Whenever this function is suspended, even for a very short time, the animal dies. The fluid respired by animals is common atmospherical air; and it has been ascertained by experiment, that no other gaseous body with which we are acquainted can be substituted for it. All the known gases have been tried; but they all prove fatal to the animal which is made to breathe them. Gaseous bodies, as far as respiration is concerned, may be divided into two classes:-1. Unrespirable gases. 2. Respirable gases. The gases belonging to the first class are of such a nature that they cannot be drawn into the lungs of an animal at all, the epiglottis closing spasmodically when ever they are applied to it. To this class belong carbonic acid, and probably all the other acid gases, as has been ascertained by the experiments of Pilatre de Rozier, who went into a brewer's tub while full of carbonic acid gas evolved by fermentation. A gentle heat manifested itself in all parts of his body, and occasioned a sensible perspiration. A slight itching sensation constrained him frequently to shut his eyes. When he attempted to breathe, a violent feeling of suffocation prevented him. He sought for the steps to get out; but not finding them readily, the necessity of breathing increased, he became giddy, and felt a tingling sensation in his ears. As soon as his mouth reached the air, he breathed freely; but for some time he could not distinguish objects; his face was purple, his limbs weak, and he understood with difficulty what was said to him. these symptoms soon left him. He repeated the experiment often; and always found, that as long as he continued without breathing he could speak and move about without inconvenience; but when ever he attempted to breathe, the feeling of suffocation came on. For the lungs of animals suffocated by it were found by Pilatre not to give a green colour to vegetable blues. The gases belonging to the second class may be drawn into the lungs and thrown out again without any opposition from the respiratory organs: of course the animal is capable of respiring them. They may be divided into four subordinate classes:-1. The first set of gases occasion death immediately, but produce no visible change in the blood. They occasion the animal's death merely by depriving him of air, in the same way as he would be suffocated by being kept under water. The only gases which belong to this class are hydrogen VOL. X. But and azotic. 2. The second set of gases occasion death immediately; but at the same time they produce certain changes in the blood, and therefore kill, not merely by depriving the animal of air, but by certain specific properties. The gases belonging to this class are carburetted hydrogen, sulphuretted hydrogen, carbonic oxide, and perhaps also nitrous gas. 3. The third set of gases may be breathed for some time without destroying the animal; but death ensues at last, provided their action be long enough continued. To this class belong the nitrous oxide and oxygen gas. 4. The fourth set may be breathed any length of time without injuring the animal. Air is the only gaseous body belonging to this class. See PHYSIOLOGY, and Thomson's Chemistry. RESPONDEAS ouster, is to answer over in an action to the merits of the cause. As if a demurrer is joined upon a plea to the jurisdiction, person, or writ, and it be adjudged against the defendant, it is a respondeas ouster. REST, the continuance of a body in the same place, or its continual application or contiguity to the same parts of the ambient or contiguous bodies; and therefore is opposed to motion. Sir Isaac Newton defines true or absolute rest to be the continuance of a body in the same part of absolute space; and relative rest to be the continuance of a body in the same part of relative space. Thus, in a ship under sail, relative rest is the continuance of a body in the same part of the ship; but absolute, is its continuance in the same part of universal space in which the ship itself is contained. It is one of the laws of nature, that matter is indifferent to motion or rest, as has been shown under the article INERTIA. REST, in poetry, is a short pause of the voice in reading, being the same with the cæsura, which, in Alexandrian verses, falls on the sixth syllable; but in verses of ten or eleven syllables, on the fourth. RESTIO, in botany, a genus of the Dioecia Triandria class and order. Natural order of Calamaria. Essential character: calyx three-leaved, two of the leaflets boat-shaped; corolla three-leaved, leaflets lanceolate, one wider: female, germ three-sided; style one, seldom two or three; stigmas one, two, three, feathered. There are twenty-eight species. These plants are all natives of the Cape of Good Hope, where some of them are used for making ropes, for brooms, or for thatching. Kk RESULTING use, in law, is when an use limited by a deed expires, or cannot vest, it returns back to him who raised it. See USES. RETAINER of debts, an executor, among debts of equal degree, may pay himself first, by retaining in his hands the amount of his debt. RETARDATION, in physics, the act of diminishing the velocity of a moving body. If bodies of equal bulk, but of dif ferent densities, be moved through the same resisting medium, with equal velocity, the medium will act equally on each, so that they will have equal resistances,, but their motions will be unequally retarded, in proportion to their densities. Retarded motion from gravity is peculiar to bodies projected upwards, and this in the same manner as a falling body is accelerated; only in the latter, the force of gravity acts in the same direction with the motion of the body: and in the former, in an opposite direction. As it is the same force which augments the motion in the falling, and diminishes it in the rising body, a body will rise till it has lost all its motion; which it does in the same time wherein a body falling would have acquired a velocity equal to that wherewith the body was projected upwards. RETE mucosum, in animal economy, is the mucous substance, situated between the cutis vera and epidermis; its composition cannot be determined with precision, because its quantity is too small to admit of examination. It is known that the black colour of negroes depends upon a black pigment, situated in this substance. Oxymuriatic acid deprives it of its black colour, and renders it yellow. A negro, by keeping his foot for some time in water impregnated with that acid, deprived it of its colour, and rendered it nearly white; but in a few days the black colour returned again with its former intensity. This experiment was first made by Dr. Beddoes, on the fingers of a negro. RETENTION. Whatever be the effect produced in the mental organs by the impressions on the organs of sense, that effect can be renewed, though in general with diminished vigour, without a repetition of the sensible impressions. In other words, sensible changes produce a tendency to similar changes, which can be repeated without the repetition of the external impressions, and may then be called ideal changes. Less generally, sensations leave relics behind them, which can be perceived without the agency of the external organs of sensation, and which are called ideas. The power or capacity of the mind, by which tendencies to ideal changes are retained, may be called the retentive power. That tendencies to a repetition of sensorial changes are thus formed, that ideas are thus retained, may be referred to the operation of the associative power, and in the human being they certainly depend upon the same organic causes, whatever those be. But in many animals it is decidedly probable that sensations leave no relics behind them; and in man there are, equally probably, numerous impressions from external objects, which leave no relics behind them. Again, these relics of sensations can re-appear without the agency of external objects. Hence it appears preferable to consider the receiv ing of sensations, and the retaining of ideas, as two separate, though intimately connected, operations, and as implying two separate powers or capacities of the mind. This is not done by Hartley, who appears to refer both to sensation; but it has subjected him to some apparently just, though in reality unfounded, animadversions of the great northern philosopher Dugald Stewart. Speaking of the phenomena of memory as not to be entirely explained by the law of association, he says, (p. 412.) "The associathoughts with each other, so as to pretion of ideas connects our various sent them to the mind in a certain order, but it presupposes a faculty of retaining the knowledge we acquire." This Hartley knew, and has according ly a distinct section on the generation of ideas. Without the retentive power, it is obvious that man would be a being of mere sensation, little, if any, superior to the lowest orders of the animal creation, and inferior to many of them. The retentive power provides materials for the agency of the associative power. Without the retentive power, the associative power would never be called into exercise, and without, the associative power, the relics of sensation, the effects of the retentive power, would be of no utility. The operations of the retentive power can scarcely be separated from those of the associative power, which together constitute the compound faculty called memory, for an account of which see PHILOSOPHY, mental, § 105. We have said that the receiving of sen sations, and the retaining the relics of This play of the organs (which how- Place about half an inch square of white paper on a black hat, and looking steadily on the centre of it for a minute, remove your eyes to a sheet of white paper; after a second or two a dark square will be seen on the white paper, which will be seen for some time. A similar dark square will be seen in the closed eye, if light be admitted through the eyelids. So, after looking at any luminous body of a small size, as at the Sun, for a short time, so as not much to fatigue the eyes, this part of the retina becomes less sensible to smaller quantities of light: hence, when the eyes are turned upon other less luminous parts of the sky, a dark spot is seen, resembling the shape of the luminous body. To the same cause Dr. R. Darwin ascribes those dark coloured floating spots, which are easily perceptible when the eyes are a little weakened by fatigue, and during illnesses which are attended with great debility. He says, that as these spectra are most easily discernible when our eyes are weakened by fatigue, it has frequently happened that people of delicate constitutions have been much alarmed at them, fearing a beginning decay of their sight, and thence have fallen into the hands of ignorant oculists. They are not, however, he observes, the preludes to any disease, and it is only from our habitual inattention to them, that we do not see them on all objects every hour of our lives. As the nerves of very weak people, he continues, lose their sensibility by a small duration of exertion, it frequently happens that sick people, in the extreme debility of fevers, are perpetually employed in picking something off from the bed-clothes, owing to their mistaking the cause of these dark spots. An Italian artist, a man of strong abilities, relates, that having passed the whole night on a distant mountain, with some companions and a conjuror, and performed many ceremonies to raise the devil, on their return in the morning to Rome, looking up when the sun began to rise, they saw numerous devils run on the tops of the houses as they passed along. So much were the spectra of their weakened eyes magnified by fear, and made subservient to the purposes of fraud or superstition. Again, make with ink, on white paper, a very black spot, about half an inch in diameter, with a tail about an inch in length, so as to represent a tadpole. Look steadily at this spot for about a minute, and on moving the eye a little, the figure of the tadpole will be seen on the white part of the paper, which figure will appear whiter or more luminous than the other part of the paper. This Dr. R. Darwin brings as one proof, that when the retina has been subjected to a less excitement, it is more easily brought into action by being subjected to a greater. A surface appears black in consequence of its absorbing all the rays of light; that part of he retina, therefore, which is unemployed while looking at the spot, is afterwards more sensible of the light from the white pa per, than those parts which had previ ously been exposed to it. On closing the eyes after viewing the black spot on the white paper, a red spot is seen of the form of the black spot; for that part of the retina on which the figure of the black spot was formed, being more sensible to the light than the other parts, is capable of being brought into action by the red rays which penetrate the eye-lids. Upon the same principle Dr. R. Darwin accounts for the following fact. A writer in the Berlin Memoirs observes that when he held a book, so that the sun shone upon his half closed eye-lids, the black letters which he had long inspected, became red. There is a similar story told by Voltaire of a Duke of Tuscany, who was playing at dice with a general of a foreign army, and believing that he saw red spots on the dice, portended dreadful events, and retired in confusion. The observer, after looking for a minute on the black spots of a die, in a bright day, and carelessly closing his eyes, would see red spots corresponding to to the black spots on the die, and if they were intense, from the fatigue or weakness of the optic organ, those appearances would continue, and on looking at the die, would be supposed to be upon it, just as before stated; persons in a very weak state often see black spots which they refer to the bed-clothes. RETICULA, or RETICULE, in astronomy, a contrivance for the exact measuring the quantity of eclipses. The reticule is a little frame, consisting of thirteen fine silken threads equidistant from each other, and parallel, placed in the focus of object-glasses of telescopes; that is, in the place where the image of the luminary is painted in its full extent; of consequence, therefore the diameter of the sun or moon is hereby seen divided into twelve equal parts or digits; so that to find the quantity of the eclipse, there is nothing to do but to number the luminous and the dark parts. As a square reticule is only proper for the diameter, not for the circumference of the luminary, it is sometimes made circular by drawing six concentric equidistant circles. This represents the phases of the eclipse perfectly. RETINA, in anatomy, the expansion of the optic nerve on the internal surface of the eye, whereupon the images of objects being painted, are impressed, and by that means conveyed to the common sensory in the brain, where the mind views and contemplates their ideas. Sce OPTICS. RETORNO habendo, in law. See RE PLEVIN. RETORT. See LABORATORY. RETRAXIT, in law, is where the plaintiff or demandant comes in person into the court, and says he will proceed no further; and this is a bar of all other actions of like or inferior nature. RETRENCHMENT, in the art of war, any kind of work raised to cover a post, and fortify it against the enemy, such as fascines loaded with earth, gabions, barrels of earth, sand-bags, and generally all things that can cover the men and stop the enemy. But retrenchment is more particularly applicable to a foss bordered with a parapet; and a post fortified thus is called post retrenched, or strong post. Retrenchments are either general or particular: general retrenchments are new fortifications made in a place besieged, to cover the besiegers when the enemy become masters of a lodgement on the fortification, that they may be in a condition of disputing the ground inch by inch, and of putting a stop to the enemy's progress, in expectation of relief. Particular retrenchments are such as are made in the bastions, when the enemy are masters of the breach. These can never be made but in new full bastions, for in empty or hollow ones there can only be made retirades. The particular retrenchments are made several ways, according to the time they have to cover themselves: sometimes they are made before-hand, which are certainly the best. ought to be five or six feet thick, and five The parapets of such retrenchments feet high, with a large and deep foss, from whence ought to run out small fougades and countermines. RETROGRADATION, in astronomy, is an apparent motion of the planets, by which they seem to go backwards in the ecliptic, and to move contrary to the order of the signs, as from Aries to Taurus; from Taurus to Gemini, &c. which, from it appears for some days in the same west to east, is said to be direct. When place or point in the heavens, it is said to be stationary; and when goes in antecedentia, or backwards, or contrary, to the order of the signs, which is from east to west, it is said to be retrograde. Saturn continues retrograde about 140 days; Jupiter 120; Mars 73; Venus 42; and Mercury 22. The interval between two retrogradations of the several planets are as follow. RETROMINGENTS, in natural history, a class or division of animals, whose characteristic it is that they stale, or make water backwards, both male and female. RETURN, is most commonly used for the return of writs, which is the certificate of the sheriff, made to the court of what he has done, touching the execution of any writ directed to him; and where a writ is executed, or the defendant cannot be found, or the like, this fact is indorsed on the writ by the officer, and delivered into the court whence the writ issued, at the day of the return thereof, in order to be filed. RETURNING stroke, in electricity, is an expression used by Lord Mahon, (now Earl Stanhope,) to denote the effect produced by the return of the electric fluid into a body, from which, under certain circumstances, it has been expelled. To understand properly the meaning of these terms, it must be premised, that according to the author's experiments, an insulated smooth body, immerged within the electrical atmosphere, but beyond the striking distance of another body, charged positively, is at the same time in a state of threefold electricity. The end next to the charged body acquires negative electricity; the further end is positively electrified; while a certain part of the body, somewhere between its two extremes, is in a natural, unelectrified, or neutral state; so that the two contrary electricities balance each other. It may further be added, that if the body be not insulated, but have a communication with the earth, the whole of it will be in a negative state. Suppose, then, a brass ball, which may be called A, to be constantly placed at the striking distance of a prime conductor, so that the conduc. tor, the, instant when it becomes fully charged, explodes into it. Let another large or second conductor be suspended in a perfectly insulated state, further from the prime conductor than the striking distance, but within its electrical atmosphere: let a person, standing on an insu lated stool, touch this second conductor very lightly with a finger of his right hand, while with a finger of his left hand he communicates with the earth, by touching very lightly a second brass ball, fixed at the top of a metallic stand, on the floor, which may be called B. Now, while the prime conductor is receiving its electricity, sparks pass (at least if the distance between the two conductors is not too great) from the second conductor to the right hand of the insulated person, while similar and simultaneous sparks pass out from the finger of his left hand into the second metallic ball, B, communicating with the earth. At length, however, the prime conductor, having acquired its full charge, suddenly strikes into the ball, A, of the first metallic stand, placed for that purpose at the striking distance. The explosion being made, and the prime conductor suddenly robbed of its elactic atmosphere, its pressure or action on the second conductor, and on the insulated person, as suddenly ceases, and the latter instantly feels a smart returning stroke, though he has no direct or visible communication (except by the floor) with either of the two bodies, and is placed at the distance of five or six feet from both of them. This returning stroke is evidently occasioned by the sudden re-entrance of the electric fire, naturally belonging to his body and to the second conductor, which had before been expelled from them by the action of the charged prime conductor upon them; and which returns to its former place in the instant when that action or elastic pressure ceases. When the second conductor and the insulated person are placed in the densest part of the electrical atmosphere of the prime conductor, or just beyond the striking distance, the effects are still more considerable; the returning stroke being extremely severe and pungent, and appearing considerably sharper than even the main stroke itself received directly from the prime conductor. Lord Mahon observes, that persons and animals may be destroyed, and particular parts of buildings may be much damaged, by an electrical returning stroke, occasioned even by some very distant explosion from a thunder cloud; possibly at the distance of a mile or more. It is certainly not difficult to conceive, that a charged extensive thunder cloud must be productive of effects similar to those produced by the prime conductor; but perhaps the effects are not so great, nor the danger so terrible, as it seems to have been apprehended. If the quantity of electric fluid naturally contained, for example, in the body of a man, were immense or indefinite, then the es |