periments. The hydrometers, or pesé-liqueurs, 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 instruments. Thus, for example, when a chymist acquaints us that a fluid indicated fourteen degrees of the pesé-liqueur 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 differences 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, independ ent 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 increments, 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 chymical readers.

M. Baumé, in his "Elemens de Pharmacie," has given a table of the degrees of his hydrometer for spirits, indicated by different mixtures of alcohol and pure water, where, he says, the spirit made use of gave

37 degrees at the freezing point of water; and in a column of the table he states the bulk of this spirit, compared with that of an equal weight of water, as 35 to 30. The last proportion answers to a specific gravity of 0.842, very nearly. A mixture of two parts, by weight, of this spirit, with thirty of pure water, gave twelve degrees of the hydrometer at the freezing point. This mixture, therefore, contained 64 parts of Blagden's standard to 100 water; and by Gilpin's excellent tables, its specific gravity must have been 0.9915. By the same tables, these specific gravities of 0.842 and 0.9915 would, at 10° Reaumur, or 55° Fahrenheit, have fallen to 0.832 and 0.9905. Here then

HYDROPHANES, OCULUS MUNDI, or Lapis Mutabilis, a kind of precious stone highly esteemed among the ancients, but little known to the moderns till Mr. Boyle made his observations upon it. Its specific gravity is about 2.018; its colour of an opaque whitish brown; it is not soluble in acids nor affected by alkalies, but is easily cut and polished. Sometimes it gives fire with steel, sometimes not. It is infusible per se; but when urged by a blow-pipe, changes to a brownish brittle substance. It is found in beds over the opals in Hungary, Silesia, and Saxony, and over the chalcedonies and agates in Iceland. These stones in general are either of a yellowish green, milky grey, or of a yellow like that of amber.

The most remarkable property of this stone is, that it becomes transparent by mere infusion in any aqueous fluid; but gradually resumes its opacity when dry. There are three of these stones in the British museum at London; the largest of them about the size of a cherry stone, but of an oval form.

HYDROPHILUS. In Zoology a genus of the class insectæ, order coleoptera. Antennas clavate, the club perfoliate; tentacles four, filiform; hind-legs formed for swimming, fringed on the inner side, and nearly unarmed with claws. Thirty-one species: chiefly natives of Europe, a few of Asia and America: six indigenous to our own country.

The insects of this genus greatly resemble the beetle; and like the Drytiscus, which also resembles the beetle, they are inhabitants of ponds and stagnant waters, where they swim with much dexterity, and are able to turn round most rapidly: by night they rise into the air and fly abroad in quest of other water. The males 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 and celerity; the larvæ remain about two and a half years before they change into pupæ, forming convenient cells, and secreting themselves in some bank; are extremely voracious and destructive to the more tender aquatic

The species most worthy of notice is the H. piceus, or water-clock: glossy-black; breast channelled with a long spire pointing backwards. It inhabits Europe, and frequents our own country. The larva appears to have the legs seated on the upper part of the body near the back, but this is only occasioned by the peculiar shape and position of the legs. The female spins a flattish circular kind of web, terminated by a long tapering horn, from which the young escape as soon as they are hatched. See Nat. Hist. Pl. CXXXI.

HYDROPICAL. a. Dropsical.

HYDROPHOBIA. (Hydrophobia, æ, f. ¿spo.6.x, from dop, water, and 6, to fear). Rabes canina. Canine madness. This genus of diseases arises in consequence of the bite of a rabid dog. It is termed hydrophobia, because persons that are thus bitten dread the sight or the falling of water when first seized. Cullen has arranged it under the class neuroses, and order spasmi.

And it is from Cullen's arrangement that the disease has of late passed under the name of hydrophobia. The term, however, is highly incorrect, for it implies a mere symptom, and that not always present in canine madness, and often present in other diseases. The Greek term Lyssa, which is now not in use, ought unquestionably to be revived and re-employed. Whether any other quadruped can originate this disease besides the dog seems doubtful. It is said to have originated occasionally with cats: but the cases referred to are few, and by no means decisive. To the dog it has been exclusively referred by, all nations and in almost all ages.

It is known by the previous history of the disease, the dread of water, painful convulsions of the pharynx, and putrid fever.

HYDROPHORA. In Botany, a genus of the class cryptogamia, order fungi. Fungus bending back, paler underneath; cymes terminal like those of the guilden-rose, composed mostly of radiate, abortive flowers; flowers green when young, and gradually changing to a beautiful rose-colour: petals generally four; stamens from six to ten; styles from one to three.

HYDRO PHYLAX. In botany, a genus of the class tetrandria, order monogynia. Corol one-petalled, funnel-form; calyx fourparted; capsule angular, two-celled, with a transverse partition; seeds solitary. One species-a native of the sea-coast of India, with red, fleshy, sweet, piliform root; coloured stem, clothed with long membranaceous sheaths; leaves opposite, ovate, entire; pale blue flowers axillary, and nearly sessile.

HYDROPHY'LLUM. Water-leaf. In botany a genus of the class pentandria, order

monogynia. Corol campanulate, with five longitudinal melliferous streaks on the inside; stygma cloven; capsule globular, twovalved. Two species natives of Virginia and Canada.

HYDROPIPER, (Hydropiper, eris, n. ὑδροπιπερις, from ὕδωρ, water, and πιπερις, pepper, so called from its biting the tongue like pepper, and being a native of marshy places.) Biting arsmart. Lake weed. Water pepper. Polygonum hydropiper of Linnæus. This plant is very common in our ditches; the leaves have an acrid burning taste, and seem to be nearly of the same nature with those of the arum. They have been recommended as possessing antiseptic, aperient, diurectic virtues, and given in scurvies and cachexies, asthmas, hypochondriacal and nephritic complaints, and wandering gout. The fresh leaves have been applied externally as a stimulating cataplasm. See POLYGENUM.

HYDROPS, (Hydrops, opis, m. v§;w↓, from idup, water.) A dropsy. Any species of dropsy may be so termed, as hydrops abdominis, thoracis, cerebri, pericardii, testis, &c. See ASCITES, HYDROTHORAX, HYDROCE PHALUS, ANASARCA, HYDROCELE, HYDROCARDIA, &c.

HYDROPHTHA'LMIA,(Hydrophthalmia, æ, f. popama, from wp, water, and fax, the eye.) There are two diseases different in their nature and consequences thus termed. The one is a mere anasarcous or adematous swelling of the eyelid. The other, the true hydrophthalmia, is a swelling of the bulb of the eye, from too great a collection of the vitreous or aqueous humours.

HYDRORA CHITIS, (Hydrorachitis,idis, f. udpopaxils, from udwg, water, and pays, the spine). Spina bifida. A small, soft, fluctuat ing tumour, mostly situated on the lumbar vertebræ of new-born children. It is a genus of diseases in the class cachexia, and order intumescentiæ of Cullen, and is always in

curable.

HYDROSARCA, (Hydrosarca, æ, f. 8poσapxa, from wf, water, and caps, the flesh.) Water in the cellular membrane. See ANA

SARCA.

HYDROSARCOCE'LE, (Hydrosarcocele, es, f. udposapxoxnan, from 8p, water, caps, the flesh, and xn, a tumour). Sarcocele, with an infusion of water into the cellular membrane.

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.

HYDRO SIDORUM, a phosphuret of iron, for a short time supposed by Meyer and Bergmann, to be a new metal. See IRON. HYDROSTATIC Balance. See HYDRO

STATICS

STATICS.

HYDROSTATIC. Paradox. See HYDROHYDROSTATICAL. a (vdwę and oratixn). Relating to hydrostatics. (Bentley). HYDROSTATICALLY. adv. According to hydrostatics (Bentley).

HYDROSTATICS, is that branch of physico-mathematical science which comprises the doctrine of the pressure and the equilibrium of non-elastic fluids, and that of the weight, pressure, stability, &c. of solids immersed in them.

DEF. A fluid is a body whose parts are very minute, yield to any force impressed upon it (however small), and by so yielding are easily moved among themselves.

This is nearly the same as the definition given by Newton, in the Principia, book 2. sect. 5. and is adopted here because, in conjunction with two or three established facts, it may serve as a basis for all which distinguishes the doctrines of hydrostatics from those of pure mechanics. The writers on the continent, however, though they admit that the minuteness of fluid moleculæ, and their excessive mobility, are characteristics common to all such bodies, yet they have recourse to a different definition. Thus the celebrated Euler in the New Commentaries of St. Peterburgh, vol. 13. takes for the basis of his analysis the following consideration: The distinguishing nature of fluids consists in this property, namely, that when it is subjected to any pressure whatever. that pressure is so distributed throughout the mass, that, while it remains in equilibrio, all its parts are equally pressed." And M. D'Alembert in his Traité de l'Equilibre et da Mouvement des Fluides, as well as M. Prony in his Architecture Hydraulique, adopt the same property as a definition. tremely consistent with experiment (though, as will soon be seen, it is rather a proposi tion, capable of proof, than a definition), and furnishes a natural foundation for an algebraical calculus, by which the whole doctrine of hydrostatics may be exhibited in a few equations. But this method, though it possesses some advantage, is not entirely pursued here, from a firm conviction that a judicious combination of the geometrical and algebraical methods is far more likely to convey distinct ideas to the student than the modern analysis merely.

It is ex

Perfect fluidity, according to the Newtonian system, arises from a want of any sensible cohesion between the constituent particles of the fluid, and this want of cohesion is commonly attributed to the spherical figure of the particles. The nature of this work does not require that we should enter into minute disquisitions on the formal cause of We shall state that the late Dr. Black, of Edinburgh, speaks of fluidity as an effect of heat: and before him Boerhaave pleaded strenuously for the same opinion. According to this view of the