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mouth, it frequently twists the arm into a spiral like a cork-screw,by which means the insect is brought to the mouth in a muchshorter time than otherwise it would be ; and so soon are the in-sects on which the polypes feed killed by them, that M. Fontanathinks they must contain the most powerful kind of poison forthe lips scarcely touch the animal when it expires, though therecannot be any wound perceived on it when dead. The worm,when swallowed, appears sometimes single, sometimes double, ac-cording to circumstances. When full, the polype contracts itself,bangs down as in a kind of stupor, but extends again in proportionas the food is digested, and the excrementitious part is discharged.The polypes produce young ones indiscriminately from all partsof their bodies, and five or six young ones have frequently beenproduced at once; nay, M. Trembley has observed nine or tenproduced at the same time. Nothing like copulation among thesecreatures was ever observed by M. Trembley, though for twoyears he had thousands of them under his inspection. When apolype is cut transversely, or longitudinally, into two or threeparts, each part in a short time becomes a perfect animal; and sogreat is this prolific power, that a new animal will be producedeven from a small portion of the skin of the old one. If the youngones be mutilated while they grow upon the parent, the parts socut off will be reproduced ; and the same property belongs to theparent. A truncated portion will send forth young ones before ithas acquired a new head and tail of its own, and sometimes thehead of the young ones supplies the place of that which shouldhave grown out of the old one. If we slit a polype longitudinallythrough the head to the middle of the body, we shall have oneformed with two heads; and by again slitting these in the samemanner, we may form one with as many heads as we please. Astill more surprising property of these animals is, that they may begrafted together. If the truncated portions of a polype be placedend to end, and gently pushed together, they will unite into a sin-gle one. The two portions are first joined together by a slenderneck, which gradually fills up and 'disappears, the food passingfrom the one part into the other ; and thus we may form polype'snot only from portions of the same, but of different animals ; wemay fix the head of one to the body of another, and the com-pound animal will grow, eat, and multiply, as if it had never beendivided. By pushing the body of one into the mouth of another,so far that their heads may lie brought into contact, and keptv there for some time, they will at last unite into one animal, onlyhaving double the number of arms which it would otherwise havehad. The fusca may be turned inside out like a glove, at thesame time that it continues to live and act as before. The liningof the stomach now forms the outer skin, and the former epider-mis constitutes the lining of the slomach. If previous to this ope-ration the polype have young ones attached to it, such as are butnewly beginning to vegetate turn themselves inside out, while thelarger ones continue to increase in size till they reach beyond themouth of the parent, and are then separated in the usual mannerfrom the body. See Adams on the Microscope .

HYDERABAD, a province of Hindoostan, now called Gol-conda.

Hydrabad, the capital of Golconda , and of the Deccan, 690miles S. of Delhi .

Hydrabad, a fort of Hindoostan Proper, in the province ofSindy, seated on the Indus, near Nusserpour.

IT*- DR AGOGU MS, [from £«og, water, and ays*’, to draw,jmedicines used in dropsies ; but the original use of the term pro-ceeded upon a mistaken supposition, that every purgative badsome particular humour which it would evacuate, and which could

1 e ' ,ac "‘ 1 ' ;e d by any tlier. It is now, however, discovered,that all strong purgatives wilt prove hydragogues, if given in largequantity, or in weak constitutions. The principal medicines, re-commended as hydragogues, are the juice of elder, the roots oftris, soldanedla, mechoacan, jalap, &c.

HYDRANGEA, in botany, a genus of the digynia order, anddecandria class of plants; natural order, Succulent *. Capsule

two-celled, two-beaked; corolla five-petaled; calyx five-cleft,superior; seeds many.

hydrachna, a genus of insects of the order aptera.Head, thorax, and abdomen, united or connate; feelers two,jointed ; eyes two, four, or six; legs eight, formed for swimming.Pne genus hydrachna are allied in the closest manner to that of

acarus. There are about fifty species. Of all the hydrachnayet discovered, by far the most elegant is the hydrachna geogra-phica, so named from the fancied map-like distribution of its va-riegations. It is one of the largest of the genus, and is occasion-ally seen in clear ponds and other stagnant waters, but is one ofthe rarer kinds: its shape is globular, and its colour a polishedblack, decorated with carmine-coloured spots and patches, which,in a certain light, are accompanied by a kind of gilded lustre.,

HYDRARGYRUM , mercury, or quicksilver; so called fropiwater, and ago;, silver; q. d. water of silver, on account ofits resembling liquid or melted silver.

HYDRASTIS, in botany, a genus of the polygamia order,and polyandria class of plants. No calyx nor nectarium ; petal*three ; berry composed of monospermous acini. It has but onespecies,

HYDRATE, in chemistry, a term lately introduced to denotethe chemical compound of water with any substance, especially ametallic oxide.

HYDRAULICS , [iJwo, water, and aax©-, a pipe,] the scienceof the motion of fluids, and t tie construction of all kinds ofinstruments and machines relating thereto. See Hydrosta-tics.

HYDRAULICO-PNEUMATICAL, a term applied to en-gines, which raise water by means of air. See Hydrosta-tics.

H YDRENTEROCELE, [Mivj, jvtsjov, and xtiXt,,] in surgery,a species of hernia, wherein the intestines descend into the scro-tum, together with a quantity of water.

HYDROCELE, [iSoMoiiw,] in surgery, a term denotingany hernia arising from water ; but is particularly used forsuch a one of the scrotum, which spmetimes grows to the size ofone’s head, without pain, but exceedingly troublesome. SeeSurgery.

HYDROCEPHALUS , [Yiivj and xspaxv,] a preternatural dis-tension of the head to an uncommon size, by a stagnation andextravasation of the lymph ; which, when collected in the insideof the cranium, is then termed internal; as that collected on theoutside is termed external. See Medicine.

HYDROCHAR1S, the Little Water-Lily ; a genus ofthe enneandria order, and dioecia class of plants; natural order,Palm*. Male, spatha two-leaved; calyx three-cleft; corollathree-petaled ; filaments bearing stiles : female, calyx three-cleft;corolla three-petaled ; styles six ; capsules six-celled ; seeds many.It has but one species.

IIYDROCOTYLE, Water Navelwort; a genus of tbsdigynia order, and pentandria class of plants ; natural order, Um-bellatx. Umbel simple; involucrum tetraphyllous; petals en-tire ; seeds half round and compressed. There are fifteenspecies.

HYDRODYNAMICS, [from 'uiwj,water, and i,-, power,]

a science which treats of the powers, force, and velocities, or fluidsin motion. See Hydrostatics.

HYDRAULICON, Water Organ ; in music, an instrumentacted upon by water, the invention of which is said to be of higherantiquity than that of the wind-organ.

HYDROGEN , [from uJum, water, and to become,]

one of the simple combustibles, and one of the constituents ofwater; it is the base of hydrogen gas, formerly called inflamma-ble air.

Hydrogen Gas, an inflammable gas , remarkable for its le-vity. Its base is hydrogen. It has been long known under thename of fire-damp. Mr. Cavendish, first completely examinedthe properties of this gas, and shewed that it is permanently elas-tic, not absorbed by water ; and that it is much lighter tlian at-mospheric air. (Philos. Trans, vol. lvi. p. 141). T his substanceforming water when combined with oxygen, and being thereforethe radical of that compound, the name hydrogen was given to it,at the formation of the new nomenclature. It is always obtainedfrom the decomposition of water, as it cannot, from other sub-stances in which it exists, be easily disengaged in perfect purity.Some substance is made to act on water, which exerts an attrac-tion to the oxygen, without combination with the hydrogen, when,of course, the'hydrogen is disengaged, and passes into the elasticform. If a coil of iron-wire, or a quantity of iron-filings be putinto an iron, coated glass, or earthen tube, which is placed across

a small