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 experiments, 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 convenient in practice; but if it be preferred, the weights may be adjusted to the hydrometer, so as to shew the specific gravity, 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 instrument alone, so will be the number 1.000, &c. to a fourth term, expressing 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 added 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 happened 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 pésé-liquers of Baumé, 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 instruVOL. VI.

ments. Thus, for example, when a chemist acquaints us that a fluid indicated fourteen degrees of the pesé-liquer of Baumé, we cannot usefully apply this result, 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 mentioned the specific gravity itself. As a considerable number of French philosophers refer to this instrument, it will be of use to explain its principles.

M. Baumé appears to have directed his attention chiefly to the acquisition of a means of making hydrometers with a graduated 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 method he adopted, by reflecting on that by which thermometers are usually graduated. See THERMOMETER.

As thermometers are graduated independent of each other, by commencing with an interval between two stationary points of temperature, so M. Baumé adopted 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 difference in the purity, and more especially the degree of dryness of the salt. Neither will it be requisite to inquire, how far the principle of measuring specific gravities by degrees, representing equal incre

ments, or decrements, in the bulk of

fluids, of equal weight, but different specific 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 further need be ascertained, than the specific gravities corresponding with its degrees, in order that such experiments as have this element among their data may be easily understood by chemical read

ers.

D d

50

35

grains of spirit

to 100 gr. of water.

HYDROPHILUS, in natural history, a genus of insects of the order Coleoptera. Antennæ clavate, the club perfoliate; feelers four, filiform; the hind legs are formed for swimming, fringed on the inner side, and nearly unarmed with claws. The insects of this genus, like those of the DYTISCUS, which see, are inhabitants of ponds and stagnant waters, where they swim with much dexterity, turning round with great velocity; they fly abroad by night in search of other waters. The male's are distinguished from the females, by having a horny concave flap or shield on the fore legs, near the setting on of the feet; the hind legs are peculiarly fitted for their aquatic situation, being furnished on the inner side with a series of long and close-set filaments, resembling a fin, by which they are enabled to swim with great ease. The larva remain about two years and a half before they change into pupæ, forming a convenient cell, and secreting themselves in some bank. They are very voracious, and destructive to the more tender aquatic insects, worms, and young fish, which they seize with their forked jaws, and destroy, by sucking out their juice. There are upwards of thirty species. The principal European species is the H. piceus, water-clock. The female of this species affords an example of a faculty, which seems to be exercised by no other insect of this order, viz. that of spinning a kind of web, or flattish circular case of silk, which it leaves floating on the water, and in which it deposits its eggs. This case, says Dr. Shaw, is terminated, on its upper surface, by a lengthened conical process, resembling a horn,

of a brown colour, and of a much stronger nature than the case itself, which is white. The larva, as soon as hatched, make their escape from the envelopement of the case, and commit themselves to the wa

ter.

HYDROPHOBIA, in medicine, an aversion or dread of water; a terrible symptom of the rabies canini. See ME

DICINE.

HYDROPHYLAX, in botany, a genus of the Tetrandria Monogynia class and order. Natural order of Rubiaceæ, Jussieu. Essential character: calyx four-parted; corolla funnel-form; fruit ancipital, oneseeded. There is only one species, viz II. maritima, found in driving sand, on the sea-shore, near Guduluhr in the East Indies.

HYDROPHYLLUM, in botany, waterleaf, a genus of the Pentandria Monogynia class and order. Natural order of Borragineæ, Jussieu. Essential character: Corolla bell-shaped, having five longitu dinal melliferous streaks on the inside; stigma bifid; capsule globular, two-valved. There are two species, viz. H. virginicum, Virginian water-leaf; and H. canadense, Canadian water-leaf.

HYDROSCOPE, an instrument anciently used for the measuring of time. The hydroscope was a kind of water-clock, consisting of a cylindrical tube, conical at bottom: the cylinder was graduated, or marked out with divisions, to which the top of the water, becoming successively contiguous, as it trickled out at the vertex of the cone, pointed out the hour

HYDROSTATICAL bulance, a kind of balance contrived for the easy and exact

918

- 1,000 - 1,028 13,595

We leave the reader to judge whether it be probable, by any apparatus of human formation, and under human guidance, to ascertain that the three millionth parts, said to have been compressed, were really so. Indeed, even the sixty-sixth millionth parts, suffered to be compressed in the spirits of wine, must appear extremely doubtful; though we cannot but conclude that, as air exists in every atom of nature, more or less, with a sufficient force, every fluid were subject to compression into a smaller space than is occupied by it when perfectly at liberty. Speaking generally, the definitions above given may be considered as applicable to all cases with which we are acquainted; and may, perhaps, be completely true.

We shall commence the detail, incident to this subject, with an account of the method of obtaining the specific gravities: of bodies: that is, by showing the comparative weights of various solids and fluids, as ascertained by the most careful and skilful chemists. The reader must, however, consider the weights as taken at a medium. See GRAVITY, specific, where is given a table of specific gravities.

The reader will observe, that the whole of the above are compared with rain-water, which appear at 1.000 parts; but it is very remarkable, that the density of that fluid varies greatly according to its temperature; and that it by no means af fords a regular scale of weight, or of bulk, in proportion to the degrees of heat. This will be seen from the following table, ta

We must suppose the water of the Dead Sea to be highly impregnated; since it appears to weigh nearly a fourth more than common sea water.

The anomalies lay between 32° and 45°, and are accounted for by the contraction which takes place in water about to freeze, and its sudden expansion afterwards; by this we understand the cause of bottles, pitchers, &c. being burst, when the water they contain freezes. The difference in bulk between water and liquors, in the winter and in the summer season, averages about three per cent : hence many great dealers have thought it worth their while to buy only in the former season, when the liquors have been most concentrated.