which, floating on the partially chymified mass, becomes rancid and occasions distressing heart burn 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 assimilated 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 atmo sphere, 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 diarrhoea 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

DIETRICH

to its use, but apt to disagree with strangers. In tropical countries rice to a great extent takes the place of the other cereals, and perhaps a larger population mainly subsist on it than on any other single article of food. It affords very little of plastic or blood-making material, and hence when taken alone is consumed in enormous quantity; as an adjunct it forms an unstimulating and digestible article of food. The leguminous seeds, peas and beans, afford a nutriment rich in plastic matter, but hard of digestion and predisposing to flatulence. Sugar is used chiefly as an addition to other articles of diet; when refined, it contains no plastic matter, and is simply a heat-producing aliment, in general abundantly wholesome; the popular prejudice that it produces caries of the teeth has no good foundation. Closely allied to sugar are the various forms of fecula, arrow root, tapioca, sago, potato starch, &c. They consist of minute granules enclosed in a membranous envelope; this membrane must be burst by heat or purification before the starch is digestible. It is then an unstimulating food, entirely respiratory in its character, it containing little or no plastic matter. Contrary to general opinion, young infants digest starch with difficulty, and when fed largely upon it, pass it unchanged by stool. Vegetables constitute an important part of our diet. With few exceptions their nutritive value is low; they consist largely of water holding organic salts in solution, of starch granules, of small quantities of albuminous matter, and of cellulose and epidermis. The cellulose, though possessing a chemical constitution identical with that of starch, when at all firm, resists the action of the gastric juice, and passes unchanged through the intestinal canal. They are valuable on account of their large quantities of organic salts, of the bulk which they give to the food, and of their stimulating effect upon the peristaltic action of the intestines. These latter qualities make them disagree where the digestive organs are feeble and irritable. They are digestible in proportion to their tenderness and the readiness with which they can be broken up into a pulp. The potato has about the same nutritive value as rice; it requires to be thoroughly masticated, and is therefore an unsuitable article for young children. St. Martin found potatoes roasted and baked disposed of more readily than when boiled, the one taking 2 hours and 30 minutes to be converted into chyme, the other an hour longer. The same rule applies to fruits as to vegetables; they are digestible just in proportion to the readiness with which they can be completely reduced to a pulp. Ripe strawberries, peaches, oranges, grapes, rarely disagree, while cherries, apples, pears, &c., are more indigestible; roasting improves the digestibility of apples by rupturing the cells in which their juices are imprisoned.

DIETRICH, CHRISTIAN WILHELM ERNST, also called DIETRICY, a German painter and engraver, born in Weimar, Oct. 30, 1712, died in Dresden'

April 24, 1774, excelled principally in the imitation of the great masters, especially Rembrandt, though he copied with great success the styles of other eminent painters.

DIFFERENTIAL CALCULUS, the science called by the English fluxions, is the most valuable of mathematical modes, from the great variety of subjects to which it is applicable, and from the strength of its solvent power. Its discovery is justly assigned to the latter part of the 17th century, although there were doubtless some hints of it among earlier writers. Archimedes had demonstrated the area of a parabola to be of its circumscribing rectangle, and also the truth of his celebrated propositions concerning the sphere and the cylinder. Kepler, seizing the spirit of his method, introduced the words infinite and infinitesimal into geometry. Cavalieri, Roberval, and Fermat enlarged the application of his mode. In the meanwhile Vieta, Cardan, Harriot, and others had improved algebra, and Descartes had applied it to geometry by his invaluable system of variable coordinates. Thus the way was prepared for Leibnitz and Newton, who, independently of each other, invented the differential calculus, although differing in the form in which they conceived of and expressed the same truths. Newton's discovery or invention was made in 1665, and that of Leibnitz several years later. The notation of the latter was so convenient, and his mode of attacking the subject had such a practical superiority for the learner, that Newton's method of fluxions has now gone completely out of use; although in a metaphysical point of view Newton's mode is not open to the objections which may be brought against that of Leibnitz. The discovery of this method originated in the investigation of curved lines, but is extended to the consideration of every species of magnitude. Newton conceived of a curved line as generated by the motion of a point; and the spirit of his method consists in determining the velocity with which the point, at each instant, is moving in a given direction different from that of the line; that is, e. g., if the point be moving in a general southwesterly direction, in determining the velocity with which it souths compared with that with which it wests. The spirit of Leibnitz's method consists in supposing the curve to be composed of infinitely short straight lines, and in determining the direction of each of these little straight arcs. What Newton called the inverse method of fluxions is now called the integral calculus. It consists in finding from the ratio of infinitesimal changes the magnitude and law of connection of the changing quantities. The whole calculus is too difficult and abstruse for any popular exposition. The reader may find general views upon the subject in Davies's "Logic of Mathemathics," and Comte's "Philosophy of Mathematics," translated by Prof. Gillespie, or in French in Carnot's Reflexions. For gaining a practical acquaintance with the science there are numerous accessible treatises, among which

Church's and Courtenay's are well adapted to ordinary students, but Peirce's conducts much more rapidly into the highest walks. Of English treatises, Price's holds the highest rank. The French have been prolific writers upon the subject; among them Duhamel perhaps holds as high a rank as any.

DIFFRACTION OF LIGHT, the deviation from a straight line which a ray of light undergoes in passing near the edge of an opaque body. In whatever way light be transmitted, the luminous influence may be regarded as propagated in the manner of a succession of hollow spheres, or shells, that spring forth from the surface of the luminary and enlarge with almost inconceivable rapidity on all sides of it through space. In the undulatory theory of light, each of these shells is considered to be a wave, or we may say, a wave-front, advancing in the form of a spherical surface, as ripples about an agitated point upon a pond of water spread outward in concentric circles. But in a homogeneous medium, the line of effect, or that in which the agitation is propagated outward from the centre of disturbance, is a straight line; and thus we say that light advances in rays, and that in a uniform medium these are straight. To this law, however, one important general exception has been found. Grimaldi, an Italian Jesuit, about the middle of the 17th century, observed that when through 2 small orifices near together 2 pencils of the sun's light-diverging, of course, in consequence of the size of the sun's disk-were admitted to fall on a screen at several feet distance in an otherwise dark room, the overlapping parts of the 2 disks of light thus obtained were brightly illuminated, while on either side of this central bright band there were alternating curved bands of less and greater illumination and showing the prismatic colors. The effect is still better seen when the pencils are made more divergent by being each brought by a convex lens to form a minute focus, beyond which the rays must again separate. These bands are known as "Grimaldi's fringes." If 2 narrow slits in the shutter are employed, the result is a bright band running longitudinally through the middle of the space occupied by their light on the screen, with alternating fringes on the 2 sides. So, if in the centre of a single divergent beam a small opaque body be held, the actual complete shadow of it on the screen is less in size than the geometrical shadow; but it is surrounded by alternating light and dark bands to a distance which again causes the shadow in part to encroach on the surrounding space. The same result, in a degree, really happens with a single small pencil; and in fact, all shadows are in this way to some extent encroached on by surrounding light, and all edges of light by shadows. Here, then, is a set of cases in which the rays of light deviate from straight lines; and it may be stated that, generally, rays of light grazing upon the edges of orifices or of bodies are bent more or less out of a straight line, being turned apparently both

within and without their previous direction. This action is the diffraction of light. Newton attempted to explain this action of the edges of bodies in accordance with the theory of emission, by supposing that the edges exerted some influence of attraction or repulsion, according to the condition in which the imagined luminous particles met them. But it was found that when the light employed in these experiments was monochromatic, as red only, or yellow, the bands produced in any case were simply light and dark, i. e., of the given color and absolutely black. And Dr. Young discovered in 1803 that in order to obliterate all the special fringes obtained in the case of 2 orifices, it was only necessary to cover up one of them; portions of the spot obtained from the other which were before crossed by dark bands immediately became light. It thus became evident that light can be added to light in such a way as to produce darkness. In water waves, a crest and a trough of equal depth, that is, 2 equal waves in opposite phases, coming together, neutralize each other, and give still water over the space thus occupied; and 2 sound waves may also so blend as to produce silence. Fresnel in 1815-'16 read before the French academy of sciences the results of his investigations of this set of phenomena, which he, as well as Dr. Young before him, judged could not be explained by the theory of emission, but which he found perfectly in harmony with consequences flowing from Huyghens's undulatory theory of light. By varying the material and shape of the orifices, he found no effect whatever upon the appearance of the fringes, except that when razor-edges were employed the rays were bent about these more than about rounded edges, an effect which has been termed inflection of the light. But he wholly disproved the Newtonian view, by throwing a diverging pencil from the focus of a lens on 2 mirrors slightly inclined to each other, so as to make the reflected rays cross in their course: here were no edges; yet, when the 2 sets of rays were received on a screen, the light and dark bands were perfectly formed; and by covering one mirror, the bands disappeared, the other giving light only. This phenomenon then, in all its forms, is due to interference, and, according to the undulatory theory, that of 2 waves or sets of waves, so managed, in the case of the mirrors, that they shall intersect each other at points along their course; where, in homogeneous light, crests conspire with crests, or troughs with troughs, producing increased brightness, but where crest and trough combine at the same point, producing rest of the vibra ting medium, that is, darkness. In compound or solar light, however, the effect of the interference is to separate the ray into its elementary colors. In the case of rays grazing the edges of orifices or bodies, the points at which the rays thus touch become points of origin of new agitations or waves, which spread out from these points as centres beyond the body, and by so doing intersect each other and produce light

DIFFRACTION OF LIGHT

and dark bands. Mathematically, it is easily proved that those surfaces of intersection along which crests will conspire to give increased light, and also those along which crests and troughs will combine to give darkness, must form along the middle line one continued plane surface, and on both sides of this, receding hyperboloid surfaces; and experiment, as in placing the screen successively at various distances, marks out exactly these curves about a middle bright band, as those actually formed. The bands thus formed are broadest in the least refrangible (red) rays, and narrowest and most crowded in the most refrangible (violet) light. The accurate measurement with a micrometer of the distances of the successive bands from the central line, together with the other known distances in the case, becomes a ready means of determining the wave lengths of the different colored rays composing white light; and it is by observing that when either of 2 pencils forming them is retarded, the fringes must shift to that side, and finding that when one of the pencils passes through a thin film of mica, or a tube of water, the fringes do actually move to the side occupied by this pencil, that it has lately been proved, in different ways severally by Arago, Foucault, and Fizeau, that light moves less rapidly in the denser of 2 media, a fact which has given to the emission theory of light its final overthrow. As consequences of this view of the production of the fringes, it follows also that the centre of the shadow of a small opaque body held in a diverging pencil of light should be a minute bright spot, while the centre of the light of the pencil without the opaque body should be a small dark spot; both these results are found to hold true. By varying the shape of the orifice, the form of the dark or light space will be changed. Shadows, as formed, do not correspond accurately with the geometrical shadows of the bodies projecting them; but in the case of large bodies or apertures the fringes are less sensible. In order to witness the effect of diffraction by a simple experiment, make a smooth pin-hole in a piece of card paper, or a clean cut down into one of its sides by looking through this, in a room otherwise dark, at a minute crevice admitting light by the shutter or door, or at the flame of a candle, either of these will present numerous light and dark bands, the candle flame being multiplied apparently into a number of flames, lessening out on either side, and showing the prismatic colors. Bring a bright star or the light of a lamp at a distance just over the edge of an intervening body, as the hand or a bar in the shutter, and a good eye will detect that in a position just preceding that of the disappearance of the light it is decomposed, showing the prismatic colors, the red and green very distinctly. Many cases of diffraction occur in nature. Among these are the colored fringes seen by looking in certain directions at or along the course of fine fibres of any kind, as the spider's web, fine wires, and the fibres upon black fab

rics, when illuminated by the sun; the fringes sometimes bordering the shadows of such bodies; the colors seen by looking through a fine dew or mist between 2 plates of glass, or upon a mirror on which lycopodium has been dusted, held in the sun; the changeable colors of the plumage of birds, and those of mother-of-pearl and other grooved or striated surfaces, the origin of the colors in the latter cases being proved by taking casts of such surfaces in black wax, which immediately become iridescent, like the natural objects, and by grooving metallic surfaces with 5,000 to 10,000 lines to the inch, as in Barton's iris buttons, in which the same result appears.

DIFFUSION OF GASES, a term applied by Priestley (who first observed the phenomenon, and published an account of it in the 4th volume of the "Transactions of the American Philosophical Society") to the property possessed by gaseous bodies of intermingling with each other, whatever may be their differences of specific gravity, or whatever their repugnance to enter into chemical combinations. Priestley found the new force so strong that the gases would in time penetrate animal membrane that separated them and that was regarded air-tight, and be found constituting similar mixtures on each side of it. To this principle he correctly attributed the uniformity of the composition of the atmosphere. Dalton, who afterward investigated the subject, explained the phenomenon on the assumption that the particles of one gas are highly repulsive to each other, but do not repel those of another gas. So, when a jar of hydrogen is inverted over another filled with carbonic acid, the light gas finds its way between the particles of the heavy gas, and this works upward into the other, till they are at last equally diffused. Thus he supposed one gas to act as a vacuum to another, with which it does not enter into chemical combination; with this difference, however, that the particles of one present a mechanical impediment to the diffusion, so that a longer time is required for it to take place. This explanation accounts also for the uniform diffusion of vapors through gases and through each other. Prof. Graham of Glasgow made some further interesting investigations as to the relative rate of diffusion of different gases. Gas contained in a glass jar slightly cracked was found to escape into the air, and the air at the same time to pass through and mingle with the gas, and the relative quantities that passed each way were found to depend upon the comparative densities of the two elastic fluids; the lightest gases passing through most rapidly, the rate of diffusion being inversely as the square root of the density of the gas. This law would seem to confirm the hypothesis that gases act as vacuums to one another; for it is found that the velocities of gases flowing into a vacuum maintain the same ratio, being inversely as the square root of the densities of the gases.

DIGAMMA (double gamma), so called from its form (F) resembling 2 gammas (r), the 6th

letter in the ancient alphabet of the Greeks, corresponding to the Hebrew and the Latin f, and probably equivalent in sound to the English w. It continued latest in the Eolic dialect, but early became obsolete in the Attic alphabet, and subsequently in the Greek language; though its original existence is indicated by the fact that the 5th letter (e) is the numerical symbol for 5, but the next letter () for 7. It does not appear in the Homeric poems, though they were composed when it was in use; but its force remained in the metre after its form had disappeared, and its latent existence at the beginning of many words and syllables apparently commencing with a vowel made preceding short syllables, if ending with a consonant, long by position, or, if ending with a vowel, prevented a hiatus. In passing into the Latin language it was written v, thus: έσπερος (FΕΣΠΕΡΟΣ), vesperus; wov (QFON), ovum.

DIGBY, a S. W. co. of Nova Scotia, bordering on the Atlantic; pop. in 1851, 12,252. It has a highly diversified surface, and comprises within its limits several small lakes, which give rise to numerous rivers. The underlying rock is sandstone of various colors. Copper and silver mines have been worked with some profit. In the N. W. part is a deep and narrow bay of the Atlantic called St. Mary's bay, enclosed on the N. by Brial's island and a narrow headland known as Digby neck. Capital, Digby.

DIGBY, SIR KENELM, an English philosopher and chemist, born in Gothurst, Buckinghamshire, in 1603, died in London in 1665. He was the son of Sir Everard Digby, who was executed for complicity in the gunpowder plot, when the subject of this sketch was about 3 years old. He was educated in the Protestant faith, and showed early tokens of remarkable talent. In 1621, having finished his education at Oxford, he visited the continent, where he travelled for about 2 years. On his return he was made gentleman of the bedchamber by Charles I., and received other marks of the royal favor. In 1628 he sailed with a squadron fitted out at his own expense, to fight the Algerines and the Venetians, with whom the English had quarrelled, and gained much credit by his courage and success on this expedition. In 1636, while in France, he became a convert to the Roman Catholic religion; and, having afterward returned to England, and taken part with the king in the civil war, was imprisoned by order of parliament. During his confinement he employed himself with literary labors, was released in 1643 in consequence of the intercession of the queen of France, and retired to that country, where he was received with great honor, and enjoyed the friendship of Descartes and other eminent Frenchmen. From this time till 1661 he lived mostly on the continent, and especially in France, employing himself with literary and scientific labors. Having returned to England, he enjoyed the favor of Charles II., and continued his philosophical studies until his death. His principal

works are: "A Conference with a Lady about the choice of a Religion;" "Observations on Religio Medici;" a "Treatise on the Nature of Bodies;" a "Treatise on the Soul, proving its Immortality;" a "Treatise of adhering to God;" "Of the Cure of Wounds by the Powder of Sympathy;" "Private Memoirs of Sir Kenelm Digby, &c., written by Himself," first published in 1827.

DIGESTION, a function peculiar to the animal kingdom, by which organic alimentary substances, introduced into the stomach and intestines, are converted into the nutritive fluid, chyle, and mixed indirectly with the blood, the excrementitious and useless matters being rejected and cast out of the body. The organs by which this function is performed in the higher animals are the mouth, pharynx, oesophagus, stomach, and intestines, with their accessory salivary glands, pancreas, liver, and mucous follicles. The first act to which food is subjected is the mechanical division by the teeth; so important is this in order that it may be influenced by the salivary secretion, that it may be said as an axiom that "food well chewed is half digested." As a people the Americans are singularly guilty of life-long and constant infraction of this rule, paying, however, the penalty of dyspepsia with its numerous train of evils and premature decay. The action of the gastric juice and of the pancreatic and biliary secretions has been described in the articles CHYME and CHYLE. While some of the nutritive matters are dissolved in and absorbed directly from the stomach, others require further preparation, and are taken up by the vessels and absorbents of the intestines; by the time that the residue arrives in the cæcum, almost all the alimentary matter has been extracted, and the insoluble portions with the excess of biliary and mucous secretions are voided at the anal termination of the canal. The digestive process, upon the proper performance of which the health of all the organs must depend, can hardly be separated from absorption, which takes up the nutritive materials, and assimilation, which converts them into a fluid resembling blood, poured into the circulation near the heart. Though inorganic substances are necessary for the support of the body, the organic alone are generally considered as food and as subjects for the digestive process. Organic substances used as food may be conveniently arranged under 4 heads: 1, the saccharine group, embracing substances composed of oxygen, bydrogen, and carbon, resembling sugar in composition, and readily convertible into it; such are starch, gum, woody fibre, and the cellulose of plants; 2, the oleaginous group, with a great preponderance of hydrogen and carbon, small proportion of oxygen, and absence of nitrogen, including vegetable oils and animal fats; 3, the albuminous group, containing a large proportion of nitrogen, comprising animal and vegetable substances allied in chemical composition to albumen and animal tissues; 4, the gelatinous group, including animal substances closely allied