The quantity of alcohol present in an aqueous solution is determined by a comparison of its specific gravity with standard tables, or directly by the use of an alcoholometer, which is a hydrometer graduated so as to read per cents by weight (degrees according to Richter) or volume per cents (degrees according to Tralles).
Acting on a principle quite different from any previously discussed is the capillary hydrometer or staktometer of Brewster, which is based upon the difference in the surface tension and density of pure water, and of mixtures of alcohol and water in varying proportions.
The capillary hydrometer consists simply of a small pipette with a bulb in the middle of the stem, the pipette terminating in a very fine capillary point.
Three types of areometer are in use: (I) the ordinary hydrometer of invariable weight with a direct reading scale, a set of from five to ten being necessary to cover the range of specific gravity from 1 000 to 1.031 so as to take account of sea-water of all possible salinities; (2) the " Challenger " type of areometer designed by J.
The hydrometer is said by Synesius Cyreneus in his fifth letter to have been invented by Hypatia at Alexandria,' but appears to have been neglected until it was reinvented by Robert Boyle, whose "New Essay Instrument," as described in the Phil.
89, Citizen Eusebe Salverte calls attention to the poem "De Ponderibus et Mensuris" generally ascribed to Rhemnius Fannius Palaemon, and consequently 300 years older than Hypatia, in which the hydrometer is described and attributed to Archimedes.
The quantity of mercury or shot inserted depends upon the density of the liquids for which the hydrometer is to be employed, it being essential that the whole of the bulb should be immersed in the heaviest liquid for which the instrument is used, while the length and diameter of the stem must be such that the hydrometer will float in the lightest liquid for which it is required.
Of liquid dis placed) when the surface of the liquid in which the hydrometer floats coincides with the lowest division of the scale, A the area of the transverse section of the stem, 1 the length of a scale division, n the number of divisions on the stem, and W the weight of the instrument.
W n be the weights of unit volume of the liquids in which the hydrometer sinks to the divisions o, I, 2 ...
The plan commonly adopted to obviate the necessity of inconveniently long stems is to construct a number of hydrometers as nearly alike as may be, but to load them differently, so that the scaledivisions at the bottom of the stem of one hydrometer just overlap those at the top of the stem of the preceding.
Long, will be equivalent to a single hydrometer with a stem of 30 in.
This difficulty may be met, as in Keene's hydrometer, by having all the weights of precisely the same volume but of different masses, and never using the instrument except with one of these weights attached.
The upper part of this wire is filed flat on one side, for the stem of the hydrometer, with a mark at m, to which it sinks exactly in proof spirits.
Clarke's hydrometer, as afterwards constructed for the purposes of the excise, was provided with thirty-two weights to adapt it to spirits of different specific gravities, and eleven smaller weights, or "weather weights" as they were called, which were attached to the instrument in order to correct for variations of temperature.
The correction for temperature thus afforded was not sufficiently accurate for excise purposes, and William Speer in his essay on the hydrometer (Tilloch's Phil.
Clarke's hydrometer, however, remained the standard instrument for excise purposes from 1787 until it was displaced by that of Sikes.
Desaguliers himself constructed a hydrometer of the ordinary type for comparing the specific gravities of different kinds of water (Desaguliers's Experimental Philosophy, ii.
The first important improvement in the hydrometer after its reinvention by Boyle was introduced by G.
Characteristics of Fahrenheit's hydrometer and of Boyle's essay instrument.
In comparing the densities of different liquids, it is clear that this instrument is precisely equivalent to that of Fahrenheit, and must be employed in the same manner, weights being placed in the top scale only until the hydrometer sinks to the mark on the wire, when the specific gravity of the liquid will be proportional to the weight of the instrument together with the weights in the scale.
The above example illustrates how Nicholson's or Fahrenheit's hydrometer may be employed as a weighing machine for small weights.
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.
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.
The universal hydrometer of G.
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.
The hydrometer therefore displaced woo grains of distilled water at 60°F.and hence the specific gravity of any other liquid was at once indicated FIG.
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.
The hydrometer of A.
Baume, which has been extensively used in France, consists of a common hydrometer graduated in the following manner.
The first of these was found by immersing the hydrometer in pure water, and marking the stem at the level of the surface.
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 I, 2, 3, ...
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 densities corresponding to the several degrees of Baume's hydrometer are given by Nicholson (Journal of Philosophy, i.
89) as follows: - Baume's Hydrometer for Spirits.
Cartier's hydrometer was very similar to that of Baume, Cartier having been employed by the latter to construct his instruments for the French revenue.
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.
If the mercury in the thermometer stand above this zero the spirit must be reckoned weaker than the hydrometer indicates by the number on the thermometer scale level with the top of the mercury, while C f if the thermometer indicate a temperature a? ?!
At the side of each of the four scales on the stem of the hydrometer is en r ' graved a set of small numbers indicating the contraction in volume which would be experienced if the requisite amount of water (or spirit) were added to bring the sample tested to the proof strength The hydrometer constructed by Dicas of Liverpool is provided with a sliding scale which FIG.
Quin's universal hydrometer is described in the Transactions of the Society of Arts, viii.
Atkins's hydrometer, as originally constructed, is described in Nicholson's Journal, 8vo, 276.
The instrument is provided with a sliding rule, with scales corresponding to the several weights, which indicate the specific gravity corresponding to the several divisions of the hydrometer scale compared with water at 55° F.
Tralles's hydrometer differs from Gay-Lussac's only in being graduated at 4° C. instead of 15° C., and taking alcohol of density 7939 at 15.5° C. for pure alcohol instead of 7947 as taken by GayLussac (Keene's Handbook of Hydrometry).
In Beck's hydrometer the zero of the scale corresponds to density 1 000 and the division 30 to density.
In the centesimal hydrometer of Francceur the volume of the stem between successive divisions of the scale is always, oath of the whole volume immersed when the instrument floats in water at 4° C. In order to graduate the stem the instrument is first weighed, then immersed in distilled water at 4° C., and the line of flotation 7.1 F marked zero.
The length of 100 divisions of the scale, or the length of the uniform stem the volume of which would be equal to that of the hydrometer up to the zero graduation, Francceur called the "modulus" of the hydrometer.
Dr Bones of Montpellier constructed a hydrometer which was based upon the results of his experiments on mixtures of alcohol and water.
Sikes's hydrometer, on account of its similarity to that of Bories, appears to have been borrowed from that instrument.
Keene, of the Hydrometer Office, London, has constructed an instrument after the model of Sikes's, but provided with twelve weights of different masses but equal volumes, and the instrument is never used without having one of these attached.