Nothing more is required on the part of the workman, than that the hydrome ter shall be light enough to float in ether, and capable of sustaining at least one third of its own weight in the dish, with out oversetting in a denser fluid., This last requisite is obtained by giving a due length to the stern beneath, to which the counterpoise is attached. With such an instrument, whatever may be its,weight, or the quantity of water it displaces, the chemist may proceed to make his experi ments, and deduce his specific gravities by the proportion before laid down. Or, to save occasional computation, he may once for all make a table of the specific gravities, corresponding to every number of the load in the dish, from one grain up to the whole number of grains, so that, by looking for the load• in one column, he may always find the specific gravity in the column opposite.
This method is very ready and conve nient in practice; but if it be preferred, the weights may be adjusted to the hy drometer, so as to sbew the specific gra vity, without computation or reference. For this purpose the hydrometer must be properly counterpoised in distilled water, at the assumed standard temperature ; suppose 60°, and the whole weight of the instrument and its load called 1.000, &c. Then the weight of the instrument and its load must be separately determined in grains and parts, or other weights, by a good pair of scales, and as the whole weight of the instrument and its load is proportioned to the weight of the instru ment alone, so will be the number 1.000, &c. to a fourth term, expressin g the weight of the instrument in such parts as make the whole 1.000, &c. Make an actual set of decimal weights, of which 1.000, &c. shall be equal to the hydrometer and its load; and it is clear, that, whatever may be the load in these weights, if it be add ed to the number denoting the weight of the instrument, the sum will denote the specific gravity of the fluid, wherein the instrument floats with that load.
By following the above easy method, it will be found that every hydrometer, wheresoever made, must give the same results. The subject is indeed in itself sufficiently simple, and would require scarcely any discussion, if it had not hap pened that many philosophers, for want of requisite attention, have made their experiments with hydrometers graduated on the stem by no certain rule, by which operators, at a distance from each other, might compare their experiments. The hydrometers, or pese-liquers of Baum6, though in reality comparable with each other, are subject, in part, to the defect, that their results, having no independent numerical measure, require explanation to those who do not know the inapt ments. Thus, for example, when a che
' mist acquaints us that a fluid indicated fourteen degrees of the peseffiquer of Morrie, we cannot usefully apply this re sult, unless we have some rule to deduce the correspondent specific gravity ; whereas we should not have been in any respect at a loss, if the author had men tioned the specific gravity itself. As a considerable number of French philoso phers refer to this instrument, it will be of use to explain its principles.
M,Baurne appears to have directed his attention chiefly to the acquisition of a means of making hydrometers with a gra duated stem, which should correspond-in their results, notwithstanding any differ ences in their balls or stems. There is little doubt but he was led into the me thod he adopted, by reflecting on that by which thermometers are usually graduat ed. See THERMOMETER.
As thermometers are graduated inde pendent of each other, by commencing with an interval between two stationary points of temperature, so M. Baurne adopt ed two determinate densities, for the sake of marking an interval on the stem of his hydrometer. These densities were those of pure water, and of water containing 83 parts of its weight of pure dry common salt in solution. The temperature was ten degrees of Reaumur above freezing, or 54.5° of Fahrenheit.. His instrument for salts was so balanced, as nearly to sink in pure water. When it was plunged in this saline solution, the Stem arose in part above the surface. The elevated portion was assumed to be fifteen degrees, and he divided the rest of the stem with a pair of compasses into similar degrees.
It is unnecessary to inquire, in this place, whether this Interval be constant, or how far it may be varied by any differ ence in the purity, and more especially the degree of dryness of the salt. Nei ther will it be requisite to inquire, how far the principle of measuring specific gravi ties by degrees, representing equal incre ments, or decrements, in the bulk of Aids, of equal weight, but different spe cific gravities, may he of value, or the contrary. It does not seem probable, that 'Nume s instrument will ever become of general use, for which reason nothing ther need he ascertained, than the speci fic gravities corresponding with its de grees, in order that such experiments as have this element among their data may be easily understood by chemical read ers.