HYDROMETER. 



length to the stem 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 shew 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.toafourthterm, expressingthe 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 Baume, 

 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 instru- 



VOL. VI. 



ments. Thus, for example, when a che- 

 mist acquaints us that a fluid indicated 

 fourteen degrees of the pese-liquer of 

 Baume", 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. Baume 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. Baume" 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 * 

 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 clryness of \he. 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 

 flfiids, of equal weight, but different spe- 

 cific gravities, may be of value, or the 

 contrary. It does not seem probable, that 

 Baume's instrument will ever become of 

 general use, for which reason nothing fur- 

 ther need be 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. 



D d 



