Part 10

=Dextrine.= Dextrine is a substance having the same chemical nature as starch, but differing in many of its properties. It may be described as a condition which starch assumes just before its change into glucose.

=Exp. to show Dextrine.= Carefully dry and then heat a little starch to about 400° Fahr. Keep it at this temperature until it turns brown, or for ten minutes. Then mix it with water, when it will dissolve, forming a gummy solution. Starch will not do this. Test it with iodine; it will not change color. The remarkable thing about the relation of dextrine to starch is that although they differ so much in properties they have the same chemical composition.

The change of starch into dextrine is an important point in cooking, because starch cannot be assimilated until the conversion has taken place, either before or after it is eaten. Now it will be seen that unless this change is either produced or approached in the cooking of starch-containing foods, they are not prepared as well as it is possible to prepare them; also, that it is not possible to cause this change at a low temperature; therefore 140° (the temperature at which the grains burst) should not be regarded as the cooking temperature of starch. It should be such a temperature as shall actually convert it into dextrine, or at least change it to such an extent that it will be more easily converted into dextrine, and ultimately into sugar, by the digestive fluids. This should be as near 401° Fahr. as practicable,--not that a potato, or a loaf of bread, or a pudding will have all the starch in it changed when it is put into an oven of that temperature. It would not be possible, on account of the water contained in each; but that in the outside may be, and the preparation of the remainder will be better than at a lower temperature.

There are other means of changing starch into dextrine than by heat, one of the most remarkable of which is diastase, a substance found in sprouting grains, which has the power to transform the starch stored in the grain by nature into soluble dextrine, in which form it can be taken up by the young plant for food. The crude starch could not thus be absorbed. The starch which we use as food is of no more value to us than it is to the young plant until it has been changed into dextrine or sugar. Now, if art outside of the body can accomplish what nature is otherwise forced to do in the alimentary canal, the body will be saved a certain amount of force,--a point of great importance, especially in the case of the sick or invalid, who can ill afford to waste energy.

Starch constitutes half of bread, our "staff of life"; nearly all of rice, the staff of life in the East; and the greater part of corn-starch, sago, arrowroot, tapioca, peas, beans, turnips, carrots, and potatoes.

Arrowroot is the purest form of starch food known. Rice is richest in starch of all the grains. Tapioca is prepared from the root of a tropical plant; it is first crushed and the grains washed out with water, then the whole is heated and stirred, thus cooking and breaking the starch grains, which on cooling assume the irregular rough shapes seen in the ordinary tapioca of commerce. Probably a part of the starch is converted into dextrine, which accounts for the peculiarly agreeable flavor which tapioca possesses. Mixed with the grains, as they are taken from the plant, is a very dangerous poison which, being soluble in water and volatile, is partially washed away and partially driven out by the heat,--in fact the heating is done for this purpose. Sago is principally starch. It is obtained from the pith of the sago-palm. Imitations of both tapioca and sago are sometimes made from common starch.

Starch may be converted into grape-sugar by treating it with acids; that of corn is generally used for the purpose. Much of the glucose of commerce is made in this way. In the United States it is estimated that $10,000,000 worth is manufactured every year. It is used for table syrup, in brewing beer, in the adulteration of cane-sugar, and in confectionery. Honey is also made from it. The nutritive value of vegetables is due largely to the starch and sugar which they contain.

In the economy of the body starch is eminently a heat producer. Pound for pound it does not give as much heat as fat, but owing to its great abundance and extensive use it, in the aggregate, produces more. (Atwater.)

Starch is an abundant and easily digested form of vegetable food, but it is incapable of sustaining life. It contains none of the nitrogenous matter needed for the nutrition of the muscles, nerves, and tissues. Indeed, it is said on good authority that many an invalid has been slowly starved to death from being fed upon this material alone.

=Sugar.= There are many kinds of sugar, the most familiar of which is cane-sugar, or sucrose (C{12}H{22}O{11}). It is obtained from the juices of various plants, for instance, sugar-cane, beet-root, the sugar-maple, and certain kinds of palms. By far the greatest amount comes from the sugar-cane. It is made by crushing the stalks of the plant (which somewhat resembles Indian corn) and extracting the sweet juice, which is then clarified and evaporated until, on cooling, crystals appear in a thick liquid; this liquid is molasses, and the grains or crystals are brown sugar. White sugar is obtained by melting this brown sugar in water, removing the impurities, and again evaporating in vacuum-pans, which are used for the purpose of boiling the liquid at a lower temperature than it could be boiled in the open air, thus avoiding the danger of burning, and otherwise preserving certain qualities of the sugar. Loaf-sugar is made by separating the crystals from the liquid by draining in molds; and granulated_ sugar by forcing out the syrup in a centrifugal machine. The process of making beet-root sugar is similar. Sugar from maple sap is obtained by simply evaporating away the excess of water. In the East a considerable quantity of sugar is made from the juices of certain varieties of palm, especially the date-palm. Maple-sugar and palm-sugar are generally not purified.

Sucrose dissolves readily in water. By allowing such a solution to stand undisturbed for a time until the water has disappeared, transparent crystals are obtained, known as rock candy. Again, sucrose melted at a temperature of 320° Fahr. forms, on cooling, a clear mass, called barley-sugar. Heated to 420° Fahr. dissociation of the carbon from the water of crystallization takes place, the carbon appearing in its characteristic black color. This dark brown, sweetish-bitter syrup is called caramel. On cooling it forms a solid, which may be dissolved in water, and is used to color gravies, soups, beer, and so forth.

=Exp. with Sulphuric Acid.= A very pretty experiment to show the separation of the water from the carbon may be made by treating a little sugar in sulphuric acid. Put a tablespoon of sugar in any vessel that will bear heat, a thin glass or stout cup. Pour over enough concentrated sulphuric acid to thoroughly moisten it, let it stand for a few minutes, when it will be seen that the mass has changed color from white to a yellowish brown. The color increases in intensity until it is perfectly black, when the whole puffs and swells up, fumes are driven off, and a mass like a cinder remains. This is charcoal, or nearly pure carbon.

The explanation is as follows: So strong is the affinity of the acid for the water that it breaks up the chemical combination between it and the carbon, unites with the water, and leaves the carbon free. So intense is the chemical change that an enormous amount of heat is evolved,--so much, in fact, that a considerable part of the water is vaporized, leaving the more or less solid charcoal. The light color noticed during the first part of the union indicates that the chemical dissociation is just beginning, and that only a small amount of carbon has been set free.

=Glucose.= Glucose or grape-sugar (C{6}H{12}O_{6}) is one of the kinds of sugar found in grapes, peaches, and other fruits. It is about two and one half times less sweet than cane-sugar. It is manufactured on a large scale from the starch of corn.

=Lactose.= Lactose or milk-sugar is the sugar found in the milk of the Mammalia. That of commerce comes chiefly from Switzerland, where it is made by evaporating the whey of cow's milk. For sweetening drinks for infants and for the sick, milk-sugar is said to be less liable to produce acid fermentation than cane-sugar, and also to be more easily digested.

Sugar is a valuable nutrient, being very easily digested and absorbed. Cane-sugar is converted into glucose in the process of digestion by the pancreatic juice, and after absorption it is completely utilized in the body, furnishing heat and probably energy.

=Effects of Heat on Sugar.= Sugar undergoes various changes, with different degrees of heat, by loss of some of its water of crystallization. One of the most remarkable of these is seen in caramel sauce, which is a rich crimson-brown syrup generally supposed to contain foreign coloring matter, but which does not. It is made by melting sugar without water, and heating it until the desired hue and thickness are reached. Nothing is added, but something is taken away; that is, some of the water is driven out, with the result of change in both color and taste.

In a recent article in "The Century Magazine" (November, 1891) Prof. Atwater touches upon the subject of the production of artificial foods from the crude materials of the earth, and states, among other things, that a sugar resembling fruit-sugar has been made artificially by synthesis, by Prof. Fischer of Würzburg, Germany.

AIR

Air is a gaseous elastic body which envelops the earth on every side, extending possibly two hundred miles from its surface, but all the while growing more and more rare as the distance increases. When pure it is tasteless and odorless. We really live at the bottom of an atmospheric ocean, and are pressed upon by its weight. At the sea-level the pressure upon every square inch of surface is equal to fifteen pound.

=Atmospheric Pressure Variable.= Atmospheric pressure diminishes and is constantly variable, according to the height above the sea-level. If we ascend into the air 5000 feet, it is perfectly evident that there are 5000 feet less of atmosphere pressing upon us than at the point from which we started. This diminution of pressure is often measured by the temperature at which water boils at different heights.

=Composition.= An average composition of the atmosphere has been previously stated. Besides nitrogen and oxygen, it always contains water in the form of vapor, and carbonic acid. The amount of aqueous vapor in the air changes according to the temperature; the amount of carbonic acid is also constantly variable. Air usually contains, in addition to these, traces of ammonia, organic matter which includes micro-organisms, ozone, salts of sodium, and other mineral matters in minute and variable quantities.

=Air in Motion.= The atmosphere is almost always in motion. We feel it in the gentle breeze and the more forcible wind. If it moves at a slower rate than two and one half feet a second this motion is not noticeable. Motion in the air is caused by the unequal heating of portions of it. If from any cause the atmosphere over a certain region becomes warm, it will expand (all bodies expand with heat), become lighter, and its tendency will be to move in the direction of least resistance,--that is, upward; so we say heated air rises. Currents of cooler air will immediately flow in to take its place, and thus we have a breeze, a wind, or a gale, according to the velocity and force with which the currents move. It is upon a knowledge of these movements that the theory of ventilation is based. It is because of the constant motion of air-currents that out of doors, except in densely populated cities, air remains constantly pure. When poisonous gases and other impurities accumulate, winds scatter them far and wide until they are so diluted as to be harmless; or under some conditions they unite with other things and form new and simple substances of a harmless nature, while under others, if they are compounds, they may be decomposed or washed down to the surface of the earth again.

=Impurities.= The chief chemical product of fires and of that slower combustion breathing is carbonic acid. Plants during the day, and under the influence of sunlight, take it up from the air for food, use the carbon for their growth, freeing the oxygen which man and the lower animals need. Thus is the balance most beautifully maintained.

Air is purest over the sea and over wind-swept heights of land. It, however, always contains some foreign substances, and always micro-organisms except over mid-ocean. Even the upper strata of atmosphere are not free from microscopic forms of life, as has been shown in experiments made with hail at the Johns Hopkins Hospital in 1890 by Dr. Abbott. Large hailstones were washed in distilled and sterilized water, and then melted, and cultures made from different layers; in all of these organisms were found, showing that they extend into the air a long distance from the earth.

Impurities of various kinds are constantly passing into the air, but so vast is the expanse of the atmosphere as compared with the impurities daily thrown into it from the lungs of man and the lower animals, from fires, manufactories, and decomposing matter, that they quickly disappear.

Air is the greatest or, as one writer says, the most immediate necessity of life. We could live without it only a few seconds. We constantly use it, whether sleeping or waking, and perhaps this accounts in part for the utter carelessness and indifference which most people have for the quality of that which they breathe. Even those persons who know something of the nature of air, make but little effort to provide themselves with a constantly pure supply.

=Effects of Breathing Bad Air.= If the effects of breathing bad air were immediate, there would then be an immediate remedy for the present total lack of any systematic means of ventilation in most houses. But the effects of breathing bad air are, like those of some slow and insidious poison, not noticeable at once, and often manifested under the name of some disease which gives no clue to the true cause.