Mary A. Boland

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PREPARATION OF FOOD

Digestibility. There are comparatively few kinds of food that can be eaten uncooked. Various fruits, milk, oysters, eggs, and some other things may be eaten raw, but the great mass of food materials must be prepared by some method of cooking. All the common vegetables, such as potatoes, turnips, carrots, beets, and the different grains, such as rice, wheat, corn, oats, etc., neither taste good nor are easily digestible until their starch, cellulose, and other constituents have been changed from their compact indigestible form by the action of heat. Some one has spoken of cooking as a sort of artificial digestion, by which nature is relieved of a certain amount of work which it would be very difficult, if not impossible, for her to perform.

Flavors. The necessity of cooking to develop, or to create, a palatable taste is important. The flesh of fowl is soft enough to masticate, but only a person on the verge of starvation could eat it until heat has changed its taste and made it one of the most savory and acceptable of meats. Coffee also well illustrates this point. When coffee is green—that is, unbrowned—it is acrid in taste, very tough, even horny in consistency, and a decoction made from it is altogether unpleasant. But when it is subjected to a certain degree of heat, for a certain time, it loses its toughness, becomes brittle, changes color, and there is developed in it a most agreeable flavor. This flavoring property is an actual product of the heat, which causes chemical changes in an essential oil contained in the bean. Heat not only develops but creates flavors, changing the odor and taste as well as the digestibility of food.

Effects of Cold. Some foods are better for being cold; for example, butter, honey, salads, and ice-cream. Sweet dishes as a rule are improved by a low temperature. The flavor of butter is very different and very much finer when cold than when warm. It is absolutely necessary to keep it cool in order to preserve the flavor.

CHEMICAL AND PHYSICAL CHANGES

Chemical Changes. Since many of the changes which cooking produces in the different food materials are of a chemical nature, it is well to consider what constitutes a chemical process. This idea may perhaps be best conveyed by a few experiments and illustrations, the materials for which may be easily obtained.

Exp. with Cream of Tartar and Bicarbonate of Soda. Mix two teaspoons of cream of tartar with one of bicarbonate of soda, in a little warm water. A union of the two substances follows and they neutralize each other; that is, the cream of tartar is no longer acid, and the soda is no longer alkaline. Owing to the power of chemical affinities a separation or breaking up of these compounds takes place, and new substances, carbonic acid and rochelle salts, are formed out of their constituents. The effervescence which is seen is caused by the escape of the carbonic acid.

Exp. with Hydrochloric Acid and Soda. Put a few drops of chemically pure hydrochloric acid into a little water; then add soda. A violent effervescence will follow. Continue putting in soda until this ceases, when the reaction should be neutral. Test it with litmus-paper. If it turns blue litmus-paper red, it is acid; if red litmus-paper blue, it is alkaline. Add acid or soda, whichever is required, until there is no change produced in either kind of litmus-paper. The results of this experiment are similar to those in the first one, namely, carbonic acid and a salt. In this case the salt is sodium chlorid or common salt, which is in solution in the liquid. Evaporate the water, when salt crystals will be found.[3]

Oxid of Iron. A piece of iron when exposed to the weather becomes covered with a brownish-yellow coating, which does not look at all like the original metal. If left long enough it will wholly disappear, being completely changed into the yellowish substance, which is oxid of iron, a compound of oxygen and iron, commonly called iron rust.

Burning of Coal. A piece of coal burns in the grate and is apparently destroyed, leaving no residue except a little ashes. The carbon and hydrogen of the coal have united with the oxygen of the air, the result of which is largely the invisible gas, carbonic acid, which escapes through the chimney.

Formation of Water. Water is formed by the union of two invisible gases, hydrogen and oxygen. It bears no resemblance whatever to either of them. Its symbol is H₂O.

All these are examples of chemical changes.

Definition of Chemical Change. Chemical changes or processes may be defined as those close and intimate actions amongst the particles of matter by which they are dissociated or decomposed, or by which new compounds are formed, and involving a complete loss of identity of the original substance.

Physical Changes. Mix a teaspoon of sugar with an equal amount of salt; the sugar is still sugar, and the salt remains salt; and they may each be separated from the mixture as such.

Water when frozen is changed from a liquid to a solid; its chemical composition, however, remains unchanged.

Water converted into steam by heat is changed from a liquid to a gas, but chemically there is no difference between the one and the other. Steam, water, and ice are forms of the same substance, the difference being physical, not chemical, and caused by a difference in temperature.

Lead melted so that it will run, and the solid lead of a bullet, are the same thing.

These illustrate physical changes.

Definition. When substances are brought together in such a way that their characteristic qualities remain the same, the change is called physical. It is less close and intimate than a chemical change. The transition from one state into another is also frequently only a physical change, as is seen in the transformation of water into steam, water into ice, etc.

ELEMENTS

One feature of the work of the chemist is to separate compound bodies into their simple constituents. These constituents he also endeavors to dissociate; and if this cannot be done by any means known to him, then the thing must be regarded as a simple substance. Such simple bodies are called elements.

Definition. An element then may be defined as a simple substance, which cannot by any known process be transformed into anything else; that is, no matter how it is treated, it still remains chemically what it was before. Gold, silver, copper, iron, platinum, carbon, phosphorus, calcium, oxygen, hydrogen, nitrogen, and chlorin are examples of elements. Once it was believed that there were but four elements in the world—earth, air, fire, and water. Then it was learned that these were not elements at all, but compounds, and the number of elements increased, until now sixty-eight are admitted to be simple primary substances. Some of these may in the future be proven to be compounds. Sulphur is at present in the doubtful list.

Oxygen. Oxygen is an element. It is an invisible gas, without taste or smell. It is the most abundant substance in the world, and an exceedingly active agent, entering into nearly all chemical changes and forming compounds with all known elements except one—fluorin. It is a necessity of life and of combustion.[4] It constitutes about two thirds of the weight of our bodies and one fifth of the weight of the air.

Hydrogen. Hydrogen is a gas. It is the lightest substance known. It unites with oxygen to form water, and, as will be seen later, enters into the composition of the human body.[5]

Nitrogen. Nitrogen is also a gas, but, unlike oxygen, is an inactive element. It supports neither fire nor life. It is not poisonous, however, for we breathe it constantly in the atmosphere, where its office is to dilute the too active oxygen. A person breathing it in a pure state dies simply from lack of oxygen.

Carbon. Carbon is a solid and an important and abundant element. It is known under three forms: diamond, graphite, and charcoal. The diamond is nearly pure carbon. Graphite (the "black-lead" of lead-pencils), coal, coke, and charcoal are impure forms of it. Carbon is combustible; that is, it burns or combines with oxygen. In this union carbonic acid is formed, and there is an evolution of heat, and usually, if the union be rapid and intense enough, of light. It is the valuable element in fuels, and in the body of man it unites with the oxygen of the air, yielding heat, to keep the body warm, and energy or muscular strength for work (Prof. Atwater). The carbonic acid formed in the body is given out by the lungs and skin.

Other Elements. There are many other elements about which it would be interesting to note something, such as calcium and phosphorus (found abundantly in the bones), magnesium, sulphur, sodium, iron, etc. Samples of these may be obtained to show to pupils, and descriptions given and experiments made, at the discretion of the teacher. Of the four most abundant elements of the body and of food,—oxygen, carbon, hydrogen, and nitrogen,—it is extremely important that some study be made, and if the apparatus can be procured, that it be of an experimental nature rather than simply descriptive.[6]

AIR

Air is made up principally of two elements, nitrogen and oxygen. It also always contains vapor of water and carbonic acid. Its average composition is as follows:

These are mixed together, not chemically united. Oxygen and nitrogen do unite chemically, but not in the proportions in which they exist in the air. Nitrous Oxid (N₂O), sometimes called "Laughing Gas," is one of the compounds of nitrogen and oxygen.

FIRE

Exp. with a Candle. Take a tallow candle, and by means of a lighted match raise its temperature sufficiently high to start an action between the carbon in the candle and the oxygen of the air; in other words, light the candle. A match is composed of wood, sulphur, and phosphorus. The latter is a substance which unites with oxygen very easily; that is, at a low temperature. By friction against any hard object, sufficient heat is aroused to effect a union between the phosphorus of a match and the oxygen of the surrounding air; the flame is then conveyed to the sulphur, or the heat thus generated causes a union between it (the sulphur) and the oxygen, sulphur burning somewhat less freely than phosphorus; this gives enough heat to ignite the wood, and with its combustion we get sufficient heat to light the candle, or to start a chemical union between the combustible portion, carbon chiefly, of the candle and the oxygen of the air. Allow the candle to burn for a time, then put over it a tall lamp-chimney; notice that the flame grows long and dim. Next place on the top of the chimney a tin cover, leaving a small opening, and make an opening into the chimney from below, with a pin or the blade of a knife placed between it and the table; note that the candle burns dimly. Then exclude the flow of air by completely covering the top; in a moment, as soon as the oxygen inside the chimney is consumed, the candle will go out.

This shows (1) that air—in other words, oxygen—is necessary to cause the candle to burn; (2) that by regulating the draft or flow of air the intensity of the combustion may be increased or diminished; (3) that by completely excluding air the candle is extinguished. This experiment with the candle illustrates the way in which coal is consumed in a stove. By opening the drafts and allowing the inflow of plenty of oxygen, combustion is increased; by partially closing them it is diminished, and by the complete exclusion of air burning is stopped.

The products of the burning of coal are carbonic acid and a small amount of ash. Twelve weights of coal, not counting the ash, will unite with thirty-two weights of oxygen, giving as a result forty-four weights of carbonic acid. Accompanying the union there is an evolution of light and heat. The enormous amount of carbonic acid given out daily from fires is taken up by plants and used by them for food. In the course of ages these plants may become coal, be consumed in combustion, and, passing into the air, thus complete the cycle of change.

Fuel and Kindlings. The common fuels are coal, coke, wood, gas, coal-oil, and peat. For kindling, newspaper is good because, being made of straw and wood-pulp, it burns easily, and also because printers' ink contains turpentine, which is highly inflammable.

COMPOSITION OF THE BODY

Before entering upon the study of foods it is well to consider the composition of the human body, that some idea of its chemical nature may be gained. In the United States National Museum at Washington may be found some interesting information on this subject. From there much that is contained in the following pages is taken.

A complete analysis of the human body has never been made, but different organs have been examined, and chemists have weighed and analyzed portions of them, and from such data of this nature as could be obtained, estimates of the probable composition of the body have been calculated. Thirteen elements united into their compounds, of which there are more than one hundred, form it.

The following table gives the average composition of a man weighing 148 pounds.

Oxygen	92.4	Sulphur	.24
Carbon	31.3	Chlorin	.12
Hydrogen	14.6	Sodium	.12
Nitrogen	4.6	Magnesium	.04
Calcium	2.8	Iron	.02
Phosphorus	1.4	Fluorin	.02
Potassium	.34

Prof. Atwater.

It will be seen from this that oxygen, carbon, hydrogen, and nitrogen constitute nearly the whole, the other elements being in very small proportions.

PRINCIPAL CHEMICAL COMPOUNDS IN
THE BODY

The following interesting table, obtained at the National Museum, gives the principal compounds of the body. Some of the more rare organic compounds are omitted.

Water:—A compound of oxygen and hydrogen.
Protein	{	Albuminoids	{	Myosin and syntonin of muscle (sometimes called "muscle fibrin").
Compounds,	{	or	{
	{	Proteids.	{
composed	{		{	Albumen of blood and milk. Casein of milk.
mainly of	{
	{
Carbon,	{		{	Collagen of bone and tendons.	}	which
	{	Gelatinoids.	{		}	yield
Oxygen,	{		{	Chondrigen of cartilage, gristle,	}	gelatin.
	{
Hydrogen,	{		{
	{	Hemoglobin.	{	The red coloring matter of blood.
Nitrogen.	{		{

Fats,	{		{	Stearin,	}	These make up the bulk of the fat of the body.
	{	Neutral	{		}
composed	{	Fats.	{	Palmitin,	}	They are likewise the chief constituents of tallow, lard, etc.
mainly of	{		{		}
	{		{	Olein,etc.	}
Carbon,	{
	{	Complex	{	Protagon,	}	Found chiefly in the brain, spinal cord, nerves, etc.
Oxygen,	{	Fats,	{		}
	{	containing	{	Lecithin,	}
Hydrogen,	{	phosphorus	{		}
	{	andnitrogen.	{	Cerebrin.	}

Carbohydrates,	{	Glycogen, "animal starch." Occurs in the liver and other organs.
composed of	{
Carbon,	{	Inosite, "muscle sugar." Occurs in various organs.
Oxygen,	{	Lactose, "milk sugar." Occurs in milk.
Hydrogen.	{	Cholesterin. Occurs in brain, nerves, and other organs.

	{	Phosphate of lime, or calcium phosphate.	}	Occurs chiefly in bones and teeth, though found in other organs.
	{	Carbonate of lime, or calcium carbonate.	}
	{	Fluorid of calcium, or calcium fluorid.	}
	{	Phosphate of magnesia, or magnesium phosphate.	}
	{
Mineral	{	Phosphate of potash, or potassium phosphate.	}
Salts.	{	Sulphate of potash, or potassium sulphate.	}	Distributed through the body in the blood, muscle, brain, and other organs.
	{	Chlorid of potassium, or potassium chlorid.	}
	{	Phosphate of soda, or sodium phosphate.	}
	{	Sulphate of soda, or sodium sulphate.	}
	{	Chlorid of sodium, or sodium chlorid.	}

Now, since the body is composed of these substances, our food, including air and water, should contain them all in due proportion, that the growth, energy, and repair of the body may be healthfully maintained.

THE FIVE FOOD PRINCIPLES

For convenience of comparison foods may be divided into five classes: Water, Protein, Fats, Carbohydrates, Mineral Matters.

Some scientists include air in the list, but it has been thought best in this work to speak of it separately as the greatest necessity of life, but not in the sense of a direct nutrient.

An average composition of three of the principles is as follows:

	{	Carbon	53
Protein	{	Hydrogen	7
	{	Oxygen	24
	{	Nitrogen	16
	{	Carbon	76.5
Fats	{	Hydrogen	12
	{	Oxygen	11.5
	{	Nitrogen	—
	{	Carbon	44
Carbohydrates	{	Hydrogen	6
	{	Oxygen	50
	{	Nitrogen	—

It will be seen from the above that the protein compounds contain nitrogen; the fats and carbohydrates do not.

WATER

We will now consider the first of the food principles—water. Water is one of the necessities of life. A person could live without air but a few minutes, without water but a few days. It constitutes by weight three fifths of the human body, and enters largely into all organic matter. Water is an aid to the performance of many of the functions of the body, holding in solution the various nutritious principles, and also acting as a carrier of waste. It usually contains foreign matter, but the nearer it is to being pure the more valuable it becomes as an agent in the body. Ordinary hydrant, well, or spring water may be made pure by filtering and then sterilizing it.

Exp. Put a little water into a test-tube, and heat it over the flame of an alcohol-lamp. In a short time tiny bubbles will appear on the sides of the glass. These are not steam, as may be proved by testing the temperature of the water; they are bubbles of atmospheric gases which have been condensed in the water from the air; they have been proved to be nitrogen, oxygen, and carbonic acid, but as they do not exist in the water in the same proportions as in the air, they are not called air, but atmospheric gases. Continue the heating, and the bubbles will continue to form. After a while, very large bubbles will appear at the bottom of the tube; they increase rapidly and rise toward the top; some break before reaching it, but as the heat becomes more intense others succeed in getting to the surface,—there they break and disappear. If the water now be tested with a thermometer, it will be found to have reached 212° Fahrenheit or 100° Centigrade, provided the experiment be tried at or near the level of the sea.

Steam. The large bubbles are bubbles of steam, or water expanded by heat until its particles are so far apart that it ceases to be a liquid and becomes a gas. True steam is invisible; the moisture which collects on the sides of the tube and is seen coming out at the mouth is partially condensed steam, or watery vapor. Watch a tea-kettle as it boils on a stove; for the space of an inch or two from the end of the spout there seems to be nothing; that is where the true steam is; beyond that, clouds of what is commonly called steam appear; they are watery vapor formed from the true steam by partial condensation which is produced by its contact with the cool air.[7]

Boiling-point of Water. Water boils at different temperatures, according to the elevation above the sea-level. In Baltimore it boils practically at 212° Fahr.; at Munich in Germany at 209½°; at the city of Mexico in Mexico at 200°; and in the Himalayas, at an elevation of 18,000 feet above the level of the sea, at 180°. These differences are caused by the varying pressure of the atmosphere at these points. In Baltimore practically the whole weight of the air is to be overcome. In Mexico, 7000 feet above the sea, there are 7000 feet less of atmosphere to be resisted; consequently, less heat is required, and boiling takes place at a lower temperature. By inclosing a vessel of water in a glass bell, and exhausting the air by means of an air-pump, water may be made to boil at a temperature of 70° Fahr., showing that much of the force (heat) that is consumed in causing water to be converted into steam is required to overcome the pressure of the air. The foregoing illustrates the point that boiling water is not of invariable temperature; consequently, that foods which in some places are cooked in it may in other places be cooked in water that is not boiling,—in other words, that it is not ebullition which produces the change in boiling substances, but heat.

Changes Produced in Water by Boiling. By boiling water for a moderate time the greater part of the atmospheric gases is driven off. The flavor is much changed. We call it "flat"; but by shaking it in a carafe or other vessel so that the air can mingle with it, it will reacquire oxygen, nitrogen, and carbonic acid, and its usual flavor can thus be restored.

Water which flows through soil containing lime is further changed by boiling.

Exp. with Lime-water. Pour a little lime-water into a test-tube. With a small glass tube blow into it for a few minutes, when it will become milky; continue the blowing for a few minutes more, when it will lose its cloudy appearance and become clear again. The following explains this: in the first place there was forced into the lime-water, from the lungs, air containing an excess of carbonic acid; this united with the lime in solution in the water and formed carbonate of lime. Carbonate of lime is insoluble in water which contains no carbonic acid, or very little,[8] but will dissolve in water which is charged with it, and this is produced by the continued blowing. Now if this water be freed of its excess of carbonic acid by boiling, the carbonate of lime will be freed from its soluble state, and will fall as a precipitate and settle on the sides of the vessel. From this we learn that water may be freed from carbonate of lime in solution in it by boiling.

Organic Matter in Water. There is another class of impurities in water of vastly more importance than either the atmospheric gases or lime. These are the organic substances which it always contains, especially that which has flowed over land covered with vegetation, or that which has received the drainage from sewers. The soluble matter found in such water is excellent food for many kinds of micro-organisms which often form, by their multiplication, poisons very destructive to animal life. Or the organisms themselves may be the direct producers of disease, as for instance the typhoid fever bacillus, the bacillus of cholera, and probably others which occur in drinking-water. These organisms are destroyed by heat, so that the most valuable effect produced in water by boiling it is their destruction. Such water is, therefore, a much safer drink to use than that which has not been boiled. Water should always be boiled if there is the slightest suspicion of dangerous impurities in the supply.

Use of Tea and Coffee. This leads us to the thought that the extensive use of tea and coffee in the world may be an instinctive safeguard against these until recently unknown forms of life. The universal use of cooked water in some form in China is a matter of history. The country is densely populated, the sewage is carried off principally by the rivers, so that the danger of contracting disease through water must be very great, and it is probable that instinct or knowledge has prompted the Chinaman to use but very little water for food except that which has been cooked. Whatever the reason, the custom is a national one. The every-day drink is weak tea made in a large teapot and kept in a wadded basket to retain the heat; the whole family use it. The very poor drink plain hot water or water just tinged with tea.

That tea and coffee furnish us each day with a certain amount of wholesome liquid in which all organic life has been destroyed, remains a fact; they may be, in addition, when properly made and of proper strength, of great value on account of their warmth, good flavor, and invigorating properties. There is no doubt that it is of the greatest importance that tea and coffee be used of proper strength; for if taken too strong, disorders of the system may be produced, necessitating their discontinuance, and thus depriving the individual of a certain amount of warm and wholesome liquid.

To Summarize. The effects produced in water by boiling which have been spoken of are: (1) the expulsion of the atmospheric gases; (2) the precipitation of lime when in solution; and (3) the destruction of micro-organisms. The most important points to remember in connection with water are, that a certain amount each day is an absolute necessity of life, and that unless the supply be above suspicion it should be filtered and then sterilized.

Filtration and Sterilization of Water. Filtration as a general thing is done by public authorities, but sterilization is not, and should be done when necessary by the nurse. For immediate use, simply boiling is said on good authority to be sufficient to destroy all organisms then in the water. Spores of organisms are, however, not killed by boiling, as they are very resistant to heat. Fortunately they are not common. As they do not develop into bacteria for some hours after the water has been boiled, they may be entirely gotten rid of by allowing them to develop and then destroying by a second boiling; but for all practical purposes, and under ordinary circumstances, water is rendered safe for use by boiling it once.[9] Should the water be very bad, boil it in a jar plugged with cotton for half an hour three days in succession, keeping it meanwhile in a temperature of 70° or 80° Fahr., so that any spores of organisms which may be in it will have an opportunity to get into such a state of existence that they will be capable of being killed by the next boiling. The third treatment is for the purpose of making sure of any that may have escaped the first and second.

PROTEIN

The second of the food principles, protein, is a complex and very important constituent of our food. The protein compounds differ from all others as to chemical composition by the presence of nitrogen; they contain carbon, oxygen, hydrogen, and nitrogen, while the fats and carbohydrates are composed principally of carbon, oxygen, and hydrogen, but no nitrogen. The so-called extractives or flavoring properties of meats are nitrogenous, and are consequently classed with the protein compounds.[10]

The body of an average person contains about eighteen per cent. of protein. The proteins of various kinds furnish nutriment for blood and muscle, hence the term "muscle-formers," which is sometimes given them. They also furnish material for tendons and other nitrogenous tissues. When these are worn out by use, it is protein which repairs the waste.

Most of the valuable work upon the analysis of food has been done in Germany. From estimates made by chemists of that country it has been decided that the amount of protein in a diet should not fall below four ounces daily. This is to represent an allowance for a man of average weight doing an average amount of work, below which he cannot go without loss in health, in work, or in both. Although protein is the most expensive of all food materials, one should endeavor to use at least four ounces each day. Meat, milk, eggs, cheese, fish of all kinds, but especially dried cod, wheat, beans, and oatmeal are all rich in this substance. The protein compounds are divided into three classes:

ALBUMINOIDS, GELATINOIDS, EXTRACTIVES.

Albuminoids. The most perfect type of an albuminoid is the white of egg. It is a viscous, glairy, thick fluid which occurs also in the flesh of meat as one of its juices, in fish, in milk, in wheat as gluten, and in other foods. It is soluble in cold water.

Exp. Mix some white of egg in a tumbler with half a cup of cold water. As soon as the viscousness is broken up it will be found to be completely dissolved. It is insoluble in alcohol.

Exp. Pour upon some white of egg double its bulk of alcohol. It will coagulate into a somewhat hard opaque mass.

Heat also has the power of coagulating albumen.

Coagulation of Albumen by Heat. Put into a test-tube some white of egg, and place the tube in a dish of warm water. Heat the water gradually over a gas-flame or an alcohol-lamp. When the temperature reaches 134° Fahr. it will be seen that little white threads have begun to appear; continue the heating to 160°, when the whole mass becomes white and firm. Now remove a part from the tube and test its consistency; it will be found to be tender, soft, and jelly-like. Replace the tube in the dish of water and raise the heat to 200° Fahr.; then take out a little more and test again; it will now be found hard, close-grained, and somewhat tough. Continue the heating, when it will be seen that the tenacity increases with rise of temperature until at 212° Fahr., the boiling-point of water, it is a firm, compact solid. When heated to about 350°, white of egg becomes so tenacious that it is used as a valuable cement for marble.

These experiments illustrate a very important point in the cooking of albuminous foods. They show that the proper temperature for albumen is that at which it is thoroughly coagulated, but not hardened; that is, about 160° Fahr. Most kinds of meat, milk, eggs, oysters, and fish, when cooked with reference to their albumen alone, we find are also done in the best possible manner with reference to their other constituents. For instance, if you cook an oyster thinking only of its albuminous juice, and endeavor to raise the temperature throughout all of its substance to, or near, 160° Fahr., and not higher, you will find it most satisfactory as to flavor, consistency, and digestibility. The same is true of eggs done in all ways, and of dishes made with eggs, such as custards, creams, and puddings. With the knowledge that albumen coagulates at a temperature of 52° below that of boiling water, one can appreciate the necessity of cooking eggs in water that is not boiling, and a little experiment like the above will impress it upon the mind as no amount of mere explanation can possibly do.

The cooking of eggs, whether poached, cooked in the shell, or in omelets, is of much importance, for albumen when hard, compact, and tenacious is very difficult of digestion; the gastric juice cannot easily penetrate it; sometimes it is not digested at all; while that which is properly done—cooked in such a way that it is tender and falls apart easily—is one of the most valuable forms of food for the sick.

Albumen should always be prepared in such manner as to require the least possible expenditure of force in digestion. Those who are ill cannot afford to waste energy. Whether they are forced to do so in the digestion of their food depends very much upon the person who prepares it.

Advantage is often taken, in cooking, of the fact that albumen hardens on exposure to certain degrees of heat, to form protecting layers over pieces of broiling steak, roast meats, etc. If a piece of meat is placed in cold water to cook, it is evident, since albumen is soluble in cold water, that some of it will be wasted. If the same piece is plunged into boiling water the albumen in its outer layers will be immediately hardened, and form a sheath over the whole which will keep in the juices and the very important flavors. When broth or soup is made, we put the meat (cut into small pieces to expose a large extent of surface) into cold water, because we wish to draw out as much as possible the soluble matter and the flavors. If, on the other hand, the meat is to be served boiled, and broth or soup is not the object, then this order should be reversed, and every effort made to prevent the escape of any of the ingredients of the meat into the liquid.

In broiling steak, we sacrifice a thin layer of the outside to form a protecting covering over the whole by plunging it into the hottest part of the fire, so that the albumen will become suddenly hard and firm, and plug up the pores, thus preventing the savory juices from oozing out. More will be said on this subject in the recipes for cooking these kinds of foods.

Gelatinoids. The second class of protein compounds comprises the gelatinoids, gelatin being their leading constituent. It is found in flesh, tendons, cartilage and bone; in fact, it exists in all the tissues of the body, for the walls of most of the microscopic cells of which the tissues are composed contain gelatin.

Exp. Boil a pound of lean meat freed from tendons, fat, and bone, in a pint of water for three hours; then set the liquid away to cool. Jelly resembling calf's-foot jelly will be the result. The cell-walls of the flesh have been dissolved by the long-continued action of heat and liquid. This is commonly called stock or glaze.

Exp. Put a piece of clean bone into a dilute solution of hydrochloric acid. In two or three days the acid will have acted upon the earthy matters in the bone to remove them, and gelatin will remain. The average amount in bone is about thirty per cent.

Calves' feet were formerly used for jelly because of the excess of gelatin which they contain. They were cooked in water for a long time and the liquid reduced by further boiling; it was then clarified, flavored, and cooled; the result was a transparent, trembling jelly. The prepared gelatin of commerce, or gelatine, has now largely displaced this, for it is much more convenient to use, and less expensive.

Extractives. The extractives or flavoring properties of meats and other substances are usually classed with the protein compounds. Their chemical nature is not well understood.

FATS

Fixed and Volatile Oils. There are two classes of fats, called fixed oils and volatile oils. All kinds of fats good for food belong to the class of fixed oils. A volatile oil is one which evaporates away, like alcohol or water, and leaves no residue. The fixed oils, at least most of them, will not do this; they do not vaporize even at very high temperatures, but they become dissociated or decomposed,—that is, their chemical structure is broken up before their boiling-point is reached. Volatile oils, on the contrary, are capable of being boiled and transformed into gases. Some one illustrates this by the changes which take place in water. When water is heated to 212° Fahr. it is converted into a gas, which on cooling below 212° returns to the liquid state again without loss. The essential oil, turpentine, if heated to 320° Fahr. ceases to be a liquid and becomes a gas, which on cooling becomes a liquid oil again without loss of weight. Other volatile oils are oil of cloves, oil of bitter almonds, orange and lemon oil, oil of cinnamon, bergamot, and patchouli.

The boiling sometimes noticed in a pot of lard is owing to the presence in it of a little water which is very soon converted into steam, when the bubbling ceases, and after that the temperature of the fat rises rapidly, reaching in a short time four or five hundred degrees Fahrenheit, when a separation of its constituents takes place, and carbon is revealed as a black mass.

Composition of Fats. Fats are hydrocarbons—that is, they are composed chiefly of carbon united with hydrogen and oxygen. They must not be confounded with the carbohydrates, which are always composed of carbon with the elements of water—that is, the proportion of hydrogen to oxygen is as two to one,—whereas in the hydrocarbons this is not the case. These elements enter into the compositions of fats as various fatty acids and glycerin; the acids are not sour, as one would suppose from the name, but are so called because they behave chemically toward bases as sour acids do, that is, they unite with them. The glycerin of commerce is obtained by decomposing fats.

Fat in Milk. The white color of milk is given to it by minute globules of fat suspended in it.

To prove this: Put a little milk into a bottle with a ground-glass stopper; pour upon it three times its bulk of ether and shake gently; let it stand for two or three days, when it will be found that the ether has dissolved the fat and left a semi-transparent yellowish white liquid resembling blood serum. By pipetting or carefully pouring off the ether, and evaporating it by placing the vessel containing it in a dish of warm water, clear oil will be obtained. Care must be taken not to put the ether near a flame or the fire, as it is highly inflammable, and an explosion might occur. Ether boils at 94.82° Fahr.

The proportion of fat in milk is from 2.8 to 8 per cent. It varies in milk from different species of cows, and from the same species at different times, according to age, feeding, and other circumstances.

Cream. When milk is allowed to stand without disturbance for a time the globules of fat, being lighter than water, rise to the surface and form cream. Cream is the most wholesome, palatable, and easily digested form of fat. Butter is obtained by beating milk or cream in a churn until the little globules of fat break and stick together in a mass.

Olive-Oil. Olive-oil is one of the most easily digested and palatable of fats. A genuine oil of the first quality is, in this country unfortunately, expensive, much of that sold under the name being adulterated with cotton-seed oil, poppy-oil, and essence of lard.[11]

Cotton-seed oil has no especially bad flavor, but it is unpleasant and indigestible when used raw as in sardines and salads. The after taste which it leaves reminds one too forcibly of castor-oil.

Olive-oil of the best quality is almost absolutely without flavor. It is prepared in several grades: the first pressing from the fruit is the best, the second is fair, the third inferior, and there is sometimes a fourth known as refuse oil. For deep fat frying nothing is so good as olive-oil, but its costliness in this country excludes it from common use.

The fat of the sheep and ox, after it has been rendered, and deprived of all membrane and fibers, is called tallow. The term is also applied to the fat of other animals, and to that of some plants, as bayberry-tallow, piny tallow, and others. The uncooked fat of any animal is called suet, but the name has come to be applied to the less easily melted kinds, which surround the kidneys or are in other parts of the loin. The fat which falls in drops from meat in roasting is called dripping.

THE CARBOHYDRATES

Starch. Starch is a substance found in wheat, corn, oats, and in fact in all grains, in potatoes, in the roots and stems of many plants, and in some fruits. In a pure state it is a white powder such as is seen in arrowroot and corn-starch. Examined by a microscope this powder is found to be made up of tiny grains of different shapes and sizes, some rounded or oval, others irregular. Those of potato-starch are ovoid, with an outside covering which appears to be folded or ridged, and looks somewhat like the outside of an oyster-shell, although its similarity extends no further than appearance, as the little ridges are true folds, and not overlapping edges.

Size of Starch Grains. Starch grains vary in size according to the source from which the starch is obtained. Those of ground rice are very small, being about 1 3000 of an inch in diameter; those of wheat are 1 1000 of an inch, and those of potato 1 300 of an inch.

Starch is a carbohydrate, being composed of six parts of carbon, ten of hydrogen, and five of oxygen. Its symbol is C₆H₁₀O₅. It is insoluble in water, but when the water is heated, the grains seem to absorb it; they increase in size, the ridges or folds disappear, and when the temperature reaches 140° Fahr. or a little over, they burst, and the contents mingle with the liquid forming the well-known paste.

Test for Starch. Mix a teaspoon of starch with a cup of cold water and boil them together for a few minutes until a paste is formed; then set it aside to cool. Meanwhile make a solution of iodine by putting a few flakes into alcohol, or use that which is already prepared, and which may be obtained at any pharmacy. Add a drop of this solution to the paste mixture; it will immediately color the whole a rich dark blue. This is known as the "iodine test," and is a very valuable one to the chemist, for by means of it the slightest trace of starch can be detected.

Exp. with Arrowroot. Make a thin paste by boiling a little arrowroot and water together. When cool test it with a drop of the iodine solution. The characteristic blue color will be very strong, showing that arrowroot is rich in starch.

Similar tests may be made with grated potato, wheat-flour, rice-flour, tapioca, and other starch-containing substances. Also powdered sugar, cream of tartar, and other substances may be tested, when it is suspected that they have been adulterated with starch.

Although starch grains burst and form a paste with water at 140° Fahr., that is not the temperature at which it should be cooked for food, and the thickening which then takes place should not be confounded, as often happens, with the true cooking of starch. In order to understand the difference between the proper cooking of starch and the simple bursting of the grains, let us consider the changes which take place in starch when it is subjected to different degrees of heat, and also those which are produced in it during the process of digestion. All starch in food is changed into dextrine and then into sugar (glucose, C₆H₁₂O₆) in the process of digestion. Glucose is a kind of sugar, resembling cane-sugar, but it is not so sweet.

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 (C₁₂H₂₂O₁₁). 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 (C₆H₁₂O₆) 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.[12]

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.

Dr. Van Rensselaer, in the Orton Prize Essay on Impure Air and Ventilation, makes the statement that statistics show that of the causes of mortality the most important and farthest-reaching is impure air.

Amount of Air Required for one Person. Sanitarians have agreed that each individual requires at least 3000 cubic feet of air every hour. A room 10 × 15 × 20 holds 3000 cubic feet of air, which should be changed once every hour in order that one individual shall have the required amount. If three persons are in the room, it must be changed three times.

The effect of bad ventilation is well illustrated by the condition of the horses in the French army some years ago. With small close stables the mortality was 197 in every 1000 annually. The simple enlargement of the stables, and consequent increase of breathing-space, reduced the number in the course of time to 68 in every 1000, and later, from 1862 to 1866, with some attention paid to the air-supply, the number fell to 28½ per 1000.[13]

Necessity for a Constant Supply of Pure Air. When we consider that the food we eat and digest cannot nourish the body until it has been acted upon by oxygen in the lungs, and that this action must be constant, never ceasing, it will help us to understand the necessity for a constant supply of air such as shall furnish us a due proportion of the life-giving principle, oxygen, and which shall not contain impurities that interfere with its absorption.

We take into the lungs a mixture of nitrogen, oxygen, and carbonic acid. We give out a mixture which has lost some of its oxygen, and gained in carbonic acid. Now, unless the amount of oxygen is what it should be, the blood will not gain from an inspiration the amount it should receive, consequently it will be but imperfectly purified and able but imperfectly to nourish the body. So the whole system suffers, and if a person for a long time continues to breathe such an atmosphere, the condition of the body will become so reduced as to produce disease. Even though in other ways one lives wisely, all the factors of health multiplied together cannot withstand the one of impure air. We eat food three or four times daily. Some of us are very particular about its quality. We breathe air every instant of our lives, but generally we give but little consideration as to whether it is pure or impure.

Ventilation. No attempt will be made here to explain different devices for ventilation, but only to touch upon the principle it involves. Its objects are (1) to remove air which has been breathed once; (2) to remove the products of combustion, whether from fires, lamps, gas, or other sources; (3) to carry away all other substances which may be generated from any cause, in a room or building, as the impurities from manufacturing, those arising from decaying matter, and micro-organisms. In a climate where artificial warmth is necessary a part of the year, it is difficult to warm and ventilate a room at the same time, without causing unpleasant drafts; but with some knowledge of the necessity of ventilation, and of the properties of air, one may in some measure work out a scheme of ventilation adapted to the circumstances in which he finds himself.

There are always the doors and windows, which may be thrown wide open at intervals, and in many houses there are fireplaces. If a window be opened at the bottom at one side of a room, and another be opened at the top on an opposite side, a current of air will be established from the first window, passing through the room and out at the second. This plan will do very well in warm weather when the temperature outside is about the same as that of the room, but it would be impracticable in cold weather. Then we may resort to the very simple plan of placing a board about eight or ten inches wide across the window at the bottom and inside of the sash. Then when the lower half of the window is raised, a space is left between the upper and lower sashes, through which the air passes freely as it enters, and, being sent into the room in an upward direction, causes no draft. The board is for the purpose of closing the window below, and should fit quite close to the sash.

Fireplaces are good, though not perfect, ventilators. Then there are the preventive measures, such as burning the gas or lamp low at night, avoiding oil- and gas-stoves, etc.; the latter are the worst possible means of heating rooms, for not only do they draw oxygen for burning from the air, but they give out the polluting carbonic acid and other products of combustion, which in a coal- or wood-stove go up the chimney.

A well-ventilated room should have an inflow of warm, pure air, and a means for the removal of the same after it has been used, the current being so controlled that, although the air is kept in motion, there is no perceptible draft.

The plan for the heating and ventilation of the Johns Hopkins Hospital, Baltimore, Maryland, is a most admirable one. Air from out of doors is conveyed by a flue into a chamber in the wall, in which are coils of pipe filled with hot water. The air in passing over these becomes warm, and, rising, passes into the room to be heated through a register. On the opposite side of the room is a chimney-like flue, running to the top of the building and containing two registers, by the opening and closing of which the movements of the air in the room can be controlled. The temperature is maintained by the temperature of the water in the pipes, and the rapidity of the flow.[14]

The ventilation by this method of heating is the most perfect known to the author, who has lived for two years in a building thus supplied with warmth and fresh air. The rooms were invariably comfortable as to temperature, and the air as invariably sweet and pure.

MILK

Milk is one of our most perfect types of food, containing water and solids in such proportions as are known to be needful for the nourishment of the body. A proof of this is seen in the fact that it is the only food of the young of the Mammalia during the time of their greatest growth. It contains those food principles in such amounts as to contribute to the rapid formation of bone and the various tissues of the body, which takes place in infancy and childhood; but after this growth is attained, and the individual requires that which will repair the tissues and furnish warmth and energy, milk ceases to be a complete food.

Composition of Cow's Milk. The composition of cow's milk varies with the breed and age, care and feeding, of the animals. Cows which are kept in foul air in stables all the year, and fed upon bad food such as the refuse from breweries and kitchens, give a quality of milk which is perhaps more to be dreaded than that from any other source; for such animals are especially liable to disease, and are often infected with tuberculosis, pneumonia, and other fatal maladies. Cows are particularly susceptible to tuberculosis, and may convey it to human beings either in their milk or flesh. According to Dr. Miller, cow's milk contains the following ingredients:

Another analysis is that of Uffelmann:

Characteristics. Milk from healthy, well-nourished cows should be of full white color, opaque, and with a slightly yellowish tinge sometimes described as "cream white." It should vary but slightly in composition from the above analyses. The fat should not be less than 2.5%. The amount of fat may be easily determined with a Feser's lactoscope (Eimer and Amend, New York), directions for the use of which come with the instruments. It will generally vary from 3% to 4% in good milk. Should it fall below 2.5% the milk should be rejected as too poor for use. Such milk has probably been skimmed, or comes from unhealthy or poorly fed cows.

The specific gravity of milk should be from 1.027 to 1.033. This may be found with a Quevenne's lactometer. If it falls below 1.027, one has a right to claim that the milk has been watered or that the cows are in poor condition.[16]

The reaction of good milk varies from slightly alkaline to slightly acid or neutral. That from the same cow will be different on different days, even under the same apparent conditions of care, varying from one to the other, probably because of some difference in the nature of the food she has eaten. However, if the reaction is decidedly alkaline, and red litmus-paper becomes a distinct blue, the milk is not good, and possibly the animal is diseased. Should the reaction be decidedly acid, it shows that the milk has been contaminated, either from the air by long exposure, or from the vessels which held it, with those micro-organisms which by their growth produce an acid, a certain amount of which causes what is known as "souring."

Milk from perfectly healthy and perfectly kept cows is neutral, leaving both red and blue litmus-paper unchanged; but as a general thing milk is slightly acid, even when transported directly from the producer to the consumer and handled by fairly clean workmen in fairly clean vessels. Such milk two or three hours old when examined microscopically is found to contain millions of organisms. Milk is one of the best of foods for bacteria, many of the ordinary forms growing in it with exceeding rapidity under favorable conditions of temperature. Now it has been found that such milk, although it may not contain the seeds of any certain disease, sometimes causes in young children, and the sick, very serious digestive disturbances, and may thus become indirectly the cause of fatal maladies.[17]

All milk, unless it is positively known to be given by healthy, well-nourished animals, and kept in thoroughly cleaned vessels free from contamination, should be sterilized before using. Often the organisms found in milk are of disease-giving nature. In Europe and America many cases of typhoid fever, scarlatina, and diphtheria have been traced to the milk-supply. In fact milk and water are two of the most fruitful food sources of disease. It therefore immediately becomes apparent that, unless these two liquids are above suspicion, they should be sterilized before using. Boiling water for half an hour will render it sterile, but milk would be injured by evaporation and other changes produced in its constituents by such long exposure to so high a degree of heat. A better method, and one which should be adopted by all who understand something of the nature of bacteria, is to expose the milk for a longer time to a lower temperature than that of boiling.

To Sterilize Milk for Immediate Use. (1) Pour the milk into a granite-ware saucepan or a double boiler, raise the temperature to 190° Fahr., and keep it at that point for one hour. (2) As soon as done put it immediately into a pitcher, or other vessel, which has been thoroughly washed, and boiled in a bath of water, and cool quickly by placing in a pan of cold or iced water. A chemist's thermometer, for testing the temperature, may be bought at any pharmacy for a small sum, but if there is not one at hand, heat the milk until a scum forms over the top, and then keep it as nearly as possible at that temperature for one hour. Do not let it boil.

To Sterilize Milk which is not for Immediate Use. Put the milk into flasks or bottles with narrow mouths; plug them with a long stopper of cotton-wool, place the flasks in a wire frame to support them, in a kettle of cold water, heat gradually to 190° Fahr., and keep it at that temperature for one hour. Repeat this the second day, for although all organisms were probably destroyed during the first process, spores which may have escaped will have developed into bacteria. These will be killed by the second heating. Repeat again on the third day to destroy any life that may have escaped the first two.

Spores or resting-cells are the germinal cells from which new bacteria develop, and are capable of surviving a much higher temperature than the bacteria themselves, as well as desiccation and severe cold.[18] Some writers give a lower temperature than 190° Fahr. as safe for sterilization with one hour's exposure, but 190 may be relied upon. Milk treated by the last or "fractional" method of sterilization, as it is called, should keep indefinitely, provided of course the cotton is not disturbed. Cotton-wool or cotton batting in thick masses acts as a strainer for bacteria, and although air will enter, organisms will not.

All persons who buy milk, or in any way control milk-supplies, should consider themselves in duty bound to (1) ascertain by personal investigation the condition in which the cows are kept. If there is any suspicion that they are diseased, a veterinary surgeon should be consulted to decide the case. If they are healthy and well fed, they cannot fail to give good milk, and nothing more is to be done except to see that it is transported in perfectly cleansed and scalded vessels. (2) If it is impossible to obtain milk directly from the producer, and one is obliged to buy that from unknown sources, it should be sterilized the moment it enters the house. There is no other means of being sure that it will not be a bearer of disease. Not all such milk contains disease-producing organisms, but it all may contain them, and there is no safety in its use until all bacteria have been deprived of life.

DIGESTION

Definition. Digestion is the breaking up, changing, and liquefying of the food in the various chambers of the alimentary canal designed for that purpose. The mechanical breaking up is done principally by the teeth in the mouth, the chemical changes and liquefying by the various digestive fluids.[19]

Digestive Fluids. The digestive fluids are true secretions. Each is formed from the blood by a special gland for the purpose which never does anything else; they do not exist in the blood as such. Their flow is intermittent, taking place only when they are needed. The liver, however, is an exception to all the others. It is both secretory and excretory, and bile is formed all the time, but is most abundant during digestion.[20]

Saliva. The fluid which is mixed with the food in the mouth is secreted by a considerable number and variety of glands, the principal of which are the parotid, submaxillary, and sublingual. Smaller glands in the roof and sides of the mouth, in the tongue, and in the mucous membrane of the pharynx contribute to the production of saliva, the digestive fluid of the mouth. The flow from the parotid gland is greatest. The flow from all the glands is greatly increased when food is taken, especially if it be of good flavor. Sometimes the amount is increased by smell alone, as when a nice steak is cooking, or a savory soup, and sometimes the saliva is made copious by thought, as when we remember the taste of dishes eaten in the past, and we say, "It makes the mouth water just to think of them."

Amount of Saliva. According to Dalton the amount of saliva secreted every twenty-four hours is 42½ oz. Its reaction is almost constantly alkaline. It is composed of water, organic matter, and various mineral salts. Ptyalin is its active principle, and is called by some authors animal diastase, or starch converter.

Gastric Juice. Gastric juice is the digestive fluid of the stomach. It is acid. Its flow is intermittent, occurring only at times of digestion. Its active principle is pepsin.

It is worthy of notice here that the character of the digestive fluids when food is taken is different from what it is when the organs are at rest. For instance, the gastric juice which flows in abundance under the stimulus of food, is not like the fluid secreted when the stomach is collapsed and empty.

Pancreatic Juice. Pancreatic juice is the digestive juice of the pancreas, and is poured into the small intestine a short distance below the pyloric opening. Its reaction is alkaline. Its flow is entirely suspended during the intervals of digestion.

Bile. Bile, the fourth in order of the digestive liquids, is the secretion of the largest gland of the body—the liver. It is poured into the small intestine by a duct which empties side by side with the duct from the pancreas. The flow of bile is constant, but is greatest during digestion.

Intestinal Juice. Intestinal juice has been to physiologists a difficult subject of study. It is mingled with the salivary and gastric juices at the times of digestion, when it is most desirable to notice its action. Nearly all authorities agree that it is alkaline, and that its function is to complete the digestion of substances which may reach it in an undigested condition.

Mucus of Large Intestine. The mucus secreted by the large intestine is for lubricating only.

Digestion in Different Parts of the Alimentary Tract. Different substances in food are digested in different portions of the alimentary canal, and by different means. Let us begin in the mouth. Taking the classes of foods, starch, one of the carbohydrates, is the one most affected by the ptyalin, or animal diastase, of the saliva. So energetic is the action of ptyalin on starch that 1 part is sufficient to change 1000 parts. Starch is not acted upon by the gastric juice of the stomach at all; however, the continued action of the saliva is not probably interrupted in the stomach. The digestion of starch is completed by the action of the pancreatic and intestinal juices, and consists in its being changed into soluble glucose, which is absorbed in solution.

Sugar. Cane-sugar, or common sugar (also called sucrose), passes through the mouth, unchanged, to the stomach, where it is converted into glucose by the slow action of the acid (hydrochloric) of the gastric juice. Dilute hydrochloric acid has the same action on sugar outside of the stomach.

The action of pancreatic juice on sugar is very marked; it immediately changes cane-sugar into glucose. The effect of intestinal fluid is not well understood, but there is the general agreement that it does not change cane-sugar, neither is cane-sugar, as such, absorbed in the intestine. Bile does not affect it, therefore cane-sugar is digested or converted into glucose either by the stomach or pancreas, or both. It will now be seen that ultimately the same substance, glucose, is obtained from both starch and sugar.

Protein. We now come to the consideration of the digestion of the protein compounds, of which albumen may be taken as a type. Possibly no action except breaking up and moistening takes place in the mouth.[21] Its digestion begins in the stomach, where its structure is broken up and a separation and dissolution of the little sacs which hold it take place. The same thing is partially accomplished outside of the stomach when white of egg is slightly beaten and strained through a cloth. Gastric juice further acts on the albumen itself, forming it into what is called albumen peptone. The digestion of raw and carefully cooked albumen has been found to be carried on very rapidly in the stomach, and the change is essentially the same in both cases, but in favor of the slightly coagulated. When the albumen is rendered hard, fine, and close in consistency by over-cooking, then it is less easy of digestion than when raw.

Absorption. It is probable that the greater portion of the process of digestion and absorption of albumen takes place in the stomach.

Fibrin. Fibrin is also digested in the stomach, and made into fibrin peptone.

Casein. Liquid casein is immediately coagulated by gastric juice, both by the action of free acid and organic matter.

Gelatin. Gelatin is quickly dissolved by gastric juice, and afterward no longer has the property of forming jelly on cooling. Gelatin is more rapidly disposed of than the tissue from which it is produced.

Vegetable Protein. The digestion of the vegetable protein compounds, such as the gluten of wheat and the protein of the various grains, such as corn, oatmeal, etc., is undoubtedly carried on in the stomach, but they must be well softened and prepared by the action of heat and water, or they will not be digested anywhere; and often corn, beans, and grains of oatmeal are rejected entirely unchanged. Partially or imperfectly digested proteins are affected by intestinal juice. It is probable that the function of this fluid is to complete digestive changes in food which have already begun in the stomach.

To summarize: The digestion and absorption of nitrogenous compounds take place in both the stomach and the intestines.

NUTRITION

One of the important points to bring to the notice of pupils in the study of cookery is the phenomenon of nutrition. It is astonishing how vague are the ideas that many people have of why they eat food, and vaguer still are their notions of the necessity of air, pure and plenty. Once instruct the mind that it is the air we breathe and the food we eat which nourish the body, giving material for its various processes, for nervous and muscular energy, and for maintaining the constant temperature which the body must always possess in order to be in a state of health, and there is much more likelihood that the dignity and importance of proper cooking and proper food will not be overlooked.

A knowledge that the health and strength of a person depend largely upon what passes through his mouth, that even the turn of his thinking is modified by what he eats, should lead all intelligent women to make food a conscientious subject of study.

In general, by the term "nutrition" is meant the building up and maintaining of the physical framework of the body with all its various functions, and ultimately the mental and moral faculties which are dependent upon it, by means of nutriment or food.

The word is derived from the Latin nutrire, to nourish. The word "nurse" is from the same root, and in its original sense means one who nourishes, a person who supplies food, tends, or brings up.

Anything which aids in sustaining the body is food; therefore, air and water, the two most immediate necessities of life, may be, and often are, so classed.

Nutriment exclusive of air is received into the body by means of the alimentary canal. The great receiver of air is the lungs, but it also penetrates the body through the pores of the skin, and at these points carbonic acid is given off as in the lungs. The body is often compared to a steam-engine, which takes in raw material in the form of fuel and converts it into force or power. Food, drink, and air are the fuel of the body,—the things consumed; heat, muscular and intellectual energy, and other forms of power are the products.

Food, during the various digestive processes, becomes reduced to a liquid, and is then absorbed and conveyed, by different channels constructed for the purpose, into the blood, which contains, after being acted upon by the oxygen of the air in the lungs, all those substances which are required to maintain the various tissues, secretions, and, in fact, the life of the system.

Some of the ways in which the different kinds of food nourish the body have been found out by chemists and physiologists from actual experiments on living animals, such as rabbits, dogs, pigs, sheep, goats, and horses, and also on man. Often a scientist becomes so enthusiastic in his search for knowledge about a certain food that he gives his own body for trial. Much valuable work has been done in this direction during the last decade by Voit, Pettenkofer, Moleschott, Ranke, Payen, and in this country by Atwater.

No one can explain all the different intricate changes which a particle of food undergoes from the moment it enters the mouth until its final transformation into tissue or some form of energy; but by comparing the income with the outgo, ideas may be gained of what goes on in the economy of the body, and of the proportion of nutrients used, and some of the intricate and complex chemical changes which the different food principles undergo in the various processes of digestion, assimilation, and use.[22] Probably hundreds of changes take place in the body, in its various nutritive functions, of which nothing is known, or they are entirely unsuspected, so that if we do our utmost with the present lights which we possess for guidance to health, we shall still fall far short of completeness. The subject of food and nutrition, viewed in the light of bacteriology and chemistry, is one of the most inviting subjects of study of the day, and is worthy of the wisest thought of the nation.

The body creates nothing of itself, either of material or of energy; all must come to it from without. Every atom of carbon, hydrogen, phosphorus, or other elements, every molecule of protein, carbohydrate, or other compounds of these elements, is brought to the body with the food and drink it consumes, and the air it breathes. Like the steam-engine, it uses the material supplied to it. Its chemical compounds and energy are the compounds and energy of the food transformed (Atwater). A proof of this is seen in the fact that when the supply from without is cut off, the body dies. The raw material which the body uses is the air and food which it consumes, the greater portion of which is digested and distributed, through the medium of the blood, to all parts of the body, to renew and nourish the various tissues and to supply the material for the different activities of life.

Ways in which Food Supplies the Wants of the Body. Food supplies the wants of the body in several ways—(1) it is used to form the tissues of the body—bones, flesh, tendons, skin, and nerves; (2) it is used to repair the waste of the tissues; (3) it is stored in the body for future use; (4) it is consumed as fuel to maintain the constant temperature which the body must always possess to be in a state of health; (5) it produces muscular and nervous energy.[23] The amount of energy of the body depends upon two things—the amount in the food eaten, and the ability of the body to use it, or free it for use.

With every motion, and every thought and feeling, material is consumed, hence the more rapid wearing out of persons who do severe work, and of the nervous—those who are keenly susceptible to every change in their surroundings, to change of weather, even to the thoughts and feelings of those about them.

We easily realize that muscular force or energy cannot be maintained without nutriment in proper quality and amount. An underfed or starving man has not the strength of a well-fed person. He cannot lift the same weight, cannot walk as far, cannot work as hard. We do not as easily comprehend the nervous organism, and generally have less sympathy with worn-out or ill-nourished nerves than muscles, but the sensibilities and the intellectual faculties, of which the nerves and brain are but the instruments, depend upon the right nutrition of the whole system for their proper and healthful exercise.

So many factors enter into the make-up of a thought that it cannot be said that any particular kind of food will ultimately produce a poem; but of this we may be sure, that the best work, the noblest thoughts, the most original ideas, will not come from a dyspeptic, underfed, or in any way ill-nourished individual.

The classification of foods has been usually based upon the deductions of Prout that milk contains all the necessary nutrients in the best form and proportions, viz., the nitrogenous matters, fat, sugar, water, and salts; the latter being combinations of magnesium, calcium, potassium, sodium, and iron, with chlorin, phosphoric acid, and, in smaller quantities, sulphuric acid.

These different classes seem to serve different purposes in the body, and are all necessary for perfect nutrition. Some of them closely resemble each other in composition, but are quite different in their physiological properties, and in the ends which they serve. For instance, starch (C₆H₁₀O₅) has almost the same chemical formula as sugar (C₁₂H₂₂O₁₁), and yet the one cannot replace the other to its entire exclusion.

The Protein Compounds. In general it may be said that the carbohydrates are changed into fats, and are used for the production of force, and that the fats are stored in the body as fat and used as fuel. The protein compounds do all that can be done by the fats and carbohydrates, and in addition something more; that is, they form the basis of blood, muscle, sinew, skin, and bone. They are, therefore, the most important of all the food compounds. The terms "power-givers" and "energy-formers" are sometimes applied to them, because wherever power and energy are developed they are present, though not by any means the only substances involved in the evolution of energy. Probably the fats and carbohydrates give most of the material for heat and the various other forces of the body. In case of emergency, where these are deficient, the proteins are used; but protein alone forms the basis of muscle, tendons, skin, and other tissues. This the fats and carbohydrates cannot do (Atwater). The different tissues are known from analysis to contain this complex nitrogenous compound, protein. Now, since the body cannot construct this substance out of the simpler chemical compounds which come to it, it becomes perfectly evident that the diet must have a due proportion of protein in order to maintain the strength of the body. We get most of our proteins from the flesh of animals, and they in turn get it from plants, which construct it from the crude materials of earth and air.

The Extractives, usually classed with the protein compounds, such as meat extract, beef tea, etc., are not generally regarded as direct nutrients, but, like tea and coffee, are valuable as accessory foods, lending savor to other foods and aiding their digestion by pleasantly exciting the flow of the digestive fluids. They also act as brain and nerve stimulants, and perhaps also in some slight degree as nutrients.

The principal proteins or nitrogenous substances are albumen in various forms, casein both animal and vegetable, blood fibrin, muscle fibrin, and gelatin. All except the last are very much alike, and probably can replace one another in nutrition.

Modern chemists agree that nitrogen is a necessary element in the various chemical and physiological actions which take place in the body to produce heat, muscular energy, and the other powers. Every structure in the body in which any form of energy is manifested is nitrogenous. The nerves, muscles, glands, and the floating cells[24] in the various liquids are nitrogenous. That nitrogen is necessary to the different processes of the system, is shown by the fact that if it be cut off, these processes languish. This may not occur immediately, for the body always has a store of nitrogen laid by for emergencies which will be consumed first, but it will occur as soon as these have been consumed. The energy of the body is measured by its consumption of oxygen. Motion and heat may be owing to the oxidation of fat, or of starch, or of nitrogenous substances; but whatever the source, the direction is given by the nitrogenous structure—in other words, nitrogen is necessary to all energy generated in the body.

Protein matter nourishes the organic framework, takes part in the generation of energy, and may be converted into non-nitrogenous substances.[25] The necessity of the protein compounds is emphasized when we realize that about one half of the body is composed of muscle, one fifth of which is protein, and the nitrogen in this protein can be furnished only by protein, since neither fats nor carbohydrates contain it. It is therefore evident that the protein-containing foods, such as beef, mutton, fish, eggs, milk, and others, are our most valued nutrients. Our daily diet must contain a due proportion.

The proteins are all complex chemical compounds, which in nutrition become reduced to simple forms, and are then built up again into flesh. The animal foods are in the main the best of the protein compounds, for they are rich in nitrogenous matter, are easily digested, and from their composition and adaptability are most valuable in maintaining the life of the body.

A diet of lean meat alone serves to build up tissue. If nothing else be taken, the stored-up fat of the body will be consumed, and the person will become thin.[26] Athletes while in training take advantage of this fact, and are allowed to eat only such food as shall furnish the greatest amount of strength and muscular energy with a minimum of fat. The lean of beef and mutton, with a certain amount of bread, constitute the foundation of the diet.

Fats. Most of the fatty substances of food are liquefied at the temperature of the body. When eaten in the form of adipose tissue, as the fat of beef and mutton, the vesicles or cells in which the fat is held are dissociated or dissolved, the fat is set free, and mingles with the digesting mass. This is done in the stomach, and is a preparation for its further change in the intestines.

Fats are not dissolved—that is, in the sense in which meats and other foods are dissolved—in the process of digestion; the only change which they undergo is a minute subdivision caused principally by the action of the pancreatic juice. In this condition of fine emulsion they are taken up by the lacteals; they may also be absorbed by the blood-vessels.

It has been found that fat emulsions pass more easily through membranes which have been moistened with bile, and it is probable that the function of bile is partly to facilitate the absorption of fat. That the pancreatic juice is the chief agent in forming fats into emulsion was discovered in 1848. Bile is, however, essential to their perfect digestion, and we may therefore say that they are digested by the united action of the pancreatic juice and the bile.[27]

Fat forms in the body fatty tissues, and serves for muscular force and heat; it is also necessary to nourish nerves and other tissues,—in fact, without it healthy tissues cannot be formed. A proper amount of fat is also a sort of albumen sparer.

It is probable that the fat which is used in the body either to be stored away or for energy, is derived from other sources than directly from the fat eaten. From experiments made by Lawes and Gilbert on pigs, it is evident that the excess of fat stored in their bodies must be derived from some other source than the fat contained in their food, and must be produced partly from nitrogenous matter and partly from carbohydrates, or, at least, that the latter play a part in its formation. It would appear from this that life might be maintained on starch, water, salts, and meat free from fat; but although the theory seems a good one, practically it is found in actual experiment[28] that nutrition is impaired by a lack of fat in the diet. The ill effects were soon seen, and immediate relief was given when fat was added to the food. Besides, in the food of all nations starch is constantly associated with some form of fat; bread with butter; potatoes with butter, cream, or gravy; macaroni and polenta with oil, and so forth. A man may live for a time and be healthy with a diet of albuminoids, fats, salts, and water, but it has not yet been proved that a similar result will be produced by a diet of albuminoids, carbohydrates, salts, and water without fat. Fat is necessary to perfect nutrition. Health cannot be maintained on albuminoids, salts, and water alone; but, on the other hand, cannot be maintained without them.

Probably the value of fats, as such, is dependent upon the ease with which they are digested. The fats eaten are not stored in the body directly, but the body constructs its fats from those eaten, and from other substances in food,—according to some authorities from the carbohydrates and proteids, and according to others from proteids alone.

Fats are stored away as fat, furnish heat, and are used for energy; at least, it is probable that at times they are put to the latter use. The fats laid by in the body for future use last in cases of starvation quite a long time, depending, of course, upon the amount. At such times a fat animal will live longer than a lean one.

Doubtless in the fat of food the body finds material for its fats in the most easily convertible form. Of the various fatty substances taken, some are more easily assimilated than others. Dr. Fothergill, in "The Town Dweller," says that the reason that cod-liver oil is given to delicate children and invalids is, that it is more easily digested than ordinary fats, but it is an inferior form of fat; the next most easily digested is the fat of bacon. When a child can take bread crumbled in a little of this fat, it will not be necessary to give him cod-liver oil. Bacon fat is the much better fat for building tissues. Then comes cream, a natural emulsion, and butter. He further says there is one form of fat not commonly looked at in its proper dietetic value, and that is "toffee." It is made of butter, sugar, and sometimes a portion of molasses. A quantity of this, added to the ordinary meals, will enable a child in winter to keep up the bodily heat. The way in which butter in the form of toffee goes into the stomach is particularly agreeable.

Carbohydrates. The principal carbohydrates are starch, dextrine, cane-sugar or common table sugar, grape-sugar, the principal sugar in fruits, and milk-sugar, the natural sugar in milk. They are substances made up, as before stated, of carbon, hydrogen, and oxygen, but no nitrogen. They are important food substances, but are of themselves incapable of sustaining life.

The carbohydrates, both starch and sugar, in the process of digestion are converted into glucose. This is stored in the liver in the form of glycogen, which the liver has the power of manufacturing; it then passes into the circulation, and is distributed to the different parts of the body as it is needed. (The liver also has the power of forming glycogen out of other substances than sugar, and it is pretty conclusively proved that it is from proteids, and not from fats. Carnivorous animals, living upon flesh alone, are found to have glycogen in their bodies.)

It is impossible to assign any especial office to the different food principles; that is, it cannot be said that the carbohydrates perform a certain kind of work in the body and nothing else, or that the proteids or fats do. The human body is a highly complex and intricate organism, and its maintenance is carried on by complex and mysterious processes that cannot be followed, except imperfectly; consequently, we must regard the uses of foods in the body as more or less involved in obscurity. It is, however, generally understood that the proteids, fats, and carbohydrates each do an individual work of their own better than either of the others can do it. They are all necessary in due amount to the nutrition of the body, and doubtless work together as well as in their separate functions. They are, however, sometimes interchangeable, as, for instance, in the absence of the carbohydrates, proteids will do their work. The carbohydrates are eminently heat and energy formers, and they also act as albumen sparers.

The body always has a store of material laid by for future use. If it were not for this a person deprived of food would die immediately, as is the case when he is deprived of oxygen. (Air being ever about us, and obtainable without effort or price, there is no need for the body to lay by an amount of oxygen; consequently only a very little is stored, and that in the blood.)

The great reserve forces of the body are in the form of fatty tissues, and glycogen, or the stored-away carbohydrates of the liver; the latter is given out to the body as it is needed during the intervals of eating to supply material for the heat and energy of daily consumption, and in case of starvation. That they are true reserves is shown by the fact that they disappear during deprivation of food. The glycogen, or liver-supply, disappears first; then the fat (Martin). The heat of the body can be maintained on these substances, and a certain amount of work done, although no food except water be taken.

The principal function of the liver is to form glycogen to be stored away. It constantly manufactures it, and as constantly loses it to the circulation. Glycogen is chemically allied to starch, having the same formula (C₆H₁₀O₅), but differing in other ways. Its quantity is greatest about two hours after a full meal; then it gradually falls, but increases again when food is again taken. Its amount also varies with the kind of food eaten: fats and proteids by themselves give little, but starch and sugars give much, for it is found in greatest quantity when these form a part of the diet.

Inorganic Matter and Vegetable Acids. Water and other inorganic matter, as the salts of different kinds, and vegetable acids, as vinegar and lemon-juice, can scarcely be said to be digested. Water is absorbed, and salts are generally in solution in liquids and are absorbed with them.

Water is found in all parts of the body, even in the very solid portions, as the bones and the enamel of the teeth; it also constitutes a large proportion of its semisolids and fluids, some of which are nearly all water, as the perspiration and the tears.

Water usually is found combined with some of the salts, which seem to act as regulators of the amount which shall be incorporated into a tissue. Water is a necessary constituent of all tissues, giving them a proper consistency and elasticity. The power of resistance of the bones could not be maintained without it. It is also valuable as a food solvent, assisting in the liquefying of different substances, which are taken up by the various absorbent tubes, conveyed into the blood, and so circulated through the body. Most of the water of the body is taken into it from without, but it is also formed in the body by the union of hydrogen and oxygen.[29]

Sodium chlorid, or common salt, is found in the blood and other fluids, and in the solids of the body, except the enamel of the teeth; it occurs in greatest proportion in the fluids. The part that this salt plays in nutrition is not altogether understood. "Common salt is intermediate in certain general processes, and does not participate by its elements in the formation of organs" (Liebig). Salt is intimately associated with water, which plays an intermediate part also in nutrition, being a bearer or carrier of nutritious matters through the body.

Salt seems to regulate the absorption and use of nutrients. It is found in the greatest quantity in the blood and chyle. It doubtless facilitates digestion by rendering foods more savory, and thus causing the digestive juices to flow more freely. Sodium chlorid is contained in most if not all kinds of food, but not in sufficient quantity to supply the wants of the body; it therefore becomes a necessary part of a diet.

Potassium chlorid has similar uses to sodium chlorid, although not so generally distributed through the body. It is found in muscle, liver, milk, chyle, blood, mucus, saliva, bile, gastric juice, and one or two other fluids.

Calcium phosphate is found in all the fluids and solids of the body, held in solution in them by the presence of CO₂; both it and calcium carbonate enter largely into the structure of the bones.

Sodium carbonate, magnesium phosphate, and other salts play important parts in nutrition.

The various salts influence chemical change as well as act in rendering food soluble. For example, serum albumen, the chief proteid of the blood, is insoluble in pure water, but dissolves easily in water which has a little neutral salts in it.[30] Salts also help to give firmness to the teeth and bones.

To recapitulate, food is eaten, digested, assimilated, and consumed or transformed in the body by a series of highly intricate and complex processes. It is for the most part used for the different powers and activities of the system; there is, however, always a small portion which is rejected as waste. The first change is in the mouth, where the food is broken up and moistened and the digestion of starch begins; these changes continue in the stomach until the whole is reduced to a more or less liquid mass. As the contents of the stomach pass little by little into the duodenum, the mass becomes more fluid by the admixture of bile, pancreatic juice, and intestinal juice, and, as it passes along, absorption takes place; the mass grows darker in color and less fluid, until all good material is taken up and only waste left, which is rejected from the body.

That portion of the food which is not affected by the single or united action of the digestive fluids is chiefly of vegetable origin. Hard seeds, such as corn, and the outer coverings of grains, such as the husk of oatmeal and those parts which are composed largely of cellulose, pass through the intestinal canal without change.

It may be remarked here that since the digestive mechanism is so perfect a structure, and will try to dissolve anything given it, and select only that which is good, why should there be the necessity of giving any special attention to preparing food before it is eaten? The answer is that the absorptive vessels cannot take up what is not there, neither can the digestive organs supply what the food lacks; therefore, the food must contain in suitable proportions all substances needed by the body. Also, food which contains a large proportion of waste, or is difficult of digestion from over or under cooking, or is unattractive by insipidity or unsavoriness, overworks these long-suffering organs (the extra power or force needed being drawn from the blood), and causes the whole system to suffer. Mal-nutrition, with the long line of evils which it entails, is the cause, direct or indirect, of most of the sickness in the world, for it reduces the powers of the system, and thus enfeebles its resistance to disease.

Ideal Diet. "The ideal diet is that combination of food which, while imposing the least burden upon the body, supplies it with exactly sufficient material to meet its wants" (Schuster).

In general the digestibility of foods may be summarized as follows:

1. The protein of ordinary animal foods is very readily and completely digestible.

2. The protein of vegetable foods is much less easily digested than that of animal foods.

3. The fat of animal foods may at times fail of digestion.

4. Sugar and starch are easy of digestion.

5. Animal foods have the advantage of vegetable foods in that they contain more protein, and that their protein is more easily digested. (Atwater.)