out the use of carbohydrates. The Eskimo and the North American Indian live entirely on the produce of the chase. The flesh of animals is rich in proteids and fats, but contains practically no carbohydrate. In the same way the Arab of the desert lives upon the flesh and milk of the camel, in districts where the date is not to be found. The use of carbohydrates is extensive in most temperate and tropical climates. Many of the inhabitants of India and Ceylon live chiefly on rice. Wheat, potatoes, and other vegetable foods, rich in carbohydrates, are staple articles of food both in Europe and America. In fact, within certain limits, man, like every other animal, is capable of adapting himself to the food produced by the district in which he lives. The French and Spanish peasants eat little meat, living on carbohydrates and vegetable proteids, and supplying necessary potential by an abundance of oil. The Sussex labourer consumes his beans, rich in vegetable proteids, and bacon, and does a good day's work on them. The Scotch labourer of a former generation lived on porridge and skimmed milk, with meat but once a week. The Arab of the Sahara, together with his family and his horse, subsist almost entirely on the date, while the Arab of the Nubian Desert hardly ever touches vegetable food. The diet of every man, under whatever circumstances, invariably contains proteids, although this may vary in its quantity. The universal use of proteid matter in some form or other suggests its importance, and this is fully borne out by the evidence of direct experiment. If proteid food be withdrawn from the diet altogether, nitrogenous matter continues to be eliminated from the body by the urine, although its supply is cut off. At the same time the body wastes, and death finally occurs. The experiments of Fick, Wislicenus, and Parkes show that during muscular exertion the body does not excrete any excess of nitrogen; and the writer's own researches show that urea-the most important excretion which contains nitrogen-is not found in abnormal extent in muscular tissue after exhaustive exercise. It would appear, therefore, that nitrogenous matter is not used by the tissues for fuel, and that when proteid food is consumed, the muscles burn a non-nitrogenous substance which is formed from it. The nitrogen, in the form of urea, &c., is carried by the blood to the kidneys, and at once excreted. We must look elsewhere than in the ordinary combustion of a muscle or brain cell for a use for the nitrogen which, as we have seen, is so necessary an article of food in the In the child, the form of a proteid substance. tissues which contain nitrogen in definite proportion are continually growing, and it will readily be seen that here, at anyrate, we have an important use for proteid diet-namely, to furnish the necessary nitrogen for the new tissue. Even during adult life we have tissue change and growth. The skin is continually growing in its deeper layers to replace its surface layers worn away by constant friction. In the same way the cells of the blood are always being broken down, and their places taken again by new ones continually forming. Although it is hardly probable that such complete changes take place in all the tissues of the body, yet, without the entire breaking down of individual cells, there are no doubt changes both integrating and disintegrating, which are constantly taking place. As a result of disintegrating changes nitrogenous matter in an effete form is being given off by the system, and it has to be replenished from without in a form which the body can use. The proteid food-stuff provides nitrogenous pabulum for the tissues which they assimilate, and build up new tissue material. For this reason proteid matter is indispensable. At the same time the system obtains

from the proteid food non-nitrogenous fuel to be burned by the tissues; this property of supplying tissue fuel is shared with the carbohydrate and fatty food-stuffs. It has been found by experiment that an ordinary diet should contain one part of nitrogenous matter (proteids) to about four of nonnitrogenous diet (fat and carbohydrates). If less nitrogenous matter be given, the tissue consump tion of nitrogen will not be supplied, and the body will waste. If a much larger quantity of nitrogenous food be taken, a nitrogenous surfeit occurs, and the body is called upon to digest this excess, and to eliminate an unnecessary quantity of the useless nitrogenous compounds which result.

We have seen then that an ordinary diet must contain sufficient potential, and in addition one part of proteid to four of non-proteid material should be present. The following table from Dr Parkes shows the proper proportions of solid waterfree food-stuffs in ounces required as daily food by an adult man :

Taking the diet table for a man performing ordinary work, we have 46 of proteid, and 17'4, or It nearly four times as much non-proteid matter. contains sufficient potential, for if the number of foot-tons which result from the combustion of an ounce of proteid, of fat, and of a carbohydrate be multiplied by the number of ounces of these substances given in the ordinary diet, it will be seen to amount to 4000. This, as we have already seen, indicates a sufficient potentiality.

It now becomes an easy matter to construct a diet table of articles of food. Most of these have been analysed, and the amount of water, proteid, carbohydrate, or fat in each calculated.

TABLE FOR CALCULATING DIETS (Parkes).

Many articles of food owe their chief importance to their action as stimulants. Such are alcohol, tea, coffee, and beef-tea. Alcohol is no doubt burned within the body, and is a source of energy. Only a small quantity, however, can be so utilised, perhaps one ounce in every twenty-four hours in the case of an ordinary individual. As a source of energy, therefore, it is of little value, and such substances as fat, meat, bread, are capable of supplying the

potential at a very much smaller cost. Alcohol taken with other food stimulates the secretion of gastric juice, assisting in that way the digestive process. At the same time, however, the gastric juice is unable to act quite so readily upon the food, though this is hardly the case with diluted spirits, which form more wholesome beverages than wines or beer. No two individuals are the same, and while alcohol, in moderate doses, promotes digestion in most persons, others suffer from its use. (See the article on the action of alcohol, Vol. I. p. 135). It should never be given in collapse and weakness without giving at the same time easily digested food when that is possible. It excites the body to great and often unnecessary activity, the potential for which it does not supply. Afterwards greater weakness ensues from want of the necessary fuel, which has not in the meanwhile been forthcoming. One should never drink without eating. Tea and coffee are both nervous stimulants, and at the same time they retard both gastric and salivary digestion.

Beef-tea is generally regarded as a food-stuff of high nutritive value. This is, however, not the case. It contains nothing besides salts and extractives, and has a very slight potential indeed. It has a stimulating effect, however, both on digestion and on the nervous system. From the mistaken ideas generally held as to the nutritive properties of this substance, thousands of invalids are annually starved to death (see BEEF-TEA). Beeftea made by infusing the beef in tepid water is more nutritious, especially if the beef be finely minced and eaten as well.

In addition to the stimulants that we have already considered are many substances known as condiments, such as mustard, pepper, pickles, sauces. These are of utility in gratifying the palate, and in addition they probably stimulate the secretory juices. Sufficient information has not as yet been obtained as to their action on gastric and pancreatic digestion. They certainly stimulate the flow of saliva, although the acid condiments will prevent the perfect action of the salivary ferment.

Common salt is a condiment, and at the same time it plays many other important parts in the animal organism. So necessary is it that both man and animals suffer great hardships if it be not supplied in sufficient quantity. It is necessary for the formation of the gastric juice; it is present in the blood and in all tissues of the body.

Inorganic salts, such as sulphates, phosphates, are required for the formation of the skeleton, and salts of iron for the colouring matter of the blood. Organic salts, such as citrates and tartrates, are also of importance. That food should be easily digested is a matter of great importance. The rapidity of digestion will depend upon the amount and quality of the digestive juices, the kind of food, and the condition in which the food is eaten. Rice, tripe, whipped eggs, sago, tapioca, barley, milk, raw eggs, lamb, parsnips, potatoes, hashes of chicken, fish, are all easily digested substances. Beef, mutton, pork, roast fowls, bread, veal, oysters, are digested more slowly. Inasmuch as perfect digestion can only be accomplished when the digestive juices have acted for some time on all parts of the food taken, it follows that fine subdivision of food is very essential. On this account liquid food rapidly disappears from the stomach, which retains for a longer time solid masses upon which the gastric juice acts with greater difficulty, When food is cooked it swells, its fibres and solid particles are separated one from another, and it is more readily permeated by the gastric and pancreatic juices. In addition, starch becomes gelat inous, and in that form is easily digested by the

809

saliva and pancreatic juice. The diet of early infantile life differs from that of the adult, inasmuch as it should contain no starch. Milk, the natural food of the infant, is rich in fat, proteids, and a sugar called milk-sugar. There is in milk, however, an entire absence of starch, an article of food which the infant is destined to consume in such large quantities in later years. Moreover, during the first few months of its extra-uterine existence the child is unable to assimilate starch given with its food. On no account, therefore, must it be supplied with bread, potatoes, rice, or other vegetable food until the cutting of the teeth suggests a more solid diet. If the mother's milk be not in sufficient plenty, cow's milk diluted with one-third of water, with a pinch of sugar, may be given, or condensed milk diluted with twelve to twenty parts of water (see MILK). Condensed milk, owing probably to its very uniform composition, cannot be given alone for more than a few weeks together. It may, how ever, be given once or twice a day for months, and children thrive on it, when they in addition are supplied with good cow's, or still better, with their own mother's milk.

In conclusion, it may be well to consider the results which follow the neglect of the most obvious rules of dietetics. As the result of deficient food, one finds loss of muscular and nervous power, wasting of the tissues, and anæmia. If the deficiency be very great, feverish symptoms and great prostration result.

There

Many persons consume large quantities of food quite out of proportion to their size or activity. In this case, owing to a 'habit of digestion,' much of the food may pass through the digestive tract without being digested or assimilated. Under these circumstances the hearty eater is a wasteful eater, and is using for his own bodily needs only a fraction of the food he consumes. In addition, dyspepsia in various forms, constipation, and diarrhoea are apt to follow, indicating functional derangements of the digestive apparatus. is often a tendency, especially in advanced years, to absorb more nourishment than is necessary for stoking the body and for replenishing ordinary tissue waste. The excess is stored up in the form of fat, which accumulates under the skin, chiefly under that of the abdomen. In addition, the muscles and internal organs are loaded with fat, the minute globules of which may be seen in the ultimate cells of which the tissues are composed. Hereditary tendency is well marked in cases of corpulency. In addition to corpulency, an excess of food is apt to engender various gastric troubles, engorgement of the liver, plethora, and an excess of effete extractions in the blood and urine. It is probable that humanity suffers more from an excess than from a deficiency of food. An excess of animal is more serious than an excess of vegetable food. The nitrogenous extractions, derived from the incomplete assimilation of meat, when present in large quantity, cause many symptoms, most of which are extremely obscure in their nature. These are provisionally spoken of as symptoms of gout. The ill effects which follow surfeit are more severe in those leading a sedentary and inactive life, bodily activity producing more efficient oxidation of the food taken. A healthy and abstemious man whose tastes have not been enslaved by the culinary art instinctively adapts his food to the requirements of his body. The cold of winter prompts the choice of substantial and energising food, while the heat of summer suggests a lighter dietary. After a country holiday, on returning to a sedentary life one at once reduces one's allowance of beef, or expects to pay the penalty that a disordered digestion is certain to exact. During the ages in which

humanity has been evolving, there has been a constant adaptation of taste and desire to the needs of the economy. The natural gustatory inclinations as a rule are a good indication of the bodily wants. As a rule wholesome things have a pleasant taste, and the reverse also holds good. It is all-important, however, that the satisfaction of mere gustatory pleasure be not allowed to monopolise too much of the energy of any individual. Under these circumstances a surfeit is certain to result. There is a well-known law in physiology to the effect that greater and greater stimuli have to be applied in order to produce a series of equal sensations. It follows that the excesses of the glutton and drunkard are out of all proportion to the actual pleasures these excesses produce, the wise man drinking and eating only in moderation.

See the articles on COOKERY, FOOD, DIGESTION, INDIGESTION; also Pavy, Food and Dietetics (1874); Sir H. Thompson, Food and Feeding (1880); Sir W. Roberts, Lectures on Dietetics and Dyspepsia (1885); Fothergill, Manual of Dietetics (1886).

Diet (Lat. dies, day'), a meeting of delegates or of dignitaries, held from day to day, for legislative or ecclesiastical purposes; the title was afterwards extended to such bodies themselves. The term is applied to the sessions of church assemblies in Scotland, but its chief use is as the specific title of the administrative assemblies of the German empire and some other continental states (see GERMANY).

The

Desertion of the Diet, in Scots law. The proceedings under a criminal libel are in Scotland spoken of technically as a diet, and when the libel is abandoned by the public prosecutor, or where he fails to appear, he is said to desert the diet. effect of a judgment of the court declaring that the diet has been deserted, is to free the accused from prosecution under the particular libel or writ, but not to prevent a new process being raised on the same grounds.

Dietrich of Bern. See THEODORIC.

Diez, FRIEDRICH CHRISTIAN, the greatest of Romance philologists, was born at Giessen, 15th March 1794, and educated at Giessen and Göttingen, with one short interval in 1813 of campaign. ing as a volunteer. In April 1818 he saw Goethe at Jena, and was directed by the sage to the lectures of Raynouard and the study of the Provençal tongue. From 1822 he lived at Bonn as a privat-docent, and in 1830 was there appointed professor of the Romance Languages, and there he died, May 29, 1876. His first work, Altspan. Romanzen (1821), was followed by a series of valuable works on the Romance languages, of which the greatest are his Grammatik der Romanischen Sprachen (3 vols. 1836-38; 4th ed. 1877), and the Etymologisches Wörterbuch der Romanischen Sprachen (2 vols. 1853; 4th ed. by A. Scheler, 1878; Eng. trans. 1864). These works discussed these languages for the first time from the comparative historical standpoint, and thus formed a sound foundation for all subsequent Romance philology. See the books on Diez, his life and work, by Sachs (1878), Breymann (1878), and Stengel (1883).

Difference, CALCULUS OF FINITE DIFFERENCES. Difference implies two quantities of the same kind, and means in arithmetic that quantity

which must be added to the smaller in order to

DIFFERENCE

added to 7 to produce 5? and the answer to this is 2.

In certain groups of problems, chiefly relating to series, differences considered in a particular manner are of peculiar importance, constituting in fact a branch of higher algebra, which took its origin in Brook Taylor's Methodus Incrementorum (1715), and is now called the Method of Differences or the Calculus of Finite Differences. This method we shall briefly illustrate.

Suppose it were required to discover the law of formation, and thence to continue the series of numbers:

We

4, 3, 0, 1, 12, 39, 88, 165. It would be wrong to assume that only one law of formation will produce these eight numbers, just as it would be wrong to assume that only one curve could be drawn through eight given points; but for the full discussion of the difficulty here raised the reader must be referred to the chapter on Interpolation in any text-book on the subject. shall, however, show how to find one law of formatary notation of the subject. The process is to tion, and use our figures to illustrate the elementake the difference between each term and the succeeding one, and so get the first series of differences, or, as it is called, the series of first differences; the process is repeated on the first differences, and so on, as follows: No. of term, 1 2 3 4 5 6 7 8 Given Series, U1 Из Из Из Ив Ug Ив 4 3 0 1 12 39 88 165 Au Au2 Aug Aus Aus Aug Au -1 -3 1 11 27 49 77

1st Differences,

2d Differences,

3d Differences,

дачи диз диз двид диз двив -2 4 10 16 22 28 дзи двиг двиз дзив двиг 6 6 6 6 6 The line of third differences suggests a law of formation, and enables us to continue the series as follows:

Aux Ax

As a final example let us suppose ux = x2. Then we have, using equation (1) above,

Aux = (x + 1)2 - x2

= 2x + 1

This is a case of the direct problem of the calculus, but there is also the inverse problem: Of what

function is 2x + 1 the difference? The solution to

this is denoted by the symbols:

(2x + 1) = x2,

DIFFERENTIATION

Between the Calculus of Finite Differences and the Differential Calculus (see CALCULUS) (a title which means the calculus of infinitesimal differences) there are many important points of contrast and of similarity, which would be not less clearly appreciated if the names were changed, as Boole all but suggested, the former to Calculus of Differences, the latter to Calculus of Limits.

The methods of the Calculus of Differences are in vogue among actuaries and others in dealing with statistics such as mortality tables; and from this calculus are derived many formulæ of approximation of great practical value, such as the rules for finding the area of surfaces bounded by curved lines.

Differentiation, that organic progress which occurs when certain parts of a uniform whole become structurally different from the others, or when, in other words, the homogeneous becomes heterogeneous. Inequality in internal and external conditions of life brings about restriction of certain vital processes and the predominance of others, and as this division of function is established, diversity of structure results. Differentiation is the structural change which is associated with the physiological division of labour,' and the process is essentially the same whether it find expression in cells, tissues, organs, or entire organisms. See DIVISION OF LABOUR, EVOLUTION, VĂRIATION, &c. Diffraction. In general, light is propagated in straight lines in a homogeneous medium; but, if it be caused to pass through an opening which is not large in comparison with the wave-length of the light, the law no longer holds. Such phenomena are said to be due to diffraction. The subject will be found treated at greater length under LIGHT.

Diffusion. The particles of all material bodies, except such as may be totally devoid of heat, are in rapid motion. In the case of solid bodies the excursions of any one particle are limited to a small space; but in fluids a particle may move more or less freely throughout the whole space occupied. This intermixture of molecules may occur also when different fluids are placed in contact with each other, but it may be prevented by the existence of tension at the common surface (see SURFACE-TENSION and CAPILLARITY). When it does occur, the fluids are said to diffuse into each other.

Diffusion of Liquids. The diffusion of dissolved salts may obviously be considered under this heading. The phenomenon may be conveniently studied by introducing a strong solution of some highly-coloured salt, such as bichromate of potash, into the bottom of a tall glass cylinder nearly filled with water. The rate of diffusion varies with the nature of the liquids. Graham was the first to investigate the subject carefully. He filled a number of similar glass vessels with solutions of different salts. The mouths of these vessels were carefully ground so that they could be closed by means of glass plates. The different vessels were then placed in equal glass jars, and covered with water to a definite extent. Next the glass covers were cautiously withdrawn, and the diffusion was allowed to go on for a certain time. The rate at which each liquid diffused was thus obtained. Graham found that, for any one solution, the rate is proportional to the gradient of concentrationi.e. to the rate at which the quantity of salt dissolved per unit-volume varies per unit-length. Thus the law regulating diffusion of liquids is analogous to that which regulates the conduction of heat in a homogeneous solid. Hence the equations obtained by Fourier in his Théorie de la Chaleur apply to the problem under consideration.

Graham found also that rise of temperature

greatly increases the effect. He divided substances into two classes, Colloids and Crystalloids, the members of the first class diffusing very much more slowly than those of the second. His investigations have been much extended by more recent observers employing various methods of observation.

If two miscible liquids be separated by a membrane of bladder or of parchment paper, &c., diffusion takes place through the septum at rates which are usually very different for different liquids. This phenomenon is known as Osmose (q.v.). It was first shown by Nollet that, if a vessel filled with alcohol be closed by a piece of bladder and placed in water, the diffusion of the water is so much more rapid than that of the alcohol that the bladder is burst because of the increase of the contents of the vessel which it closes. By this means the various constituents of a mixture of colloid and crystalloid substances may be separated to any desired extent. The rate at which liquids diffuse into each other through a septum depends greatly upon the molecular action between them and the septum.

Diffusion of Gases.-If two flasks, each filled with a different gas at a given pressure and each other, the gases will be found to interdiffuse. temperature, be placed in communication with The rate of interdiffusion is shown by theory to be nearly in inverse proportion to the square root of the product of the densities of the two gases, and the experimental results are in accordance with the theory.

Effusion of Gases.-This is exhibited in the passage of a gas into vacuum under constant pressure through a small opening in a very thin plate otherwise impervious to it. The work done in the passage of a given volume of the gas is proportional to the pressure, and the equivalent kinetic energy is proportional to the product of the density and the square of the speed of effusion. Hence the speed for a given pressure varies inversely as the square root of the density. Graham showed that this result of theory is closely realised by experiment. He showed, further, that when a discrepancy exists, it is due to the finite thickness of the plate.

Transpiration of gases is the term to the passage of gases under pressure through a fine capillary tube. This subject was also investigated by Graham, who found that the rate of passage is not affected by the material of the tube. This seems to indicate that the tube becomes coated internally with a thin film of gas, so that the opposition to the flow of gas is due to Viscosity (q.v.).

The rates at which different gases pass through fine unglazed earthenware are inversely as the square roots of their densities. Hence we have a means of separating gases the densities of which are different (see ATMOLYSIS). If the septum be made of caoutchouc, which is not porous, the passage of gases still occurs. The gas seems to combine with the matter of the septum on the one side, to diffuse through it, and finally to be given off on the other side. The passage of some gases, such as carbonic oxide, through hot cast-iron is analogous.

Digamma is the name given by the grammarians of the 1st century to vau, the sixth letter of the primitive Greek alphabet, which had become obsolete, and was only known to them from inscriptions. The name was given owing to a fancied resemblance of its form to a double gamma. Its sound was something like that of our w. It is found in Peloponnesian inscriptions as late as the 6th century B.C., but it had disappeared from the alphabet of Attica before the date of the oldest inscriptions-i.e. before the

middle of the 7th century B.C., although, as Bentley has proved, it must have been in use at the time when most of the Homeric poems were composed. It appears as the letter F in the Latin alphabet, which was derived at a very early period from the alphabet of Euboea. In later Greek, though discarded as a letter, it is retained in the See the articles on

form 5 as the numeral for 6. F and V.

Digby, a small seaport of Nova Scotia, on St Mary's Bay, reputed for its curing of a variety of small herrings or pilchards ('Nova Scotia sprats'). Pop. 1951.

Digby, SIR KENELM, was born at Gayhurst, near Newport Pagnell, 11th July 1603. His father, Sir Everard Digby (1578-1606), in 1592 came into a large estate, but seven years later turned Catholic, and was hanged for his part in the Gunpowder Plot (q.v.). Kenelm himself was bred a Catholic, but in 1618, after a half-year in Spain, entered Gloucester Hall, Oxford (now Worcester College). He left it in 1620 without a degree, and spent nearly three years abroad, in Florence chiefly. At Madrid he fell in with Prince Charles, and following him back to England, was knighted, and entered his service. În 1625, after a singular courtship, he secretly married that celebrated beautie and courtezane,' Venetia Stanley (1600-33), who had been his playmate in childhood. With two privateers he sailed in 1628 to the Mediterranean, and on 11th June vanquished a French and Venetian squadron off Scanderoon. On his beloved wife's death he withdrew to Gresham College, and there passed two hermit-like years, diverting himself with chemistry and the professors' good conversation. Meanwhile he had professed the Protestant faith, but, 'looking back,' in 1636 he announced his reconversion to Archbishop Laud; and his tortuous conduct during the Great Rebellion was dictated, it seems, by his zeal for Catholicism. He was imprisoned by the Parliament (1642-43), and had his estate confiscated; was at Rome (1645-47), where he finished by hectoring at his Holiness;' and thrice revisited England (1649-51-54), the third time staying two years, and entering into close relations with Cromwell. At the Restoration, however, he was well received, and retained his office of chancellor to Queen Henrietta Maria. one of the first members of the Royal Society (1663), and died 11th June 1665.

He was

'The very Pliny of our age for lying,' said Stubbes of Digby, whom Evelyn terms an arrant mountebank. Yet he was a friend of Descartes and Sir Thomas Browne (q.v.); he could appreciate the discoveries of Harvey, Bacon, and Galileo. In the Dictionary of National Biography (vol. xv. 1888) Mr S. L. Lee points out, that as a philosopher-an Aristotelian-Sir Kenelm undoubtedly owed much to Thomas White;' and he questions whether his much-vaunted powder of sympathy' was not really invented by Sir Gilbert Talbot. This powder-Digby professed to have learned the secret from a Carmelite who had travelled in the farthest East-was made of vitriol, and applied to a bandage, not to the wound itself. Anyhow, Digby's Discourse thereon (1658), like his Treatise of Bodies and of Man's Soul (1644), contains much that is curious, if little of real value; whilst in his Discourse concerning the Vegetation of Plants (1660), the chief of his other twelve works, he is said to have been the first to notice the importance of vital air or oxygen to plants.' See his bombastic Memoirs, dealing with his courtship (edited by Sir Harris Nicolas, 1827), and his Journal of the Scanderoon Voyage (edited for the Camden Society by John Bruce, 1868).

DIGESTION

Digby, KENELM HENRY, was born in 1800, youngest son of the dean of Clonfert. Having entered Trinity College, Cambridge, he took his B.A. in 1819, and three years later published the Broad Stone of Honour that noble manual for gentlemen,' as Julius Hare called it, 'that volume which, had I a son, I would place in his hands, charging him, though such admonition would be needless, to love it next to his Bible.' It was much altered and enlarged in the 1828 and subsequent editions (the latest 1877), its author having in the where most of his long life was spent, on 22d meantime turned Catholic. He died in London, March 1880. Of fourteen other works (32 vols. 1831-74) all the last eight were poetry.

Digest, a name often given to the Pandects (q.v.) of the civil or Roman law, because they contained Legalia præcepta excellenter digesta.'

Digester, PAPIN'S, is a strong boiler with a closely fitting cover, in which articles of food may be boiled at a higher temperature than 212° (100° C.). As its name implies, it was invented by Papin (q.v.), and a common form is the Autoclave, fig. 1, where the lid can be turned round under clamps or ears, and thus be rendered steam-tight. Another form is given in fig. 2, where a portion of the side

is removed to exhibit the interior. The lid, A, is fastened down by a screw, B, and the steam generated in the boiler is allowed to escape at a stopcock, C, or by raising the weighted valve, D. The increased pressure to which the contents of the boiler are exposed causes the boiling-point of the water to rise to 400° (204° C.), and occasionally higher. The digester is of great value as a means of preparing soups of various kinds, and especially in the extraction of gelatin from bones.

Digestion is the change which food undergoes in order to prepare it for the nutrition of the animal frame, and is carried on in the higher animals in the DIGESTIVE SYSTEM. In some of the lowest forms of animal life (amoeba) particles of food may be drawn into the body (which possesses no special organs at any part of its surface), and may then be digested. In higher organisms, however, parts have become evolved, which serve more especially the function of digestion. Thus in the common sea anemone there is a simple pouch which leads inwards from the centre of the cluster of tentacles. Into this, fish and other food are drawn and digested, while the undigested parts are afterwards voided through the same aperture by which they entered. In still higher organisms, man himself included, this simple pouch is changed into a complex and greatly elongated tube, which is provided with one aperture (the mouth) by which food enters, and another aperture (the anus) through which undigested matter leaves the body. The whole digestive system is lined with a soft mem