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which, floating on the partially chymified mass, becomes rancid and occasions distressing heartburn and nausea, or causes eructations of acrid matter which leave a peculiarly disagreeable taste upon the palate. The mode of dressing meat has a great influence upon its digestibility; that which agrees best with the majority of stomachs is broiling. The fire should be brisk, so that the albumen on the surface of the meat may be rapidly coagulated; this preserves the juices of the meat, and it is rendered at once more savory and more tender. The same rule applies to boiling and roasting. When the meat is to be cooked, if boiled, it should be at once plunged into boiling water; the coagulation of the albumen on the surface thus produced, protects the interior from loss; while if soup is to be made, the meat should be put into cold water and the temperature slowly and graduly raised, thus extracting its nutritious fluids to the greatest possible extent. Of all methods of cooking, frying is the most objectionable; not only is the meat rendered harder than when boiled, and thus more indigestible, but it becomes imbued with boiling fat, and is thus rendered still more refractory to the gastric juice. Rich stews are objectionable on the same account; the fat set free by the heat penetrates and is absorbed by the meat, and renders it liable to offend delicate stomachs. By the action of salt on muscular flesh, the juices of the meat are abstracted; in this manner not only is its nutritive value impaired (see ALIMENT), but it is rendered harder and drier and consequently more indigestible; the longer the flesh is exposed to the action of salt, the harder and drier it becomes. Perhaps all fats form an exception to the fact that meat is rendered more indigestible by salting; they have little water to lose, and their texture cannot consequently become consolidated; fat pork is even rendered more digestible by salting. St. Martin, according to Dr. Beaumont's observations, digested recently salted pork when raw or broiled in from 3 hours to 3 hours and 15 minutes; the same article fried occupied him 4 hours 15 minutes for its reduction; while fresh pork, fat and lean, roast ed, required 5 hours 15 minutes. On the other hand, boiled fresh beef with a little salt was digested in 2 hours 45 minutes, while old salted beef required 4 hours 15 minutes when dressed in the same manner. All empyreumatic substances impair digestion by interfering with the action of the animal matter, the pepsin, which is the principal solvent agent of the gastric juice. In this manner smoking impairs the digestibility of meat; few things are more difficult of management by a feeble stomach than old and well-smoked beef. Of poultry, the turkey is most digestible. St. Martin found fowls, roasted or boiled, of slower digestion than beef; ducks and geese, as might be supposed from the amount of fat they contain, are assim ilated with difficulty. Fish furnishes an abundant and digestible variety of food. The dry, white sorts, cod, haddock, bass, &c., are the

most digestible; while the richer kinds, salmon, shad, mackerel, eels, &c., are less apt to agree with the stomach. St. Martin digested boiled or fried salmon trout in 13 hours, boiled dried cod in 2 hours, fried catfish in 3 hours 20 minutes, and boiled pickled salmon in 4 hours. Milk, the only food during the earlier months of infancy, contains from 12 to 13 per cent. of solid matter, about of what is contained in flesh; it is poorer in plastic and richer in respiratory food; its ash furnishes but 0.47 per cent. of iron, while those of flesh and wheat flour yield 1 per cent. It is not digested so quickly as would be supposed, and in this respect boiled has the advantage of unboiled milk; the one took St. Martin 2 hours, the other 21, to convert into chyme. Milk disagrees with a great many persons; this is often connected with the readiness with which it undergoes change when exposed to the atmosphere, and this change commences long before it can be recognized by the taste. Milk just drawn from the cow agrees perfectly with many persons who are unable to take it when a few hours old. When cows are kept in an impure and confined atmosphere, it has been conclusively shown that their milk produces disturb ance of the digestive organs and diarrhea in infants who are fed upon it, and there is good reason to believe that constitutional diseases, scrofula and phthisis, may be thus developed. The caseine of milk, coagulated, generally mixed with more or less butter, and pressed so as to free it from the whey, constitutes cheese. Its richness varies with the quantity of butter it contains; some varieties, Stilton for instance, are made from milk to which an additional quantity of cream has been added. Salt is used to preserve it, and some kinds, as Dutch cheese, are very highly salted. When cheese is kept for a length of time, it undergoes a number of changes, partly dependent on the liberation of the volatile fatty acids existing in the butter, partly in the richer varieties on the commencement of putrefactive fermentation. The firm, close texture of cheese renders it always hard of digestion, and the rich and strong-smelling varieties are particularly to be avoided by delicate stomachs. Fresh sweet butter is, perhaps, the most wholesome and digestible of fatty matters; by heating or rancidity its digestibility is greatly impaired.-Of farinaceous articles, light well-made wheaten bread, from 12 to 24 hours old, is the most generally digestible; warm bread is indigestible, because it forms a tough mass not readily penetrated by the saliva and rebellious to the gastric juices. Unleavened bread, maccaroni, and vermicelli are wholesome, and agree well with the stomach; on the other hand, flour combined with fatty matter, whether in the form of pastry, cake, or pudding, is more or less indigestible, according to its texture and richness. Next to wheat flour, rye affords the best and most wholesome bread. In various countries oatmeal, barley, and maize are used as substitutes for wheat; they form kinds of bread wholesome enough for those habituated




to its use, but apt to disagree with strangers. April 24, 1774, excelled principally in the imitaIn tropical countries rice to a great extent tion of the great masters, especially Rembrandt, takes the place of the other cereals, and per- though he copied with great success the styles haps a larger population mainly subsist on it of other eminent painters. than on any other single article of food. It af- DIFFERENTIAL CALCULUS, the science fords very little of plastic or blood-making ma- called by the English fluxions, is the most terial, and hence when taken alone is consumed valuable of mathematical modes, from the great in enorinous quantity; as an adjunct it forms variety of subjects to which it is applicable, and an unstimulating and digestible article of food. from the strength of its solvent power. Its The leguminous seeds, peas and beans, afford a discovery is justly assigned to the latter part of nutriment rich in plastic matter, but hard of di- the 17th century, although there were doubtless gestion and predisposing to flatulence. Sugar is some hints of it among earlier writers. Archiused chiefly as an addition to other articles of medes had demonstrated the area of a parabola diet; when refined, it contains no plastic matter, to be of its circumscribing rectangle, and and is simply a heat-producing aliment, in gen- also the truth of his celebrated propositions eral abundantly wholesome; the popular preju- concerning the sphere and the cylinder. Kepdice that it produces caries of the teeth has no ler, seizing the spirit of his method, introduced good foundation. Closely allied to sugar are the words infinite and infinitesimal into gethe various forms of fecula, arrow root, tapioca, ometry. Cavalieri, Roberval, and Fermat ensago, potato starch, &c. They consist of mi- larged the application of his mode. In the nute granules enclosed in a membranous enve- meanwhile Vieta, Cardan, Harriot, and others lope ; this membrane must be burst by heat or had improved algebra, and Descartes had appurification before the starch is digestible. It plied it to geometry by his invaluable system of is then an unstimulating food, entirely respira- variable coördinates. Thus the way was pretory in its character, it containing little or no pared for Leibnitz and Newton, who, independplastic matter. Contrary to general opinion, ently of each other, invented the differential young infants digest starch with difficulty, and calculus, although differing in the form in when fed largely upon it, pass it unchanged by which they conceived of and expressed the same stool. Vegetables constitute an important part truths. Newton's discovery or invention was of our diet. With few exceptions their nutri- made in 1665, and that of Leibnitz several years tive value is low; they consist largely of water later. The notation of the latter was so conholding organic salts in solution, of starch gran- venient, and his mode of attacking the subject ules, of small quantities of albuminous matter, had such a practical superiority for the learner, and of cellulose and epidermis. The cellulose, that Newton's method of fluxions has now gone though possessing a chemical constitution iden- completely out of use; although in a metaphystical with that of starch, when at all firm, re- ical point of view Newton's mode is not open sists the action of the gastric juice, and passes to the objections which may be brought against unchanged through the intestinal canal. "They that of Leibnitz. The discovery of this method are valuable on account of their large quantities originated in the investigation of curved lines, of organic salts, of the bulk which they give to but is extended to the consideration of every the food, and of their stimulating effect upon species of magnitude. Newton conceived of a the peristaltic action of the intestines. These curved line as generated by the motion of a latter qualities make them disagree where the point; and the spirit of his method consists in digestive organs are feeble and irritable. Thoy determining the velocity with which the point, are digestible in proportion to their tenderness at each instant, is moving in a given direction and the readiness with which they can be bro- different from that of the line; that is, e. g., if ken up into a pulp. The potato has about the the point be moving in a general southwesterly same nutritive value as rice; it requires to be direction, in determining the velocity with thoroughly masticated, and is therefore an un- which it souths compared with that with which suitable article for young children. St. Martin it wests. The spirit of Leibnitz's method confound potatoes roasted and baked disposed of sists in supposing the curve to be composed of more readily than when boiled, the one taking infinitely short straight lines, and in determining 2 hours and 30 minutes to be converted into the direction of each of these little straight chyme, the other an hour longer. The same What Newton called the inverse method rule applies to fruits as to vegetables; they are of fluxions is now called the integral calculus. digestible just in proportion to the readiness It consists in finding from the ratio of infiniteswith which they can be completely reduced to imal changes the magnitude and law of conneca pulp. Ripe strawberries, peaches, oranges, tion of the changing quantities. The whole calgrapes, rarely disagree, while cherries, apples, culus is too difficult

and abstruse for any popular pears, &c., are more indigestible; roasting exposition. The reader may find general views improves the digestibility of apples by rup- upon the subject in Davies's “ Logic of Mathetaring the cells in which their juices are im. mathics," and Comte's "Philosophy of Matheprisoned.

matics,” translated by Prof. Gillespie, or in DIETRICH, CHRISTIAN WILIELM Ernst, also French in Carnot's Reflexions. For gaining a called DIETRICY, a German painter and engraver, practical acquaintance with the science there born in Weimar, Oct. 30, 1712, died in Dresden are numerous accessible treatises, among which


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Church's and Courtenay's are well adapted to within and without their previous direction. ordinary students, but Peirce's conducts much This action is the diffraction of light. Newton more rapidly into the highest walks. Of English attempted to explain this action of the edges of treatises, Price's holds the highest rank. The bodies in accordance with the theory of emis. French have been prolific writers upon the sub- sion, by supposing that the edges exerted some ject; among them Duhamel perhaps holds as influence of attraction or repulsion, according high a rank as any.

to the condition in which the imagined lamiDIFFRACTION OF LIGHT, the deviation nous particles met them. But it was found from a straight line which a ray of light under- that when the light employed in these experigoes in passing near the edge of an opaque body. ments was monochromatic, as red only, or yel. In whatever way light be transmitted, the lumi- low, the bands produced in any case were sim. nous influence may be regarded as propagated ply light and dark, i. e., of the given color and in the manner of a succession of hollow spheres

, absolutely black. And Dr. Young discovered or shells, that spring forth from the surface of in 1803 that in order to obliterate all the special the luminary and enlarge with almost incon- fringes obtained in the case of 2 orifices, it was ceivable rapidity on all sides of it through space. only necessary to cover up one of them; portions In the undulatory theory of light, each of these of the spot obtained from the other which were shells is considered to be a wave, or we may before crossed by dark bands immediately besay, a wave-front, advancing in the form of a came light. It thus became evident that light spherical surface, as ripples about an agitated can be added to light in such a way as to produce point upon a pond of water spread outward in darkness. In water waves, a crest and a trough concentric circles. But in a homogeneous me- of equal depth, that is, 2 equal waves in opposite dium, the line of effect, or that in which the phases, coming together, neutralize each other, agitation is propagated outward from the cen- and give still water over the space thus ocectre of disturbance, is a straight line; and thus pied; and 2 sound waves may also so blend as we say that light advances in rays, and that in to produce_silence. Fresnel in 1815–'16 read a uniform medium these are straight. To this before the French academy of sciences the relaw, however, one important general exception sults of his investigations of this set of phenohas been found. Grimaldi, an Italian Jesuit, mena, which he, as well as Dr. Young before about the middle of the 17th century, observed him, judged could not be explained by the that when through 2 small orifices near to- theory of emission, but which he found perfectgether 2 pencils of the sun's light-diverging, ly in harmony with consequences flowing from of course, in consequence of the size of the sun's Huyghens's undulatory theory of light. By disk—were admitted to fall on a screen at sev- varying the material and shape of the orifices, eral feet distance in an otherwise dark room, he found no effect whatever upon the appear. the overlapping parts of the 2 disks of light thus ance of the fringes, except that when razor-edges obtained were brightly illuminated, while on were employed the rays were bent about these either side of this central bright band there inore than about rounded edges, an effect which were alternating curved bands of less and great- has been termed inflection of the light. Bat er illumination and showing the prismatic colors. he wholly disproved the Newtonian view, by The effect is still better seen when the pencils throwing a diverging pencil from the focus of a are made more divergent by being each brought lens on 2 mirrors slightly inclined to each other, by a convex lens to form a minute focus, beyond so as to make the reflected rays cross in their which the rays must again separate. These course: here were no edges; yet, when the 9 bands are known as “Grimaldi's fringes.” If sets of rays were received on a screen, the light 2 narrow slits in the shutter are employed, the and dark bands were perfectly formed; and by result is a bright band running longitudinally covering one mirror, the bands disappeared, the through the middle of the space occupied by other giving light only. This phenomenon then, their light on the screen, with alternating in all its forms, is due to interference, and, acfringes on the 2 sides. So, if in the centre of & cording to the undulatory theory, that of 9 single divergent beam a small opaque body be waves or sets of waves, so managed, in the case held, the actual complete shadow of it on the of the mirrors, that they shall intersect each screen is less in size than the geometrical sha- other at points along their course; where, in dow; but it is surrounded by alternating light homogeneous light, crests conspire with crests

, and dark bands to a distance which again causes or troughs with troughs, producing increased the shadow in part to encroach on the surround- brightness, but where crest and trough combine ing space. The same result, in a degree, really at the same point, producing rest of the vibrahappens with a single small pencil; and in fact, ting medium, that is, darkness. In compound all shadows are in this way to some extent en- or solar light, however, the effect of the intercroached on by surrounding light, and all edges ference is to separate the ray into its elementof light by shadows. Here, then, is a set of ary colors. In the case of rays grazing the cases in which the rays of light deviate from edges of orifices or bodies, the points at which straight lines; and it may be stated that, gener- tho rays thus touch become points of origin of ally, rays of light grazing upon the edges of new agitations or waves, which spread out from orifices or of bodies are bent more or less out these points as centres beyond the body, and by of a straight line, being turned apparently both so doing intersect each other and produce light DIFFRACTION OF LIGHT



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and dark bands. Mathematically, it is easily rics, when illuminated by the sun; the fringes proved that those surfaces of intersection along sometimes bordering the shadows of such bodies; which crests will conspire to give increased the colors seen by looking through a fine dew or light, and also those along which crests and mist between 2 plates of glass, or upon a mirtroughs will combine to give darkness, must ror on which lycopodium has been dusted, held form along the middle line one continued plane in the sun; the changeable colors of the plusurface, and on both sides of this, receding hy. mage of birds, and those of mother-of-pearl and perboloid surfaces; and experiment, as in plac- other grooved or striated surfaces, the origin of ing the screen successively at various distances, the colors in the latter cases being proved by marks out exactly these curves about a middle taking casts of such surfaces in black wax, bright band, as those actually formed. The which immediately become iridescent, like the bands thus formed are broadest in the least re- natural objects, and by grooving metallic surfrangible (red) rays, and narrowest and most faces with 5,000 to 10,000 lines to the inch, as crowded in the most refrangible (violet) light. in Barton's iris buttons, in which the same re. The accurate measurement with a micrometer sult appears. of the distances of the successive bands from DIFFUSION OF GASES, a term applied by the central line, together with the other known Priestley (who first observed the phenomenon, distances in the case, becomes a ready means of and published an account of it in the 4th voldetermining the wave lengths of the different ume of the “Transactions of the American colored rays composing white light; and it is Philosophical Society") to the property possessby observing that when either of 2 pencils form- ed by gaseous bodies of intermingling with each ing them is retarded, the fringes must shift to other, whatever may be their differences of that side, and finding that when one of the pen- specific gravity, or whatever their repugnance cils passes through a thin film of mica, or a tube to enter into chemical combinations. Priestley of water, the fringes do actually move to the found the new force so strong that the gases side occupied by this pencil, that it has lately would in time penetrate animal membrane that been proved, in different ways severally by separated them and that was regarded air-tight, Arago, Foucault, and Fizeau, that light moves and be found constituting similar mixtures on less rapidly in the denser of 2 media, a fact each side of it. To this principle he correctly which has given to the emission theory of light attributed the uniformity of the composition of its final overthrow. As consequences of this the atmosphere. Dalton, who afterward invesview of the production of the fringes, it follows tigated the subject, explained the phenomenon also that the centre of the shadow of a small on the assumption that the particles of one gas opaque body held in a diverging pencil of light are highly repulsive to each other, but do not should be a minute bright spot, while the cen- repel those of another gas. So, when a jar of tre of the light of the pencil without the opaque hydrogen is inverted over another filled with body should be a small dark spot; both these carbonic acid, the light gas finds its way beresults are found to hold true. By varying the tween the particles of the heavy gas, and this shape of the orifice, the form of the dark or works upward into the other, till they are at last light space will be changed. Shadows, as equally diffused. Thus he supposed one gas formed, do not correspond accurately with the to act as a vacuum to another, with which it geometrical shadows of the bodies projecting does not enter into chemical combination; with them; but in the case of large bodies or aper- this difference, however, that the particles of tures the fringes are less sensible. In order to one present a mechanical impediment to the difwitness the effect of diffraction by a simple ex- fusion, so that a longer time is required for it to periment, make a smooth pin-hole in a piece take place. This explanation accounts also for of card paper, or a clean cut down into one the uniform diffusion of vapors through gases and of its sides : by looking through this, in a room through each other. Prof. Graham of Glasgow otherwise dark, at a minute crevice admitting made some further interesting investigations as light by the shutter or door, or at the flame of to the relative rate of diffusion of different gases. a candle, either of these will present numerous Gas contained in a glass jar slightly cracked was light and dark bands, the candle flame being mul- found to escape into the air, and the air at the tiplied apparently into a number of flames, less- same time to pass through and mingle with the ening out on either side, and showing the pris- gas, and the relative quantities that passed each matic colors. Bring a bright star or the light way were found to depend upon the comparative of a lamp at a distance just over the edge of an densities of the two elastic fluids; the lightest intervening body, as the hand or a bar in the gases passing through most rapidly, the rate of shutter, and a good eye will detect that in a diffusion being inversely as the square root of position just preceding that of the disappear- the density of the gas. This law would seem ance of the light it is decomposed, showing the to confirm the hypothesis that gases act as vaprismatic colors, the red and green very dis- cuums to one another; for it is found that the tinctly. Many cases of diffraction occur in na- velocities of gases flowing into a vacuum mainture. Among these are the colored fringes seen tain the same ratio, being inversely as the square by looking in certain directions at or along the root of the densities of the gases. course of fine fibres of any kind, as the spider's DIGAMMA (double gamma), so called from web, fine wires, and the tibres upon black fab- its form (F) resembling 2 gammas (r), the 6th letter in the ancient alphabet of the Greeks, works are: “A Conference with a Lady about corresponding to the Hebrew 7 and the Latin

the choice of a Religion;" “ Observations on f, and probably equivalent in sound to the Eng- Religio Medici;" a "Treatise on the Nature of lish x. It continued latest in the Æolic dialect, Bodies;" a "Treatise on the Soul, proving its but early became obsolete in the Attic alphabet, Immortality;"1" Treatise of adhering to God;" and subsequently in the Greek language; though“Of the Cure of Wounds by the Powder of its original existence is indicated by the fact Sympathy;" “ Private Memoirs of Sir Kenelm that the 5th letter (e) is the numerical symbol Digby, &c., written by Himself,” first published for 5, but the next letter (S) for 7. It does in 1827. not appear in the Homeric poems, though they DIGESTION, a function peculiar to the aniwere composed when it was in use; but its force mal kingdom, by which organic alimentary remained in the metre after its form had disap- substances, introduced into the stomach and peared, and its latent existence at the beginning intestines, are converted into the nutritive fluid, of many words and syllables apparently com- chyle, and mixed indirectly with the blood, the mencing with a vowel made preceding short excrementitious and useless matters being resyllables, if ending with a consonant, long by jected and cast out of the body. The organs by position, or, if ending with a vowel, prevented which this function is performed in the higher a hiatus. In passing into the Latin language animals are the mouth, pharynx, csophagus, stoit was written o, thus: cotepos (FESNEPOE), mach, and intestines, with their accessory salivesperus ; wov (OFON), ovum.

vary glands, pancreas, liver, and mucous follicles. DIGBY, a S. W. co. of Nova Scotia, border- The first act to which food is subjected is the ing on the Atlantic; pop. in 1851, 12,252. It mechanical division by the teeth; so important has a highly diversified surface, and comprises is this in order that it may be influenced by the within its limits several small lakes, which give salivary secretion, that it may be said as an rise to numerous rivers. The underlying rock axiom that “food well chewed is half digested." is sandstone of various colors. Copper and sil- As a people the Americans are singularly guilty ver mines have been worked with some profit. of life-long and constant infraction of this rule, In the N. W. part is a deep and narrow bay of paying, however, the penalty of dyspepsia with the Atlantic called St. Mary's bay, enclosed on its numerous train of evils and premature dethe N. by Brial's island and a narrow headland cay. The action of the gastric juice and of the known as Digby neck. Capital, Digby. pancreatic and biliary secretions has been de.

DIGBY, Sir KENELM, an English philoso- scribed in the articles Chyme and CHYLE. While pher and chemist, born in Gothurst, Bucking- some of the nutritive matters are dissolved in hamshire, in 1603, died in London in 1665. He and absorbed directly from the stomach, others was the son of Sir Everard Digby, who was ex- require further preparation, and are taken up by ecuted for complicity in the gunpowder plot, the vessels and absorbents of the intestines; by when the subject of this sketch was about 3 the time that the residue arrives in the cæcum, years old. He was educated in the Protestant almost all the alimentary matter has been exfaith, and showed early tokens of remarkable tracted, and the insoluble portions with the talent. In 1621, having finished his education excess of biliary and mucous secretions are voidat Oxford, he visited the continent, where he ed at the anal termination of the canal. The travelled 'for about 2 years. On his return digestive process, upon the proper performance he was made gentleman of the bedchamber by of which the health of all the organs must deCharles I., and received other marks of the pend, can hardly be separated from absorption, royal favor. In 1628 he sailed with a squad- which takes up the nutritive materials, and ron fitted out at his own expense, to fight the assimilation, which converts them into a fluid Algerines and the Venetians, with whom the resembling blood, poured into the circulation English had quarrelled, and gained much cre- near the heart. Though inorganic substances dit by his courage and success on this expedi- are necessary for the support of the body, the tion. In 1636, while in France, he became organic alone are generally considered as food a convert to the Roman Catholic religion; and as subjects for the digestive process. Organic and, having afterward returned to England, substances used as food may be conveniently arand taken part with the king in the civil war, ranged under 4 heads: 1, the saccharine group, was imprisoned by order of parliament. Dur- embracing substances composed of oxygen, bying his confinement he employed himself with drogen, and carbon, resembling sugar in compoliterary labors, was released in 1643 in con- sition, and readily convertible into it; such are sequence of the intercession of the queen of starch, gum, woody fibre, and the cellulose of France, and retired to that country, where he plants; 2, the oleaginous group, with a great was received with great honor, and enjoyed the preponderance of hydrogen and carbon, small friendship of Descartes and other eminent proportion of oxygen, and absence of nitrogen, Frenchmen. From this time till 1661 he lived including vegetable oils and animal fats; 3, the mostly on the continent, and especially in France, albuminous group, containing a large proportion employing himself with literary and scientific of nitrogen, comprising animal and vegetable labors. Having returned to England, he enjoyed substances allied in chemical composition to the favor of Charles II., and continued his philo- albumen and animal tissues; 4, the gelatinous sophical studies until his death. His principal group, including animal substances closely allied

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