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the place pointed out where a cut, C, should principle may be followed, though the prac. be made, whereby the whole of the inclined tice is different, in places watered by plane, D), might be irrigated to the greatest drains. In such, the greatest care ought to advantage ; the surplus-water draining off be taken to avoid throwing on the water into the lower level of still-water, E, front while the air is frosty ; but so soon as the which it would not be possible to raise the weather opens, the ground ought to be mowater to the superior parts of the inclined derately moistened. The sun's power should plane, CD, without the aid of expensive guide us to the frequency and quantity of machinery. This section will, we trust, water; nor should its quality be overlookprove completely satisfactory, by shewing ed: water from warm soils will produce ef. how necessary it is to look back to superior fects widely different from the streams flow. levels, often within reach.

ing out of clay lands, or such as are impregUnder the head of componnd irrigation, nated with iron, &c. The best water we consider the various changes of direc. usually rises out of gravelly or chalky lands. tion, attended with an intermixture of the It is better to throw the water on early in several modes laid down for simple irriga- the day, during cold weather, in order that tion. In the former, we occasionally find the grass may dry well, and the danger apthe water caught several times by the same prehended from frosty nights be obviated; stream, which, being obstructed at its seve, but in summer, the watering should take ral turns by weirs, sluices, &c., enables us place late in the evening, whereby the to abbreviate the succession of ridges. This ground will be cold, without danger of is a matter of great importance, because it scorching the plants. renders a less body of water, in the branch- We have dwelt thus long on the subject drains of the first level, equal to every par- of irrigation, under the conviction of its expose, and obviates the mischiet that some- treme importance : the reader may, under times attends upon a numerous succession the bead of AGRICULTURE, find a few addiof levels, when the quantity of water re- tional remarks, which were given with the quired for the whole is forced through the view to bringing all matters relating to first, in which, by its weight and volume, farming under one general head, while we the roots of the grass are denudated, and reserved this mechanical part to be sepathe finer parts of the soil completely wasli- rately treated, under its proper designaed away. It is far better to give the stream tion. a second, or even a third, turn through the IRRITABILITY, in physiology, is the land, than to allow all the water, necessary property peculiar to the muscles, by which to moisten six or seven successive levels, to they contract upon the application of certain pass through the first. A reference to fig. 9, stimuli, without a consciousness of action. will give some idea of this mode; by the Haller and other physiologists denominate courses of the dotted lines, and arrows, the that part of the human body irritable, which various descents may be understood. becomes shorter by being touched : very

With respect to the season for watering irritable, if it contracts upon a slight touch. land, so many varieties prevail, in conse. They call that a sensible part of the human quence of soil, and of locality, that we can body, which, upon being touched, transonly observe, in general terms, that, where mits the impression of it to the mind : on lands are to be inundated completely, by the contrary, they call that insensible, which letting the water assame an unlimited being barnt, torn, cnt, &c. occasions no range, and to expand over all parts which sign of pain or convulsion, nor any sort of come under its level, such places require, change in the sitaation of the body. It is during the winter season, to be kept well inferred that the epidermis is insensible; covered, that the frost may not attack the that the true skin is the most sensible part plants while saturated with moisture: if of the body; that the fat and cellular memthat were to happen, the whole would be brane are insensible; and the muscular flesh destroyed; whereas, by a periodical inunda- sensible, the sensibility of which he ascribes tion the grass is sheltered from frost; and, rather to the nerves than the flesh itself. by drawing off the water as the spring ad. The tendons, having no nerves distributed vances, and at intervals of about ten days, among them, are deemed insensible. Irri. when the weather is fair, such grass will tability then is the distinguishing characshoot out vigorously, and afford a very early teristic between the muscular and cellular bite for cattle, at that season when green fibres. Irritability differs from sensibility, food is both valuable and scarce. The same and is not proportioned to it: the intestines are less sensible than the stomach, but more ther considerable portion of time is required irritable: the heart is very irritable, though before they regain the irritability they lost; it has but a small degree of sensation. but the heart and arteries have a more The laws of irritability, according to Dr. short and sudden action, and their state of Crichton, are: 1. After every action in an rest is equally so. The circular muscles of irritable part a state of rest, or cessation the intestines have also a quick action and from motion must take place before the ir- short rest. 6. All stimuli produce action sitable part can be again incited to action. in proportion to their irritating powers. As If by an act of volition we throw any of our a person approaches his hand to the fire, the muscles into action, that action can only be action of all the vessels in the skin is incontinued for a certain space of time; the creased, and it glows with heat; if thic hand muscle becomes relaxed, notwithstanding be approached still nearer, the action is inall our endeavours to the contrary, and re- creased to such an unusual degree as to oc. mains a certain time in that relaxed state, casion redness and pain; and if it be contibefore it can be again thrown into action, nued too long, real inflammation takes 2. Each irritable part has a certain portion place; but if this heat be continued, the or quantity of the principle of irritability part at last loses it irritability, and a sphacewhich is natural to it, part of which it loses lus or gangrene ensues. 7. The action of during action, or from the application of every stimulus is in an inverse ratio to the stimuli. 3. By a process wholly unknown frequency of its application. A small quan. to us it regains this lost quantity during its tity of spirits taken into the stomach inrepose or state of rest. In order to express creases the action of its muscular coat, and the different quantities of irritability in any also of its various vessels, so that digestion part, we say that it is either more or less is thereby facilitated. If the same quantity, redundant, or more or less defective. It bowever, be taken frequently, it loses its becomes redundant in a part when the stic effect. In order to produce the same efmuli which are calcnlated to act on that fect as at first, a larger quantity is necessary; part are withdrawn, or withheld for a certain and hence the origin of dram-drinking. length of time, because then no action can 8. The more the irritability of a part is take place: while, on the other hand, the ap. accumulated, the more that part is disposed plication of stimuli causes it to be exhausted, to be acted upon. It is on this account or to be deficient, not only by exciting ac that the activity of all animals, while in per. tion, but by some secret influence, the na- fect health, is much livelier in the morning ture of which has not yet been detected; than at any other time of the day; for dur. for it is a circumstance extremely deserving ing the night the irritability of the whole of attention, that an irritable part or body frame, and especially that of the muscles may be snddenly deprived of its irritability destined for labour, riz. the muscles of vo. by powerful stimuli, and yet no apparent luntary action, is re-accumulated. The cause of muscular or vascular action takes same law explains why digestion goes on place at the time. Thus a certain quantity more rapidly the first hour after food is of spirits taken at once into the stomach swallowed than at any other time; and it kills almost as instantaneously as lightning also accounts for the great danger that acdoes: the same thing may be observed of crues to a famished person upon first taking some poisons, as opium, laurel-water, the in food. 9. If the stimuli which keep up juice of some poisonous vegetables, &c. the action of any irritable body be with4. Each irritable part has stimuli which are drawn for too great a length of time, that peculiar to it; and which are intended to process on which the formation of the prinsupport its natural action: thus blood, which ciple depends is gradually diminished, and is the stimulus proper to the heart and arte. at last entirely destroyed. When the irri. ries, if by any accident it gets into the sto tability of the system is too quickly exmach, produces sickness or vomiting. 5. hausted by heat, as is the case in certain Each irritable part differs from the rest in warm climates, the application of cold invi. regard to the quantity of irritability which gorates the frame, because cold is a mere it possesses. This law explains to us the diminution of the overplus of that stimulus reason of the great diversity which we ob- which was causing the rapid consumption of serve in the action of various irritable parts: the principle. Under such, or similar eir. thus the muscles of voluntary motion can cumstances, therefore, cold is a tonic re. remain a long time in a state of action, and medy; but if in a climate naturally cold, a if it be continued as long as possible, ano person were to go into a cold bath, and not soon return into a warmer atmosphere, it slightly attracted by the magnet. It is would destroy life just in the same pianner composed of as many poor people, who have no com

Oxide of menachine...... 59.1 fortable dwellings, are often destroyed fivm

iron ............. 30.1 being too long exposed to the cold in win

yran ............ 10.2 ter. Upon the first application of cold the irritability is accumulated, and the vascuar

99.4 system therefore is disposed to great action;

Los .............. but after a certain time all action is so

100.0 much diminished, that the process, whatever it be, on which the formation of the It bears a great resemblance to iron-sand, irritable principle depends, is entirely lost. in colour, but in specific gravity it differs, as See Dr. Crichton on Mental Derangement also in its being very slighly attractable by for more on this subject.

a powerful magnet. It is found on high ISATIS, in botarry, a genus of the Tetra- mountains in Germany. dynamia Siliculosa class and order. Natu- SERTIA, in botany, a genus of the ral order of Siliqnosæ or Cruciformes. Cru- Hexandria Monogynia class and order. Esciferæ, Jussieu. Essential character: silicle sential character : calyx coloured, four lanceolate, one-celled, one-seeded, decidu- or six-toothed ; corolla six-cleft, funnel ous, bivalve; valves navicular. There are form ; pome sub-globular, six-celled, many five species, of which I. tinctoria, dyer's seeded. There is but one species, viz. I. woad, is a biennial plant, with a fusiform, coccinea, a tree with a trunk ten or twelve fibrous root: stem npright, round and feet in height, and about eight inches in sinooth, woody at bottom, branched at topi diameter ; the bark is wrinkled, and of a stem leaves from two to three inches long, russet colour ; the wood light, and of a and scarcely half an inch in breadth; flowers loose texture ; branches quadrangular, small, terminating the stem and branches straight, with opposite brancblets, chanin a close raceme; both corolla and calyx nelled and covered with a russet down ; yellow; petals notched at the end; seed each branchlet has three flowers, of which vessels on slender peduncles, hanging down, that in the middle is sessile; calyx purplish; chesnut coloured or dark brown, shining tube of the corolla two inches long, of a when ripe, of an oblong elliptic form, com- bright red; border yellow, covered on the pressed at top and on the sides into a sharp inside with hairs of the same colour ; fruit a edge, swelling like a convex lens in the succulent red berry or pome, the size of a middle; cotyledops ovate, fleshy, plano cherry, sweet and good to eat. The wood convex ; radicle subcylindrical, bent in np. is bitter ; a decoction of the leaves is used wards. It is a native of most parts of Eu- by the Creoles in fomentations. It is com rope. Woad is much nsed by dyers for its mon in the island of Cayenne; and on the blue colour; it is the basis of black and continent of Guiana, flowering and bearing many other colours.

fruit a great part of the year. ISCHÆMUM, in botany, a genus of the ISINGLASS, used in medicine and do. Polygamia Monoecia class and order. Na mestic economy, is a preparation formerly tural order of Gramina, or Grasses. Gra- made only from a fish named huso, a spemineæ, Jussieu. Essential character: her- cies of the Accipenser genus. We have, in maphrodite calyx; glume two-flowered; co the sixty-third volume of the transactions of rolla two-valved; stamens three; styles the Royal Society, a full account of the three; seed one: male, calyx and corolla mode of preparing this substance, of which as in the other; stamens three. There are we shall give an extract. eight species.

The sounds, or air-bladders, of fresh waISERINE, in mineralogy, a species of the ter fish in general, are preferred for this Menachine genus : it is of an iron-black, in- purpose, as being the most transparent, clining a little to the brownish-black; it flexible, delicate substances. These conoccurs in small, obtuse, angular grains, and stitute the finest sorts of isinglass; those in rolled pieces, with a rongh glimmering called book and ordinary staple are made of surface. Internally it is glistering, and its the intestines, and probably of the peritoInstre is semi-metallic. Specific gravity vænm of the fish. The belluga yields the 4.5. Before the blow-pipe, it melts intu a greatest quantity, as being the largest and blackish brown coloured glass, which is most plentiful fish in the Muscovy rivers ; but the sounds of all fresh water fish yield, to retain its form, when the pegs and stick, more or less, fine isinglass, particularly the are taken out, and the drying completed; smaller sorts, found in prodigions quantities Lastly, the pieces of isinglass are colligated in the Caspian sea, and several bundred in rows, by running pack-thread throngli miles beyond Astracan, in the Wolga, Yaik, the peg-holes, for convenience of package Don, and even as far as Siberia, where it is and exportation. That called cake-isincalled kle or kla by the natives, which im.' glass is formed of the bits and fragments of plies a glutinous matter; it is the basis of the the staple-sorts, put into a flat metalline Russian glue, which is preferred to all other pan, with a very little water, and heated kinds for its strength. The sounds, which just enough to make the parts cohere like a yield the finer isinglass, consist of parallel pancake when it is dried; but frequently it fibres, and are easily rent longitudinally; is overheated, and such pieces, as before but the ordinary sorts are found composed observed, are useless in the business of of double membranes, whose fibres cross fining. Experience has taught the coneach other obliquely, resembling the coats sumers to reject them. of a bladder ; hence the former are more Isinglass is best made in the summer, as readily pervaded and divided with subacid frost gives it a disagreeable colour, deliquors; but the latter, through a peculiar prives it of weight, and impairs its gelatinous kind of interwoven texture, are with great principles ; its fashionable forms are unne. difficulty torn asunder, and long resist the cessary, and frequently injurious to its napower of the same menstruum ; yet, when tive qualities. It is common to find oily duly resolved, are found to act with equal putrid matter, and exuviæ of insects, beenergy in clarifying liquors.

tween the implicated membranes, which, Isinglass receives its different shapes in through the inattention of the cellarman, the following manner. The parts of which often contaminale wines and malt-liquors in it is composed, particularly the sounds, are the act of clarification. taken from the fish while sweet and fresh, These peculiar shapes might probably be slit open, washed from their slimy sordes, introduced originally with a view to conceal divested of every thin membrane which and disguise the real substance of isinglass, envelopes the sound, and then exposed to and preserve the monopoly ; but, as the stiffen a little in the air. In this state, they mask is now taken off, it cannot be doubted. are formed into rolls about the thickness of to answer every parpose more effectually a finger, and in length according to the in- in its native state, without any subsequent tended size of the staple: a thin membrane manufacture whatever, especially to the is generally selected for the centre of the principal consumers, who hence will be roll, round which the rest are solded alter enabled to procure sufficient supply from nately, and about half an inch of each ex. the British colonies. Until this laudable tremity of the roll is turned inwards. The end can be fully accomplished, and as a dae dimensions being thus obtained, the species of isinglass, more easily produceable two ends of what is called short staple are from the marine fisheries, may probably be pinned together with a small wooden peg; more immediately encouraged, it may be the middle of the roll is then pressed a little manufactured as follows. The sounds of downwards, which gives it the resemblance cod and ling bear great analogy with those of a heart-shape, and thus it is laid on of the accipenser genus of Linnæus and boards, or hung up in the air to dry. Artedi ; and are in general so well known

The sounds, which compose the long as to require no particular description. The staple, are longer than the former ; but the Newfoundland and Iceland fishermen split operator lengthens this sort at pleasure, by open the fish as soon as taken, and throw the interfolding the ends of one or more pieces back bones with the sounds annexed, in a of the sound with each other. The extre- heap; but previously to incipient putrefacmities are fastened with a peg, like the for. tion, the sounds are cut out, washed from mer ; but the middle part of the roll is bent their slimes, and salted for use. In cutting more considerably downwards, and, in or out the sounds, the intercostal parts are left der to preserve the shape of the three ob behind, which are much the best; the Icetuse angles thus formed, a piece of round land fishermen are so sensible of this, that stick, about a quarter of an inch diameter, they beat the bone upon a block with a is fastened in each angle with small wooden thick stick, till the pockets, as they term pegs, in the same manner as the ends. In them, come ont easily, and thus preserye this state, it is permitted to dry long enough the sound entire. If the sounds have been

cured with salt, that must be dissolved by scribed the body at large, but have acsteeping them in water before they are pre counted it either a crystal or a talc ; errors pared for isinglass; the freslı sound must which could not have bappened, had the crithen be laid upon a block of wood, whose terions of fossils been at that time fixed; surface is a little elliptical, to the end of since Sir Isaae Newton has recorded its which a small hair-brush is nailed, and with property of making an ebullition with aquaa saw knife the membranes on each side of fortis, which alone must prove that it is the sound must be scraped off. The knife neither tale nor crystal, both those bodies is rubbed upon the brush occasionally, to being wholly unaffected by that menstruum. clear its teeth; the pockets are cut open See CRYSTAL, ORYCTOLOGY, and Talc. with scissars, and perfectly cleansed of the It is always found in form of an oblique mucous matter with a coarse cloth; tlie parallelopiprd, with six sides, and is found sounds are afterwards washed a few mi- of various sizes, from a quarter of an inch to nutes in lime-water in order to absorb their three inches or more in diameter. It is oily principle, and lastly in clear water. pellucid, and not much less bright than They are then laid upon nets to dry, but if the purest crystal, and its planes are all to. intended to resemble the foreign isinglass, lerably smooth, though, when nicely viewed, the sound of the cod will only admit of that they are found to be waved with crooked called book, but those of ling both shapes. lines made by the edges of imperfect The thicker the sounds are the better the plates. isinglass.

What appears very singular in the strucISIS, coral, in natural bistory, a genus of ture of this body, is, that all the surfaces the Vermes Zooplyta class and order. Anis are placed in the same manner, and conse mal growing in the form of a plant ; stem quently it will split off into thin plates, eistony, jointed, the joints longitudinally ther horizontally or perpendicularly ; but striate, united by spongy or horny junctures, this is found on a miscroscopic examination, and covered by a soft porous cellular flesh to be owing to the regularity of figure, or bark; mouth beset with oviparous po. smoothness of surface, and nice joining of Jypes. There are six species. I. hippuris; the several small parallelopiped concretions, with white striate joints and black junctures; of which the whole is composed; and to the it is found chiefly in the Indian seas, grow same cause is probably owing its remarking to rocks, and is from two inches to two able property in refraction. See Optics, feet long. I. entrocha ; stem testaceous, and ReFRACTION. round, with orbicular perforated joints and It is very soft, and easily scratched with verticillate dichotomous branches. Inha- the point of a pin; it will not give fire on bits the ocean. The stem is about the being struck against steel, and ferments thickness of a finger, with crowded flat or- and is perfectly dissolved in aquafortis. It bicular joints perforated in the centre, the is found in Iceland, from whence it has its perforation is pentangular, with the disk name; and in France, Germany, and many substriate from the centre ; outer bark or other places. In England fragments of flesh unequal, and surrounded with a row other spars are very often mistaken for it, of tubercles ; branches thin, dichotomous, many of them having in some degree the continned, not jointed. Hence it is thought same property. that those fossils, called entrochi, are speci. ISNARDIA, in botany, so named in mens of this species of coral.

memory of Mons. Antoine Danti d'Isnard, ISLAND, or ICELAND, crystal, a body fa. member of the Academy of Sciences, a mous among the writers of optics, for its pro genus of the Tetrandria Monogynia class perty of a double refraction; but impro- and order. Natural order of Calycanperly called by that name, as it has none themæ. Salicariæ, Jussieu. Essential cha. of the distinguishing characters of crystal, racter : calyx four-cleft; corolla none; cap. and is plainly a body of another class. Dr. sale four-celled, covered by the calyx. Hill has reduced it to its proper class, and There is but one species, riz, I. palustris, determined it to be of a genus of spars, which bears a great resemblance to peplis which he has called, from their figure, pa. portulaca; it is creeping and floating; the rallelopipedia, and of which he has de flowers are axillary, opposite, sessile, and scribed several species, all of which, as well green. It is a native of Italy, France, as some other bodies of a different genus, Alsace, Russia, Jamaica, and Virginia, in have the same properties. Bartholine, rivers. Huygens, and Sir Isaac Newton, have de. ISOCHRONAL, IsoCHRONE, or Iso

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