity -8. Thus deprived of its additional weight it may be usedfor spirits. l' 0 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 differin specific gravities by drawing out a sliding tube, thus changing the volume of the hydrometer while its weight remains constant. The hydrometer of A. Baumé, which has been extensively used in France, consists of a common hydrometer raduated in the l/ollowing manner. Certain fixed points were first determined upon the stern 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-FIG. 4.
ing respectively 1, 2, 3, . . . I5'% (by weight) of dry salt. The hydrometer was plunged in these solutions i11 order, and the stem having been marked at the several surfaces, the degrees so obtained were numbered I,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 instrument 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% 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 IOO 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 Baumé's hydrometer are given by Nicholson (Journal of Philosophy, i. 89) as follows:—
Baumélt Hydromeler for Spirits. Temperature 10° R. Degrees. Density. Degrees. Density. Degrees. Densityi I0 ' 1-000 21 '922 31 -861
II -990 22 -915 32 -856
I2 '985 23 '909 33 '852
13 '977 24 '903 34 '847
14 970 25 '897 35 '842
1 5 -963 26 -892 36 -837
16 -955 27 -886 37 -832,
I7 °949 28 -880 38 -827 1
18 -943 29 '874 39 -822 .
19 '935 3° '867 40 '3¥7
20 -928 3
= 4 Baumé's H hydrometer for Salts. -, Degrees Density. Degrees." Density. Degrees. Density. N 0 1-000 27 1-230 l 51 l 1-547
3 1-020 30 1-261 54 F594
6 1-940 33 I-295 57 1-659
9 1-064 36 1-333 60 i 1-717
12 1-089 39 V373 63 1-779,
I5 I-114 42 1-414 66 1-848
18 I'140 45 1-455 69 1-920,
21 I'I7O 48 1-500 72 2-000,
24 ' I~200 »
Cartier's hydrometer was very similar to that of Baumé, Cartier having been employed by the latter to construct his instruments for the French revenue. The point at which the instrument floated in distilled water was marked 10° by Cartier, and 30° on Cartier's scale corresponded to 32° on Baumé's. Perhaps the main object- for which hydrometers have been constructed is the determination of the value of spirituous liquors, chiefly for revenue purposes. To this end an immense variety of hydrometers have been devised, differing mainly in the character of their scales. In Speer's hydrometer the stem has the form of an octagonal prism, and upon each of the eight faces a scale is engraved, indicating the percentage strength of the spirit corresponding to the several divisions of the scale, the eight ' scales being adapted respectively to the temperature 35°, 40°, 45°, 50°, 55, 60°, 65° and 70° F. Four small pins, which can be inserted into the counterpoise of the instrument, serve to adapt the instrument to the temperatures intermediate between those for which the scales are constructed. William Speer was supervisor and chief assayer of spirits in the port of Dublin. For a more complete account of this V
instrument see Tilloch's Phil. Illag., xiv. I 51. The hydrometer constructed by Jones, of Holborn, consists of~ a s heroidal bulb with a rectangular stern (fig. 5§ . Between the bulb a nd counterpoise is placed a thermometer, which serves to indicate the temperature of the liquid, and the instrument is provided with three weights which can be attached to the top of the stem. On the four sides of the stem AD are engraved-four 5%
ra 0. 7
39' -F"[11T1fiIf"""*' 1 narczrlizlgiteciiaall § l -5, L-~»- — .
1-1 ' ' M ' Tfééiii-#T
fi -O v
respective weights. The instrument when unloaded serves for the range from 74 to 47 over proof; when loaded with the first weight it indicates from 46 to 13 over proof, with the second
weight from 13 over proof to 29 under proof, and with the third