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shell-fish which we possess in the work of an eye-witness, Eudocia Macrembolitissa, daughter of the Emperor Constantine VIII., who lived in the eleventh century.

The moderns have obtained from the New World several dye-drugs unknown to the antients; such as cochineal, quercitron, Brazil wood, logwood, annatto; and they have discovered the art of using indigo as a dye, which the Romans knew only as a pigment. But the vast superiority of our dyes over those of former times must be ascribed principally to the employment of pure alum and solution of tin as mordants, either alone or mixed with other bases; substances which give to our common dye-stuffs remarkable depth, durability, and lustre. Another improvement in dyeing of more recent date is the application to textile substances of metallic compounds, such as Prussian blue, chrome yellow, manganese brown, &c.

Indigo, the innoxious and beautiful product of an interesting tribe of tropical plants, which is adapted to form the most useful and substantial of all dyes, was actually denounced as a dangerous drug, and forbidden to be used, by our parliament in the reign of Queen Elizabeth. An act was passed authorizing searchers to burn both it and logwood in every dye-house where they could be found. This act remained in full force till the time of Charles II.; that is, for a great part of a century. A foreigner might have supposed that the legislators of England entertained such an affection for their native woad, with which their naked | sires used to dye their skins in the old times, that they would allow no outlandish drug to come in competition with it. A most instructive book might be written illustrative of the evils inflicted upon arts, manufactures, and commerce, in consequence of the ignorance of the legislature.

Mr. Delaval made many ingenious experiments to prove that the particles of dye-stuffs possess no power of reflecting light, and that therefore when viewed upon a dark ground they all appear black, whatever colour they may exhibit when seen by light transmitted through them. He hence inferred that the difference of colour shown by dyed cloths is owing to the white light which is reflected from the textile fibres being decomposed in its passage through the superinduced colouring particles. We think it more than probable that this conclusion is in some respects incorrect, and that the aluminous, iron, and tin bases form combinations with dye-stuffs which are capable of reflecting light, independent of the reflection from the fibre itself. There can be no doubt however that this latter reflected light adds greatly to the brightness of the tints, and that the whiter the textile substance is the better dye it will, generally speaking, receive. It is for this reason that scouring or bleaching of the stuffs is usually prescribed as a process preliminary to dyeing.

Bergman appears to have been the first who referred to chemical affinities the phenomena of dyeing. Having plunged wool and silk into two separate vessels, containing solution of indigo in sulphuric acid diluted with a great deal of water, he observed that the wool abstracted much of the colouring matter, and took a deep blue tint, but that the silk was hardly changed. He ascribed this difference to the greater affinity subsisting between the particles of sulphate of indigo and wool, than between these and silk; and he showed that the affinity of the wool is sufficiently energetic to render the solution colourless by attracting the whole of the indigo, while that of the silk can separate only a little of it. He thence concluded that dyes owed both their permanence and their depth to the intensity of that attractive force.

Dye-stuffs, whether of vegetable or animal origin, though susceptible of solution in water, and, in this state, of penetrating the pores of fibrous bodies, seldom possess alone the power of fixing their particles so durably as to be capable of resisting the action of water, light, and air. For this purpose they require to be aided by another class of bodies, already alluded to, which bodies may not possess any colour in themselves, but serve in this case merely as a bond of union between the dye and the substance to be dyed. These bodies were supposed, in the infancy of the art, to seize the fibres by an agency analogous to that of the teeth of animals, and were hence called mordants, from the Latin verb mordere, to bite. However preposterous this comparison is now known to be, the term derived from it has gained such a footing in the language of the dyer that all writers upon his art are compelled to adopt it.

Mordants may be regarded, in general, as not only fixing but also occasionally modifying the dye, by forming with the colouring particles an insoluble compound, which is deposited within the textile fibres. Such dyes as are capable of passing from the soluble into the insoluble state, and of thus becoming permanent, without the addition of a mordant, have been called substantive, and all the others have been called adjective colours. Indigo and tannin are perhaps the only dyes of organic origin to which the title substantive can be applied, and even they probably are so altered by atmospheric oxygen, in their fixation upon stuffs, as to form no exception to the true theory of mordants. Mordants are of primary importance in dyeing; they enable us to vary the colours almost indefinitely with the same dye, to increase their lustre, and to give them a durability which they otherwise could not possess. A mordant is not always a simple agent, but in the mixture of which it consists various compounds may be formed, so that the substances may not act directly, but through a series of transformations. The China blue process [CALICO PRINTING] affords a fine illustration of this truth. Sometimes the mordant is mixed with the colouring matters, sometimes it is applied by itself first of all to the stuff, and at others both these methods are conjoined. We may dye successively with liquors which contain different substances, which will act differently according to the different mordants successively employed. One solution will give up its base to the stuff only when aided by heat; another acts better and more uniformly when cold, though this is

a rarer case.

When a mordant consists of a changeable metallic oxide, as of iron or tin, unless great nicety be used in its application, either no effect or an injurious one may be produced upon the dye. All these circumstances prove how necessary it is for the dyer to be thoroughly versed in chemical science. Each of the great dye-works in Alsace, celebrated for the beauty and fixity of their colours, is superintended in the laboratory department by a gentleman who has studied chemistry for two or more sessions in the universities of Paris or some other eminent schools. The numerous complaints which for some time back have been made in foreign markets of the fugitiveness of our calico, but especially of our cloth dyes, ought to rivet the attention of our great manufacturers and merchants on this important desideratum, and to lead them to supply it by consulting qualified persons as to the best means of improving this great branch of national industry.

The first principle of dyeing fast colours, we have seen, consists in causing the colouring matter to undergo such a change, when deposited upon the wool or other stuffs, as to become insoluble in the liquor of the dye-bath. The more powerfully it resists the action of other external agents, the more solid or durable is the dye. Generally speaking, a piece of well-dyed cloth should not be materially affected by hot water, by soap and water, by exposure to air and light, by dilute nitric acid, or even by very dilute aqueous chlorine.

In the following details concerning the art of dyeing we shall consider principally its application to wool and silk, having already treated, in the article CALICO PRINTING, Of what is peculiar to cotton and linen.

We have therefore to consider in dyeing the play of affinities between the liquid medium in which the dye is dissolved and the fibrous substance to be dyed. When wool is plunged in a bath containing cochineal, tartar, and salt of tin, it readily assumes a beautiful scarlet hue, but when cotton is subjected to the same bath it receives only a feeble pink tinge. Dufay took a piece of cloth woven of woollen warp and cotton weft, and having exposed it to the fulling-mill in order that both kinds of fibres might receive the same treatment, he then subjected it to the scarlet dye; he found that the woollen threads became of a vivid red, while the cotton continued white. By studying these difparatory to being dyed are intended, 1, to separate certain ferences of affinity, and by varying the preparations and processes, with the same or different dye-stuffs, we may obtain an indefinite variety of colours of variable solidity and depth of shade.

The operations to which wool and silk are subjected pre

foreign matters from the animal fibre; 2, to render it more apt to unite with such colouring particles as the dyer wishes to fix upon it, as also to take there from a more lively and agreeable tint, as well as to be less liable to soil in uso

The matters foreign to the fibre are either such as are natu- pure black. The pyrolignite of iron, which contains a brown rally associated with it during its production by the animal, empyreumatic matter, occasions a brown inclining to greensuch as have been added to it in the spinning and weaving | ish yellow in light shades, and to chesnut brown in dark operations, or such as have been accidentally applied. hues. By galling cotton and silk, after a bath of pyrolignite of iron, and reiterating the processes several times, a tolerably pure black may be procured. Galls, logwood, and a salt of iron (copperas) produce merely a very deep violet blue; but if they be applied to wool in a hot bath, with frequent exposure to air, the logwood induces a brownness which is favourable to the formation of black.

Silk is scoured by means of boiling in soap and water, whereby it is freed from a varnish improperly called gum. This consists of an azotized compound, which may be separated in a gelatinous form by cooling the hot water saturated with it. It constitutes about a fourth part of the weight of most raw silks, and contains a little colouring matter of an orange or yellow hue. When silk is required to be extremely white, either to be woven in that state, or to receive the brightest and purest dyes, it should be exposed to the action of humid sulphurous acid. [SULPHURATION] For dark dyes, silk need not be scoured at all, in which case it preserves its whole weight. Wool is first washed in running water to separate its coarser impurities; it is then deprived of its yolk (a species of animal soap secreted from the skin of the sheep) either by the action of ammoniacal urine, by soap and water, or by a weak lye of carbonate of soda. Common wools lose in this way from 20 to 50 per cent. of their weight, and Merino wools still more. They receive their final bleaching by the fumes of burning sulphur, or by aqueous sulphurous acid.

Wools present remarkable differences in their aptitude of combining with dye-stuffs, which depend upon the different structure of the imbrications of the filaments. (Ure's Philosophy of Manufactures, p. 91.) The colouring particles seem to insinuate themselves at these pores with greater or less facility, and to be retained with greater or less force, according to the magnitude and form of the orifices. This difference in dyeing, therefore, is not due to the repulsive action of fatty matter, as has been commonly supposed, since it still exists in wool even when every particle of grease has been removed from it by alcohol and æther. A bran boil is often had recourse to, in order to make wool take the dye more readily and equally; but a hot lye containing onehalf per cent. of crystallized carbonate of soda answers much better. When heated to the temperature of 140° or 150° Fahr., four parts of wool should be immersed in that liquor, and turned about for half an hour. The wool receives a faint yellowish tint from this bath, but it speedily becomes white on exposure to air, or it may be whitened at once by passing it through tepid water containing a very small quantity of muriatic acid. The yellow colour is most probably occasioned by the reaction of the sulphur and iron contained in

the wool.

According to the experiments of Thenard and Roard, alum combines with wool in the state of a salt, without separation of its acid constituent. Wool boiled with a solution of tartar decomposes a portion of it completely; some of the acid and a little of the tartar combine with the wool, while a neutral tartrate of potash remains in the bath. This fact is interesting in reference to the scarlet dye, showing the important part which tartaric acid here performs.

Tinctorial colours are either simple or compound. The simple are black; brown, or dun; blue; yellow; and red; the compound are gray; purple; green; orange; and other numerous modifications, all producible by the mixture of simple colours. We shall treat here of only black and brown, to supply an omission in the previous part of the Cyclopædia.' The other dyes will be discussed in their alphabetical places.

Black.-If we apply to a white stuff blue, red, and yellow, in certain proportions, the resulting colour will be black. Proceeding on this principle, Father Castel asserted that 15 parts of blue, 5 of red, and 3 of yellow will produce a perfeet black; but in making this statement he was influenced rather by theoretical than practical considerations. In fact he has afforded us no means of procuring these simple colours in an absolute state. It is undoubtedly true, however, that red, yellow, and blue, employed in adequate quantities, will produce black: because they will together absorb, or obstruct the passage of all coloured light, or, in other words, cause its total privation, whence blackness must result. If we suppose a piece of cloth, to which these three colours have been communicated, but not in such proportions as to produce a pure black, we shall have a tint corresponding to the colour that is in excess; as, for example, a blue, violet, red, or greenish black; and with paler tints we shall have bluish, violet, red, or greenish gray.

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Gall-nuts, and a salt of iron, so generally employed for the black dye, give merely a violet or greenish gray, and never a

The black dye for hats is communicated by logwood, copperas, and verdigris mixed in certain proportions in the same bath; from that mixture there results a vast quantity of an ochreous muddy precipitate, amounting to twenty-five per cent. of the copperas employed. This mud forms a deposit upon the hats which not only corrodes the fine beaver filaments, but causes both them and the felt to turn speedily of a rusty brown. A well-dyed black hat should retain its original tint as long as it lasts. There is no process in dyeing so defective as that of hats, or which stands so much in need of scientific amelioration. The hatter tries to wash away this ochreous mud by dilute sulphuric acid, and then counteracts the acid by a weak alkaline bath, thus introducing two adventitious evils as remedies for the first and main evil, which a very little chemical science could obviate.

Since gall-nuts give a blue precipitate with the peroxide salts of iron, they are occasionally replaced by sumach, bablah, &c. ; but account should be taken in this substitution of the proportions of red or yellow colouring matter in these substances, relatively to the tannin which alone forms the blue precipitate. When a black of the best possible shade is to be given, the wool should be first grounded with indigo, then passed through a bath of logwood, sumach, and protosulphate of iron (green copperas). Sumach and nutgalls may also be employed in the proportion of 6 to 24; or the sumach may be replaced by nut-galls, if they be equal to one-third of the sumach prescribed. A good black may be dyed upon an indigo ground with 100 pounds of wool, by taking 200 pounds of logwood, 60 pounds of sumach, 24 pounds of galls, and 20 pounds of green copperas; and giving three heats of two hours each to the wool, with airings between. A good black, without an indigo blue ground, may be given to 100 pounds of wool, by boiling it in a bath of 25 pounds of alum and 674 of tartar; grounding it with weld and madder; then passing it through a bath of 200 pounds of logwood, 60 of sumach, and 24 of galls; taking it out, adding to the bath 20 pounds of copperas; lastly, giving it three heats of two hours each time.

The best French black, according to Hellot, may be given to wool by first dyeing it a dark blue in the indigo vat, washing and fulling it; then for every 50 pounds, putting into the copper 8 pounds of bruised galls, and as much logwood tied up in a coarse canvas bag, and boiling them for twelve hours. One-third of the bath thus prepared is to be transferred into another copper with one pound of verdigris, and the wool or stuff is to be worked in this solution without intermission for two hours, the bath being kept hot, but not boiling. After taking out the stuff, another third part of the first bath is to be added along with four pounds of green copperas; the fire must be lowered while this salt is being dissolved, and the bath being refreshed with a little cold water, the stuff is to be worked through it for half an hour, and then aired. Lastly, the residuary third of the first bath is to be now introduced, taking care to squeeze the contents of the bag. From eight to ten pounds of sumach are added, the liquor is just made to boil, then refreshed with some cold water, after which a pound of green copperas being dissolved in it, the stuff is again passed through it for an hour. It is now taken out, aired, washed, then returned to the copper, and worked in the bath for another hour. It is next washed at the river and fulled. A finish is prescribed in the madder-bath,

The ordinary proportions used by the English black dyers for 100 pounds of cloth, previously treated in the indigo vat, are about 5 pounds of copperas, as much nut-galls bruised, and 30 pounds of logwood. They first gall the cloth, and then pass it through the decoction of logwood in which the copperas has been dissolved. A finish of weld is often given after fulling; but this is of doubtful utility, especially when a little soap has been used in the fulling-mill.

Vitalis prefers the pyrolignite of iron to the sulphate for

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the black dye, and says it produces a softer and more velvety colour. We by no means join in this opinion, having found the pyrolignite apt to communicate a brown tint to the blue black, an effect producible also by using old copperas peroxidized by exposure to air.

The black dye vat, as it gets exhausted, is employed to dye greys of various shades.

Silk is dyed black in two methods, according to the market for which it is made. When sold by weight, as was formerly the practice at Tours, and is now with silk thread in this country, it is an object with the dyer to load it with as much colouring or other matter as possible. Sugar is at present much employed to falsify the weight of English silk thread, as any person may discover by applying a hank of it to his tongue. We have seen thread more than doubled in weight by this fraudulent device. Such silk is called English black by the French, who are not suffered to practise this deception. When silk is sold by superficial measure, on the other hand, it becomes the dyer's object to give it a black colour with as little weight of materials as possible. Hence the distinction well known in the trade of heavy and light silks.

or the stuff dyed red in the madder bath may be turned about in the black dye vat till the required shade be hit.

The finest browns are produced by boiling each pound of the wool with two ounces of alum, dyeing it in a cochineal bath, and then transferring it into a bath containing a little cochineal darkened with acetate of iron. Instead of cochi neal the archil or cudbear bath may be used with a little sumach or galls. This forms a cheaper but a more fugitive colour.

A beautiful brown tint, on wool or silk, may be had by first giving a pale blue shade in the indigo vat, then mordanting with alum, washing and finishing in a madder bath till the proper brown be brought up. The Saxon blue vat may also be used. If the stuff be mordanted with alum and tartar, then boiled in a madder bath, afterwards in one of weld or fustic, to which more or less copperas has been added, we shall have a mordore, cinnamon, or chestnut brown. By the combination of olive shades with red, bronze tints may be produced. For twenty-five pounds of stuff a bath containing four pounds of fustic will suffice. Boil the wood two hours, then turn the stuff in the bath for an hour, take it out, and drain. Add to the bath four or six ounces of copperas and a pound of madder or sandal wood; then work the stuff in it till the wished-for shade is attained.

Silk may receive a ground of annatto, and then be dyed in a bath of logwood or Brazil wood, whereby a fine brown tint is obtained.

The 25 per cent. of weight which silk has lost in scouring may be in a great measure recovered, by giving it a sufficient dose of galls. For this purpose a bath is made by boiling galls equal to two-thirds or three-fourths the weight of the silk for three or four hours in a sufficient quantity of water, and then letting the decoction settle for two hours. The silk must be steeped in this bath from twenty to thirty-six hours, and then washed in the river. The first galling is however commonly given with a bath somewhat spent ; and Blue colours are dyed with indigo, Prussian blue, and for heavy blacks generally upon unscoured silk. Several woad. Yellows with fustic, Persian berries, quercitron, successive immersions in gall-baths, and of considerable turmeric, and weld. Reds, with archil or cudbear, Brazil duration, are usually given to silk, with intervening wash-wood, cochineal, kermes, lac, logwood, madder, safflower, or ings and wringings at the peg.

The silk dyers keep up from year to year a black vat, often of very complex composition. The essential constituents of the vat are sulphate of iron and gum; but many vegetable matters, as well as filings of iron, are usually added. This bath being heated short of boiling, and then allowed to settle for about an hour, the silks are worked in it with much manipulation, occasional wringing out, airing, and re-dipping. As the copperas and gum get exhausted, the bath must be replenished with these ingredients in due proportions. The addition of logwood and verdigris is very useful to the black silk dye, and is now generally made. A ground of walnut peels is a good and cheap preparation for this dye.

We have entered into these theoretical and practical details concerning the black dye, as we conceive them likely to prove useful to our cloth manufacturers, many of whom have hitherto followed too much a blind routine. Every wearer of a black coat or trowsers is soon convinced to his cost that great improvements remain to be made in this department of dyeing.

II. Brown or dun colour.-This dye is not so common in this country as on the continent, where the colouring matter is generally produced at very cheap rate by steeping ripe walnuts with their peels in water for a year or two till the vat acquires a deep brown colour and a fetid smell. This infusion affords very agreeable and permanent brown tints without arv mordant, while it preserves the downy softness of the wool, and requires but a simple and economical process. In dyeing with this infusion, a quantity of it proportional to the shade required is to be put into the copper, diluted with water, and made to boil. The cloth or yarn needs merely to be moistened beforehand with tepid water, to be then plunged in the bath, and turned about till sufficiently dyed. Some dyers, however, give the stuff a preparatory mordant of alum, and leave it to drain for twenty-four hours before subjecting it to the bath of walnutpeels.

Sumach is usually employed in this country to dye fawns, and some browns; but more beautiful browns may be given to woollen stuffs by boiling them first with one-fourth their weight of alum and some tartar and copperas; washing, and afterwards dyeing them in a madder bath. The shade of colour depends upon the proportion which the copperas bears to the alum.

A good brown may also be obtained by mordanting every pound of the stuff with two ounces of alum and one ounce of common salt in a boiling bath; and then dyeing it in a bath of logwood to which some copperas has been added:

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Catechu is used for giving a bronze and brown to cotton goods. [CALICO-PRINTING.]

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The purple, green, and orange dyes may be conveniently considered under the heads of SCARLET DYE, INDIGO, and QUERCITRON.

We shall conclude this article with a few practical remarks. M. Roard, long the skilful director of the Gobelins' dyeworks, has observed that copper boilers exercise a considerable influence upon delicate dyes. He found that ammonia causes a blue precipitate in the alum bath made in such boilers, while it causes merely a white precipitate in the same bath made in vessels of glass, porcelain, and tin. When wool is kept for some hours in boiling-water contained in a copper vessel, it acquires a greenish gray tint; a result increased by the ordinary mixture of alum and tartar. If into this bath white wool be plunged, it receives a greenish yellow, or sometimes an ochrey hue.

These observations of M. Roard are of considerable importance, and should lead dyers to employ tin or at least brass boilers instead of copper ones for all vivid colours. Heating with steam, either by double vessels, by straight or spiral tubes, ought on all occasions to be preferred in the dye-house to naked fires, which seldom fail to carbonize some portions of the vegetable or animal matters, and thereby to degrade the colours. The top edge, or surface of the boilers should be about three feet and a half above the floor; this being a height which the workmen find most convenient for their manipulations when they stand upon a step 8 or 10 inches high.

The stuffs mordanted with alum should not be transferred to the bath immediately, but be allowed to drain and air for 24 hours. The colours are thereby rendered more lively than when dyed soon after the aluming. As experience has proved that an old alum bath is better than one fresh made, it should not be thrown away, but be strengthened or refreshed by the requisite additions of alum and tartar. It is certain that wools boiled in alum the second time, are more beautiful than those boiled in it the first time.

DYER, JOHN, born in 1700, was the second son of a respectable solicitor of Aberglasney in Caermarthenshire. He received his education at Westminster school, and when that was completed, began the study of the law. An early taste for poetry and painting led him to relinquish his legal pursuits, and he travelled about South Wales in the capacity of an itinerant painter. At this period he wrote his poem Grongar Hill,' which was published in 1727. Though he scems to have made but small proficiency in painting, he went to Italy to study, where he wrote the Ruins of Rome,' a descriptive poem, published in 1740. On his return to

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England, having a small independence, he retired into the country, entered into holy orders, and married a lady named Ensor, said to be a descendant of Shakspeare. He died in 1758, shortly after the publication of his longer poem The Fleece,' having gradually improved his fortune.

'The Fleece' is a long unreadable poem, of a purely didactic kind. The middle of the last century was remarkably prolific in poems which took for their model Virgil's Georgics. Dyer's Fleece,' Grainger's 'Sugar-cane,' and Phillips's 'Cyder,' are all of this class. By selecting subjects essentially unpoetical, whatever might be the ingenuity of the writers, they could do no more than make a tolerable poem of a bad kind; for they did not confine themselves to a mere outline of the subject, which they might fill up with what colouring they pleased, but essayed to give, in a poetical form, the intricacies and minutiae of various branches of manufacture. The selection of Virgil's Georgics' for a model was in itself a fallacy, as we question whether this work, with all its beauties, would be much read at the present time were it not for the opportunity which it affords of studying one of the most elegant writers of the Augustan age, and for the light it throws on the agriculture of the antients. The Ruins of Rome,' with here and there a fine line, seldom rises above mediocrity, and is a very heavy performance.

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It is on the poem of his youth, Grongar Hill,' that Dyer's reputation depends. There is, perhaps, no depth of thought, no new idea in this work, but it is a most vivid and brilliant combination of pleasing images. The poet invokes the muse to draw the landskip bright and strong,' and the muse seems to grant his request. We may conceive the poem to be the work of a man walking up-hill, and struck with the succession of scenery which opens all around, he says the first thing that comes into his head; and as he is affected by none but beautiful prospects, what he says is sure to be pleasing. Grongar Hill' will always be a general favourite.

DYKE (in Geology), a fissure caused by the dislocation of strata, commonly also termed a fault. Dykes are of frequent occurrence, and often extend several miles, penetrating generally to an unknown depth. They must have been produced by some violent disturbances, and the amount of dislocation of necessity would vary in proportion to the intensity of the disturbing force. Accordingly there are many dykes of great width and extent, which materially affect the face of the country in which they occur, while there are others so slight that it requires much care and observation to ascertain their existence. The strata are in most cases uplifted on one side of the dyke much higher (varying many fathoms) than those on the other side, and produce an apparent irregularity of strata most perplexing to the geologist. Sometimes it happens that, without any irregularity of surface, two distinct strata appear to form a continuous line, as in the Black Down Hills in Devonshire. [CRETACEOUS GROUP.] In some cases, however, dislocation is found without any alteration of the level of the strata on either side, but the appearance of the strata immediately adjacent to the fault sometimes affords proof of the action of fire. [COAL FIELDS.] Dykes are of two distinct characters, depending upon the manner in which they have been filled up, and the substance of which they are composed. Dykes of the first description are those into which igneous rocks are supposed to have been injected in a state of fusion, and now appear as a consolidated mass. [BASALT.] In the second the fissures are filled with the debris, sometimes mixed with clay, of the dislocated strata through which they pass. In some cases the fissure has evidently remained unoccupied for a long period, and the filling up has proceeded gradually from the sides inwards. This is observed very evidently in the carboniferous limestones of England and Wales. Sometimes, in consequence of the great length of time intervening between the production of each coating of calcareous matter, the outside of each is covered with crystals, upon which the next layer has been formed: in the central portions of such fissures cavities are by no means uncommon.

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Dynamics may be divided into two distinct parts: the mathematical consideration of motion, without reference to any connexion with its cause; and the experimental investigation of the connexion between pressure and the motion produced by it, together with the mathematical exhibition of the laws under which the second is a consequence of the first. The former branch is purely mathematical, and will be further treated under the head MOTION, RELATIVE; the latter will be found, as to its experimental part, under MOTION, LAWS OF; and as to the mathematical part, under FORCES, IMPRESSED and EFFECTIVE, and VIRTUAL VELOCITIES, PRINCIPLE OF. We need not suggest that such articles as FORCE, GRAVITATION, ATTRACTION, PERGUSSION, FRICTION, &c. &c. contain the details of matters connected with the general term dynamics. The history of dynamics is particularly connected with the names of Galileo, Huyghens, Newton, D'Alembert, and Lagrange. See also on this point MECHANICS, the general term under which statics and dynamics are included.

DYNAM'OMETER (measurer of power), a term which has been applied to an instrument which measures any thing to which the name of power has been given, whether that of an animal, or (to take a very different instance) of a telescope. We have also seen the incorrect term dynometer.

DYNO'MENE, a genus of brachyurous crustaceans belonging to the division Notopoda, founded by Latreille.

Character-Ocular pedicles longer than those of Dromia. Shell wide, nearly heart-shaped and truncated posteriorly, hairy or bearded. Two posterior feet only dorsal, and much smaller than the others.

Example, Dynomene hispida, the only species known to M. Latreille. Locality, Isle of France.

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Dynomene hispida.

DYRRA'CHIUM. [DURAZZO.]

DY'SENTERY (AvGevrepia, Dysenteria, from dis, with difficulty, and Evrepov, intestine; difficultas intestinorum, bloody flux), a disease in which there is difficulty and pain in passing the stools, which consist of mucus and blood, containing little or no fæculent matter, and generally attended with fever. The desire to evacuate the bowels is frequent and urgent; but the effort is accompanied with severe pain, and is often altogether ineffectual, constituting the affection called tenesmus. What scanty stools are passed consist, as has been stated, of mucus mixed with blood, or of pure blood in considerable quantity; and if any fæculent matter be present, it is commonly in the form of round and hard balls called scybalæ. There is always griping pain in the abdomen. More or less fever is invariably present. The seat of the disease is chiefly in the large intestines: the disease itself consists essentially of inflammation of the mucous membrane.

The forms of this disease, the causes which produce it, the circumstances under which it prevails, the pathological conditions on which its essential characters depend, and its degrees of intensity, are infinitely various; and these modifying influences cause it to assume at different seasons, in different climates, and in different constitutions, the most diversified aspects. It is sometimes a primary, sometimes a consecutive, and sometimes a symptomatic disease. It is now sporadic, now endemic, and occasionally both endemic and epidemic. It is sometimes inflammatory and sthenic, at other times typhoid and asthenic, at one time acute, and at another chronic. These differences are attended with essential differences in the nature of the disease, which not only communicate to it different external aspects, dependent on different internal conditions, but which require totally different remedies.

In the acute form of dysentery, when purely inflammatory, and when mild in character, constipation commould The liquid precedes for some days the attack of diarrhoea. and frequent stools which at length succeed to this state

of constipation soon become streaked with blood; the griping pains which accompany the evacuations, and the straining and tenesmus which follow them, are often attended with distinct chills. The stools may be from eight or ten to sixteen or twenty in the twenty-four hours. The pulse is commonly quick and small, the tongue loaded, and the appetite little impaired.

When the attack is more severe, it is generally attended at the very commencement with diarrhoea, often accompanied with nausea and vomiting, quickly succeeded by scanty, mucous, or gelatinous stools, streaked with blood, preceded by tormina, and followed by tenesmus. The pain in the course of the large intestines may be either severe, or it may not be urgent, but rather a sense of heat and aching than acute pain. Pain, however, is always induced by full pressure over the tract of the colon; and if, in any particular part of this tract, there be urgent pain, some degree of fulness may generally be perceived there. The progress of the disease is indicated by the increasing severity of all the symptoms, and more especially by the increasing frequency of the stools, by the increasing tormina and tenesinus, and the augmentation of the general febrile symptoms. It is not uncommon for from twenty to forty efforts at stool to be made in the twenty-four hours, with the effect of passing only a very small quantity of mucus and blood. In all cases the evacuations are exceedingly offensive; in the worst they are of a cadaverous odour, and the clots of blood are sometimes mixed with pieces of coagulated lymph or fibrin.

In hot climates the disease is still more intense. The heat, the tormina, and the tenesmus, are more urgent and distressing; the thirst becomes excessive, the urine scanty or altogether suppressed, the stools slimy, streaked with blood, and attended with prolapsus ani, or watery and ichorous, resembling the washings of raw beef, in which float particles or even large shreds of coagulable lymph, thrown off from the acutely-inflamed surface.' In these cases the prostration of strength is extreme, and is increased by most distressing and exhausting vomiting. When, as sometimes happens in this form of the disease, portions of the mucous coat of the intestine slough away, the countenance of the patient is sunk and cadaverous, and the odour of the stools, and in some degree, indeed, of the whole body, is putrid.

In the asthenic form of dysentery, the tormina, tenesmus, and mucous and bloody stools are attended with great depression of all the organic functions, and extreme prostration of strength. The local dysenteric symptoms, exceedingly urgent from the commencement, are rapidly followed by fever of a low nervous or typhoid type. This form of the disease often prevails as an epidemic; and under circumstances favourable to their accumulation and concentration, exhalations from the stools of the sick seem capable of producing dysentery in persons directly exposed to them, previously in a state of sound health. These forms of the disease are very apt to occur in hot seasons and in hot cli mates, where great numbers of persons are collected together in close and ill-ventilated apartments, in damp and unhealthy situations, as in barracks, garrisons, camps, crowded ships, &c. It is this form of dysentery which rages among the poor in seasons of scarcity, which sometimes destroys whole armies in countries laid waste by war, and which so constantly, in besieged towns, anticipates the havoc of the sword.

The duration of dysentery is as various as its types. It may prove fatal in a few days or hours, or last for weeks and even months, and ultimately destroy life by inflammation and gangrene of the bowels. In some cases the disease ceases spontaneously, the frequency of the stools, the griping and the tenesmus gradually diminishing, while natural stools return; but in other cases the disease with moderate symptoms continues long, and ends in protracted and exhausting diarrhœa.

The causes which predispose to dysentery appear to be long-continued exposure to a high temperature, or alternations of heat and cold; hence the disease is generally most prevalent in summer or autumn, after considerable heats have prevailed for some time, and especially after very warm and at the same time very dry states of the weather. It is certainly more frequent, as well as much more severe, in hot than in cold or even in temperate climates. All observation and experience show that a powerful predisposition to the disease is formed by the habitual use of a high and stimulating diet, and especially by indulgence in spi

rituous liquors, by excessive fatigue; and by all causes which enfeeble the constitution in general, at the same time that they over excite the alimentary canal in particular. The exciting causes are long-continued exposure to intense heat, or to sudden and great alternations from heat to cold; exhalations from vegetable and animal matters in a state of decomposition, as from marsh, stagnant, river or sea water, from animalcula and minute insects, or from the flesh of deceased animals; noxious exhalations from the bodies of persons crowded together in close and confined situations, and more especially, as would appear, from the discharges from the bowels of persons labouring under dysentery; scanty and bad food, consisting more especially of vegetable or animal matter in a state of decay, as tainted meat, stale fish, unwholesome bread, unripe rice, rye, &c. The inflammatory affection of the mucous membrane of the large intestine in which dysentery essentially consists, passes, in the severe forms of the disease, into ulceration and even gangrene. On the examination of the large intestine in fatal cases after death, there is often found effusion of coagulable lymph, ulcers of various forms, and patches more or less extensive of mortification. In the most malignant varieties the internal surface of the whole alimentary canal is of a livid, purple, or dark colour, with patches of excoriation, ulceration, and gangrene.

In the acute form of dysentery, when the fever is high, the pain intense, and the inflammation active, blood-letting from the arm is indispensable, which must be repeated to the subdual of the acute inflammatory symptoms. After a moderate general blood-letting, however, the local abstraction of blood by leeching or cupping is more efficacious; the number and the repetition of the leeches must of course depend on the urgency of the pain and the strength of the patient. The employment of purgative remedies in dysentery requires the greatest discrimination and caution. If the colon be distended with feculent matter which it cannot discharge, no remedies will succeed until this accumulation is removed; if, on the contrary, there have been already frequent and copious discharges of feculent matter, the administration of purgatives is absurd, for all purgatives are irritants, and the diseased membrane is already in a state of intense excitement. The practitioner should therefore carefully examine the state of the bowels with regard to their fullness or emptiness of fæcal matter, and their actual state in this respect can almost always be ascertained with a great degree of certainty if due pains be taken to discover it. If there be reason to suppose that there is any accumulation of faces, the mildest purgatives should be given, of which the best is castor oil, and this should be cautiously repeated until the irritating matter is wholly removed. Great relief is at the same time afforded to the distressing tormina and tenesmus by emollient and opiate enemas injected in very small quantities. After the subdual of the inflammatory state by blood-letting, and the evacution of the accumulated fæces by mild purgatives, the great object is to soothe the irritated membrane by opiates, on the judicious employment of which, and the skilful combination and alternation of this class of remedies with mild purgatives, the successful treatment of ordinary dysentery mainly depends. The acute forms of dysentery in hot climates require a prompt and decided combination of remedies, the best selection and administration of which it is impossible to discuss here. The asthenic forms with typhoid symptoms need a guarded yet active treatment, nearly the same as that which is proper to typhus fever with abdominal affection. [FEVER.]

DYSPEPSIA (Avoñevía, dyspepsia), Indigestion, the difficult and imperfect conversion of the food into nutriment. Digestion is a part of the great function of nutrition; its ultimate object is to convert the aliment into blood. Between the articles taken as food and the nutrient fluid of the body-the blood, there is no obvious analogy, and there is a wide difference in nature. Hence the function of digestion consists of a succession of stages, at each of which the food undergoes a specific change. Each change is effected by a peculiar process, for the accomplishment of which a special apparatus is provided. Of these processes the chief are mastication, deglutition, chymification, chylification, and fæcation. The delicacy and complexity of the apparatus by which each of these processes is carried on has been already shown. [DIGESTION.] The healthy condition and the natural action of every individual organ belonging to the portion of the

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