is an instrument, of which so much has been said in the Encyclopædia under that title, and in the article Specific Gravity, that we certainly should not again introduce it in this place, but to guard our readers against error, when studying the works of the French chemists. These gentlemen, who are so strongly attached to every thing which is new, as to believe that their ancestors have for ages been wandering in the mazes of ignorance, refer very frequently to the pse-liqueur de Baume; and as that instrument has never been generally used in this country, it becomes our duty to describe its construction.

Instead of adopting the simpler method of immediate numerical reference to the density of water expressed by unity, as is done in all modern tables of specific gravity, he had recourse to a process similar to that of graduating the stems of thermometers from two fixed points. The first of these points was obtained by immersing his instrument, which is the common arcometer, consisting of a ball, stem, and counterpoise, in pure water. At that point of the stem which was intersected by the surface of the fluid, he marked zero, or the commencement of his graduations. In the next place, he provided a number of solutions of pure dry common salt in water; these solutions contained respectively one, two, three, four, &c. pounds of the salt; and in each solution the quantity of water was such, as to make up the weight equal to one hundred pounds in the whole; so that in the solution containing one pound of salt, there were ninety-nine pounds of water; in the solution containing two pounds of salt, there were ninety-eight pounds of water, and so of the rest. The instrument was then plunged in the first solution, in which of course it floated with a larger portion of the stem above the fluid, than when pure water was used. The fluid, by the intersection of its surface upon the stem, indicated the place for marking his first degree; the same operation repeated, with the fluid containing two pounds of salt, indicated the mark for the second degree; the solution of three pounds afforded the third degree; and in this manner his enumeration was carried as far as fifteen degrees. The first fifteen degrees afterwards, applied with the compasses repeatedly along the stem, served to extend the graduation as far as eighty degrees, if required.

This instrument, which is applicable to the measurement of densities exceeding that of pure water, is commonly distinguished by the name of the Hydrometer for salts.

The hydrometer for spirits is constructed upon the same principle; but in this the counterpoise is so adjusted, that most part of the stem rises above the fluid when immersed in pure water, and the graduations to express inferior densities are continued upwards. A solution of ten parts by weight of salt in ninety parts of pure water, affords the first point, or zero, upon the stem; and the mark indicated by pure water is called the tenth degree; whence, by equal divisions, the remaining degrees are continued upwards upon the stem as far as the fiftieth degree.

These experiments, in both cases, are made at the tenth degree of Réaumur, which answers very nearly to fifty-five of Fahrenheit.