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the place pointed out where a cut, C, should be made, whereby the whole of the inclined plane, D, might be irrigated to the greatest advantage; the surplus-water draining off into the lower level of still-water, E, from which it would not be possible to raise the water to the superior parts of the inclined plane, CD, without the aid of expensive machinery. This section will, we trust, prove completely satisfactory, by shewing how necessary it is to look back to superior levels, often within reach.

Under the head of compound irrigation, we consider the various changes of direc. tion, attended with an intermixture of the several modes laid down for simple irrigation. In the former, we occasionally find the water caught several times by the same stream, which, being obstructed at its several turns by weirs, sluices, &c., enables us to abbreviate the succession of ridges. This is a matter of great importance, because it renders a less body of water, in the branchdrains of the first level, equal to every purpose, and obviates the mischief that sometimes attends upon a numerous succession of levels, when the quantity of water required for the whole is forced through the first, in which, by its weight and volume, the roots of the grass are denudated, and the finer parts of the soil completely washed away. It is far better to give the stream a second, or even a third, turn through the land, than to allow all the water, necessary to moisten six or seven successive levels, to pass through the first. A reference to fig. 9, will give some idea of this mode; by the courses of the dotted lines, and arrows, the various descents may be understood.

With respect to the season for watering laud, so many varieties prevail, in consequence of soil, and of locality, that we can only observe, in general terms, that, where lands are to be inundated completely, by letting the water assume an unlimited range, and to expand over all parts which come under its level, such places require, during the winter season, to be kept well covered, that the frost may not attack the plants while saturated with moisture: if that were to happen, the whole would be destroyed; whereas, by a periodical inunda tion the grass is sheltered from frost; and, by drawing off the water as the spring advances, and at intervals of about ten days, when the weather is fair, such grass will shoot out vigorously, and afford a very early bite for cattle, at that season when green food is both valuable and scarce. The same

principle may be followed, though the prac tice is different, in places watered by drains. In such, the greatest care ought to be taken to avoid throwing on the water while the air is frosty; but so soon as the weather opens, the ground ought to be moderately moistened. The sun's power should guide us to the frequency and quantity of water; nor should its quality be overlooked: water from warm soils will produce effects widely different from the streams flow. ing out of clay lands, or such as are impreg nated with iron, &c. The best water usually rises out of gravelly or chalky lands. It is better to throw the water on early in the day, during cold weather, in order that the grass may dry well, and the danger ap prehended from frosty nights be obviated; but in summer, the watering should take place late in the evening, whereby the ground will be cold, without danger of scorching the plants.

We have dwelt thus long on the subject of irrigation, under the conviction of its extreme importance: the reader may, under the head of AGRICULTURE, find a few additional remarks, which were given with the view to bringing all matters relating to farming under one general head, while we reserved this mechanical part to be separately treated, under its proper designa

tion.

IRRITABILITY, in physiology, is the property peculiar to the muscles, by which they contract upon the application of certain stimuli, without a consciousness of action. Haller and other physiologists denominate that part of the human body irritable,which becomes shorter by being touched: very irritable, if it contracts upon a slight touch. They call that a sensible part of the human body, which, upon being touched, transmits the impression of it to the mind: on the contrary, they call that insensible, which being burnt, torn, cut, &c. occasions no sign of pain or convulsion, nor any sort of change in the situation of the body. It is inferred that the epidermis is insensible; that the true skin is the most sensible part of the body; that the fat and cellular membrane are insensible; and the muscular flesh sensible, the sensibility of which he ascribes rather to the nerves than the flesh itself. The tendons, having no nerves distributed among them, are deemed insensible. Irritability then is the distinguishing characteristic between the muscular and cellular fibres. Irritability differs from sensibility, and is not proportioned to it; the intestines

are less sensible than the stomach, but more irritable: the heart is very irritable, though it has but a small degree of sensation. The laws of irritability, according to Dr. Crichton, are: 1. After every action in an irritable part a state of rest, or cessation from motion must take place before the irritable part can be again incited to action. If by an act of volition we throw any of our muscles into action, that action can only be continued for a certain space of time; the muscle becomes relaxed, notwithstanding all our endeavours to the contrary, and remains a certain time in that relaxed state, before it can be again thrown into action. 2. Each irritable part has a certain portion or quantity of the principle of irritability which is natural to it, part of which it loses during action, or from the application of stimuli. 3. By a process wholly unknown to us it regains this lost quantity during its repose or state of rest. In order to express the different quantities of irritability in any part, we say that it is either more or less redundant, or more or less defective. It becomes redundant in a part when the stimuli which are calculated to act on that part are withdrawn, or withheld for a certain length of time, because then no action can take place: while, on the other hand, the application of stimuli causes it to be exhausted, or to be deficient, not only by exciting action, but by some secret influence, the nature of which has not yet been detected; for it is a circumstance extremely deserving of attention, that an irritable part or body may be suddenly deprived of its irritability by powerful stimuli, and yet no apparent cause of muscular or vascular action takes place at the time. Thus a certain quantity of spirits taken at once into the stomach kills almost as instantaneously as lightning does the same thing may be observed of some poisons, as opium, laurel-water, the juice of some poisonous vegetables, &c. 4. Each irritable part has stimuli which are peculiar to it; and which are intended to support its natural action: thus blood,which is the stimulus proper to the heart and arteries, if by any accident it gets into the stomach, produces sickness or vomiting. 5. Each irritable part differs from the rest in regard to the quantity of irritability which it possesses. This law explains to us the reason of the great diversity which we observe in the action of various irritable parts: thus the muscles of voluntary motion can remain a long time in a state of action, and if it be continued as long as possible, ano

ther considerable portion of time is required before they regain the irritability they lost; but the heart and arteries have a more short and sudden action, and their state of rest is equally so. The circular muscles of the intestines have also a quick action and short rest. 6. All stimuli produce action in proportion to their irritating powers. As a person approaches his hand to the fire, the action of all the vessels in the skin is increased, and it glows with heat; if the hand be approached still nearer, the action is increased to such an unusual degree as to occasion redness and pain; and if it be continued too long, real inflammation takes place; but if this heat be continued, the part at last loses it irritability, and a sphacelus or gangrene ensues. 7. The action of every stimulus is in an inverse ratio to the frequency of its application. A small quantity of spirits taken into the stomach increases the action of its muscular coat, and also of its various vessels, so that digestion is thereby facilitated. If the same quantity, however, be taken frequently, it loses its effect. In order to produce the same effect as at first, a larger quantity is necessary; and hence the origin of dram-drinking. 8. The more the irritability of a part is accumulated, the more that part is disposed to be acted upon. It is on this account that the activity of all animals, while in perfect health, is much livelier in the morning than at any other time of the day; for during the night the irritability of the whole frame, and especially that of the muscles destined for labour, viz. the muscles of voluntary action, is re-accumulated. same law explains why digestion goes on more rapidly the first hour after food is swallowed than at any other time; and it also accounts for the great danger that accrues to a famished person upon first taking in food. 9. If the stimuli which keep up the action of any irritable body be withdrawn for too great a length of time, that process on which the formation of the principle depends is gradually diminished, and at last entirely destroyed. When the irritability of the system is too quickly exhausted by heat, as is the case in certain warm climates, the application of cold invigorates the frame, because cold is a mere diminution of the overplus of that stimulus which was causing the rapid consumption of the principle. Under such, or similar circumstances, therefore, cold is a tonic remedy; but if in a climate naturally cold, a person were to go into a cold bath, and not

The

*oon return into a warmer atmosphere, it would destroy life just in the same nianner as many poor people, who have no comfortable dwellings, are often destroyed from being too long exposed to the cold in winter. Upon the first application of cold the irritability is accumulated, and the vascular system therefore is disposed to great action; but after a certain time all action is so much diminished, that the process, whatever it be, on which the formation of the - irritable principle depends, is entirely lost. See Dr. Crichton on Mental Derangement for more on this subject.

ISATIS, in botany, a genus of the Tetradynamia Siliculosa class and order. Natural order of Siliquosæ or Cruciformes. Cruciferæ, Jussieu. Essential character: silicle lanceolate, one-celled, one-seeded, deciduous, bivalve; valves navicular. There are five species, of which I. tinctoria, dyer's woad, is a biennial plant, with a fusiform, fibrous root: stem upright, round and sinooth, woody at bottom, branched at top; stem leaves from two to three inches long, and scarcely half an inch in breadth; flowers small, terminating the stem and branches in a close raceme; both corolla and calyx yellow; petals notched at the end; seed vessels on slender peduncles, hanging down, chesnut coloured or dark brown, shining when ripe, of an oblong elliptic form, compressed at top and on the sides into a sharp edge, swelling like a convex lens in the middle; cotyledons ovate, fleshy, plano convex; radicle subcylindrical, bent in upwards. It is a native of most parts of Europe. Woad is much used by dyers for its blue colour: it is the basis of black and many other colours.

ISCHÆMUM, in botany, a genus of the Polygamia Monoecia class and order. Na tural order of Gramina, or Grasses. Gramineæ, Jussieu. Essential character: hermaphrodite calyx; glume two-flowered; corolla two-valved; stamens three; styles three; seed one: male, calyx and corolla as in the other; stamens three. There are eight species.

ISERINE, in mineralogy, a species of the Menachine genus; it is of an iron-black, inclining a little to the brownish-black; it occurs in small, obtuse, angular grains, and in rolled pieces, with a rough glimmering surface. Internally it is glistering, and its lustre is semi-metallic. Specific gravity 4.5. Before the blow-pipe, it melts into a blackish-brown coloured glass, which is

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It bears a great resemblance to iron-sand, in colour, but in specific gravity it differs, as also in its being very slighly attractable by a powerful magnet. It is found on high mountains in Germany.

ISERTIA, in botany, a genus of the Hexandria Monogynia class and order. Essential character: calyx coloured, four or six-toothed; corolla six-cleft, funnel form; pome sub-globular, six-celled, many seeded. There is but one species, viz. I. coccinea, a tree with a trunk ten or twelve feet in height, and about eight inches in diameter; the bark is wrinkled, and of a russet colour; the wood light, and of a loose texture; branches quadrangular, straight, with opposite branchlets, channelled and covered with a russet down; each branchlet has three flowers, of which that in the middle is sessile; calyx purplish ; tube of the corolla two inches long, of a bright red; border yellow, covered on the inside with hairs of the same colour; fruit a succulent red berry or pome, the size of a cherry, sweet and good to eat. The wood is bitter; a decoction of the leaves is used by the Creoles in fomentations. It is com mon in the island of Cayenne; and on the continent of Guiana, flowering and bearing fruit a great part of the year.

ISINGLASS, used in medicine and domestic economy, is a preparation formerly made only from a fish named huso, a species of the Accipenser genus. We have, in the sixty-third volume of the transactions of the Royal Society, a full account of the mode of preparing this substance, of which we shall give an extract.

The sounds, or air-bladders, of fresh water fish in general, are preferred for this purpose, as being the most transparent, flexible, delicate substances. These constitute the finest sorts of isinglass; those called book and ordinary staple are made of the intestines, and probably of the peritonæum of the fish. The belluga yields the greatest quantity, as being the largest and most plentiful fish in the Muscovy rivers;

but the sounds of all fresh water fish yield, more or less, fine isinglass, particularly the smaller sorts, found in prodigious quantities in the Caspian sea, and several hundred miles beyond Astracan, in the Wolga, Yaik, Don, and even as far as Siberia, where it is called kle or kla by the natives, which im-' plies a glutinous matter; it is the basis of the Russian glue, which is preferred to all other kinds for its strength. The sounds, which yield the finer isinglass, consist of parallel fibres, and are easily rent longitudinally; but the ordinary sorts are found composed of double membranes, whose fibres cross each other obliquely, resembling the coats of a bladder; hence the former are more readily pervaded and divided with subacid liquors; but the latter, through a peculiar kind of interwoven texture, are with great difficulty torn asunder, and long resist the power of the same menstruum; yet, when duly resolved, are found to act with equal energy in clarifying liquors.

Isinglass receives its different shapes in the following manner. The parts of which it is composed, particularly the sounds, are taken from the fish while sweet and fresh, slit open, washed from their slimy sordes, divested of every thin membrane which envelopes the sound, and then exposed to stiffen a little in the air. In this state, they are formed into rolls about the thickness of a finger, and in length according to the intended size of the staple: a thin membrane is generally selected for the centre of the roll, round which the rest are folded alter nately, and about half an inch of each extremity of the roll is turned inwards. The due dimensions being thus obtained, the two ends of what is called short staple are pinned together with a small wooden peg; the middle of the roll is then pressed a little downwards, which gives it the resemblance of a heart-shape, and thus it is laid on boards, or hung up in the air to dry.

The sounds, which compose the longstaple, are longer than the former; but the operator lengthens this sort at pleasure, by interfolding the ends of one or more pieces of the sound with each other. The extremities are fastened with a peg, like the former; but the middle part of the roll is bent more considerably downwards, and, in order to preserve the shape of the three obtuse angles thus formed, a piece of round stick, about a quarter of an inch diameter, is fastened in each angle with small wooden pegs, in the same manner as the ends. In this state, it is permitted to dry long enough

to retain its form, when the pegs and sticks are taken out, and the drying completed; lastly, the pieces of isinglass are colligated in rows, by running pack-thread through the peg-holes, for convenience of package' and exportation. That called cake-isinglass is formed of the bits and fragments of the staple-sorts, put into a flat metalline pan, with a very little water, and heated just enough to make the parts cohere like a pancake when it is dried; but frequently it is overheated, and such pieces, as before observed, are useless in the business of fining. Experience has taught the consumers to reject them.

Isinglass is best made in the summer, as frost gives it a disagreeable colour, deprives it of weight, and impairs its gelatinous principles; its fashionable forms are unnecessary, and frequently injurious to its native qualities. It is common to find oily putrid matter, and exuviæ of insects, between the implicated membranes, which, through the inattention of the cellarman, often contaminate wines and malt-liquors in the act of clarification.

These peculiar shapes might probably be introduced originally with a view to conceal and disguise the real substance of isinglass, and preserve the monopoly; but, as the mask is now taken off, it cannot be doubted to answer every purpose more effectually in its native state, without any subsequent manufacture whatever, especially to the principal consumers, who hence will be enabled to procure sufficient supply from the British colonies. Until this laudable end can be fully accomplished, and as a species of isinglass, more easily produceable from the marine fisheries, may probably be more immediately encouraged, it may be manufactured as follows. The sounds of cod and ling bear great analogy with those of the accipenser genus of Linnæus and Artedi; and are in general so well known as to require no particular description. The Newfoundland and Iceland fishermen split open the fish as soon as taken, and throw the back bones with the sounds annexed, in a heap; but previously to incipient putrefaction, the sounds are cut out, washed from their slimes, and salted for use. In cutting out the sounds, the intercostal parts are left behind, which are much the best; the Iceland fishermen are so sensible of this, that they beat the bone upon a block with a thick stick, till the pockets, as they term them, come out easily, and thus preserve the sound entire. If the sounds have been

cured with salt, that must be dissolved by steeping them in water before they are prepared for isinglass; the fresh sound must then be laid upon a block of wood, whose surface is a little elliptical, to the end of which a small hair-brush is nailed, and with a saw knife the membranes on each side of the sound must be scraped off. The knife is rabbed upon the brush occasionally, to clear its teeth; the pockets are cut open with scissars, and perfectly cleansed of the mucous matter with a coarse cloth; the sounds are afterwards washed a few minutes in lime-water in order to absorb their oily principle, and lastly in clear water. They are then laid upon nets to dry, but if intended to resemble the foreign isinglass, the sound of the cod will only admit of that called book, but those of ling both shapes. The thicker the sounds are the better the isinglass.

ISIS, coral, in natural history, a genus of the Vermes Zoophyta class and order. Animal growing in the form of a plant; stem stony, jointed, the joints longitudinally striate, united by spongy or horny junctures, and covered by a soft porous cellular flesh or bark; mouth beset with oviparous polypes. There are six specics. I. hippuris; with white striate joints and black junctures; it is found chiefly in the Indian seas, grow. ing to rocks, and is from two inches to two feet long. I. entrocha; stem testaceous, round, with orbicular perforated joints and verticillate dichotomous branches. Inhabits the ocean. The stem is about the thickness of a finger, with crowded flat orbicular joints perforated in the centre, the perforation is pentangular, with the disk substriate from the centre; outer bark or flesh unequal, and surrounded with a row of tubercles; branches thin, dichotomous, continued, not jointed. Hence it is thought that those fossils, called entrochi, are specimens of this species of coral.

ISLAND, or ICELAND, crystal, a body famous among the writers of optics, for its property of a double refraction; but improperly called by that name, as it has none of the distinguishing characters of crystal, and is plainly a body of another class. Dr. Hill has reduced it to its proper class, and determined it to be of a genus of spars, which he has called, from their figure, parallelopipedia, and of which he has described several species, all of which, as well as some other bodies of a different genus, have the same properties. Bartholine, Huygens, and Sir Isaac Newton, have de

scribed the body at large, but have accounted it either a crystal or a talc; errors which could not have happened, had the criterions of fossils been at that time fixed; since Sir Isaac Newton has recorded its property of making an ebullition with aquafortis, which alone must prove that it is neither tale nor crystal, both those bodies being wholly unaffected by that menstruum. See CRYSTAL, ORYCTOLOGY, and TALC.

It is always found in form of an oblique parallelopiped, with six sides, and is found of various sizes, from a quarter of an inch to three inches or more in diameter. It is pellucid, and not much less bright than the purest crystal, and its planes are all tolerably smooth, though, when nicely viewed, they are found to be waved with crooked lines made by the edges of imperfect plates.

What appears very singular in the stracture of this body, is, that all the surfaces are placed in the same manner, and conse quently it will split off into thin plates, either horizontally or perpendicularly; but this is found on a miscroscopic examination, to be owing to the regularity of figure, smoothness of surface, and nice joining of the several small parallelopiped concretions, of which the whole is composed; and to the same cause is probably owing its remark. able property in refraction. See OPTICS, and REFRACTION.

It is very soft, and easily scratched with the point of a pin; it will not give fire on being struck against steel, and ferments and is perfectly dissolved in aquafortis. It is found in Iceland, from whence it has its name; and in France, Germany, and many other places. In England fragments of other spars are very often mistaken for it, many of them having in some degree the same property.

ISNARDIA, in botany, so named in memory of Mons. Antoine Danti d'Isnard, member of the Academy of Sciences, a genus of the Tetrandria Monogynia class and order. Natural order of Calycanthemæ. Salicariæ, Jussieu. Essential cha. racter: calyx four-cleft; corolla none; cap. sule four-celled, covered by the calyx. There is but one species, viz. I. palustris, which bears a great resemblance to peplis portulaca; it is creeping and floating; the flowers are axillary, opposite, sessile, and green. It is a native of Italy, France, Alsace, Russia, Jamaica, and Virginia, in rivers.

ISOCHRONAL, ISOCHRONE, or Iso

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