The body is constantly in need of new material. Oxidation, as shown in the
preceding chapter, rapidly destroys substances at the cells, and these
have to be replaced. Upon this renewal depends the supply of energy.
Moreover, there is found to be an actual breaking down of the living
material, or protoplasm, in the body. While this does not destroy the
cells, as is sometimes erroneously stated, it reduces the quantity of the
protoplasm and makes necessary a process of repair, or rebuilding, of the
tissues. This also requires new material. Finally, substances, such as
water and common salt, are required for the aid which they render in the
general work of the body. Since these are constantly being lost in one way
or another, they also must be replaced. These different needs of the body
for new materials are supplied through
*The Foods.*--Foods are substances that, on being taken into the healthy
body, are of assistance in carrying on its work. This definition properly
includes oxygen, but the term is usually limited to substances introduced
through the digestive organs. As suggested above, foods serve at least
three purposes:
1. They, with oxygen, supply the body with energy.
2. They provide materials for rebuilding the tissues.
3. They supply materials that aid directly or indirectly in the general
work of the body.
*The Simple Foods, or Nutrients.*--From the great variety of things that
are eaten, it might appear that many different kinds of substances are
suitable for food. When our various animal and vegetable foods are
analyzed, however, they are found to be similar in composition and to
contain only some five or six kinds of materials that are essentially
different. While certain foods may contain only a single one of these,
most of the foods are mixtures of two or more. These few common materials
which, in different proportions, form the different things that are eaten,
are variously referred to as simple foods, food-stuffs, and _nutrients_,
the last name being the one generally preferred. The different classes of
nutrients are as follows:
Nutrients:
Proteids
(Albuminoids)
Carbohydrates
Fats
Mineral salts
Water
It is now necessary to become somewhat familiar with the different
nutrients and the purposes which they serve in the body.
*Proteids.*--The proteids are obtained in part from the animal and in part
from the plant kingdom, there being several varieties. A well-known
variety, called _albumin_, is found in the white of eggs and in the plasma
of the blood, while the muscles contain an abundance of another variety,
known as _myosin_. Cheese consists largely of a kind of proteid, called
_casein_, which is also present in milk, but in a more diluted form. If a
mouthful of wheat is chewed for some time, most of it is dissolved and
swallowed, but there remains in the mouth a sticky, gum-like substance.
This is _gluten_, a form of proteid which occurs in different grains.
Again, certain vegetables, as beans, peas, and peanuts, are rich in a kind
of proteid which is called _legumen_.
Proteids are compounds of carbon, hydrogen, oxygen, nitrogen, and a small
per cent of sulphur. Certain ones (the nucleo-proteids from grains) also
contain phosphorus. All of the proteids are highly complex compounds and
form a most important class of nutrients.
*Purposes of Proteids.*--The chief purpose of proteids in the body is to
rebuild the tissues. Not only do they supply all of the main elements in
the tissues, but they are of such a nature chemically that they are
readily built into the protoplasm. They are absolutely essential to life,
no other nutrients being able to take their place. An animal deprived of
them exhausts the proteids in its body and then dies. In addition to
rebuilding the tissues, proteids may also be oxidized to supply the body
with energy.
*Albuminoids* form a small class of foods, of minor importance, which are
similar to proteids in composition, but differ from them in being unable
to rebuild the tissues. Gelatin, a constituent of soup and obtained from
bones and connective tissue by boiling, is the best known of the
albuminoid foods. On account of the nitrogen which they contain, proteids
and albuminoids are often classed together as _nitrogenous foods_.
*Carbohydrates.*--While the carbohydrates are not so essential to life as
are the proteids, they are of very great value in the body. They are
composed of carbon, hydrogen, and oxygen, and are obtained mainly from
plants. There are several varieties of carbohydrates, but they are similar
in composition. All of those used as food to any great extent are starch
and certain kinds of sugar.
*Starch* is the carbohydrate of greatest importance as a food, and it is
also the one found in the greatest abundance. All green plants form more
or less starch, and many of them store it in their leaves, seeds, or roots
(Fig. 60). From these sources it is obtained as food. _Glycogen_, a
substance closely resembling starch, is found in the body of the oyster.
It is also formed in the liver and muscles of the higher animals, being
prepared from the sugar of the blood, and is stored by them as reserve
food (Chapter XI). Glycogen is, on this account, called _animal starch_.
Starch on being eaten is first changed to sugar, after which it may be
converted into glycogen in the liver and in the muscles.
[Fig. 60]
Fig. 60--*Starch grains* in cells of potato as they appear under the
microscope. (See practical work.)
*Sugars.*--There are several varieties of sugar, but the important ones
used as foods fall into one or the other of two classes, known as _double
sugars_ (disaccharides) and _single sugars_ (monosaccharides). To the
first class belong _cane sugar_, found in sugar cane and beets, _milk
sugar_, found in sweet milk, and _maltose_, a kind of sugar which is made
from starch by the action of malt. The important members of the second
class are _grape sugar_, or dextrose, and _fruit sugar_, or levulose, both
of which are found in fruits and in honey.
The most important of all sugars, so far as its use in the body is
concerned, is _dextrose_. To this form all the other sugars, and starch
also, are converted before they are finally used in the body. The close
chemical relation between the different carbohydrates makes such a
conversion easily possible.
*Fats.*--The fats used as foods belong to one or the other of two classes,
known as solid fats and oils. The solid fats are derived chiefly from
animals, and the oils are obtained mostly from plants. Butter, the fat of
meats, olive oil, and the oil of nuts are the fats of greatest importance
as foods. Fats, like the carbohydrates, are composed of carbon, hydrogen,
and oxygen. They are rather complex chemical compounds, though not so
complex as proteids. Since neither fats nor carbohydrates contain
nitrogen, they are frequently classed together as _non-nitrogenous_ foods.
*Purpose Served by Carbohydrates, Fats, and Albuminoids.*--These classes of
nutrients all serve the common purpose of supplying energy. By uniting
with oxygen at the cells, they supply heat and the other forms of bodily
force. This is perhaps their only purpose.(50) Proteids also serve this
purpose, but they are not so well adapted to supplying energy as are the
carbohydrates and the fats. In the first place they do not completely
oxidize and therefore do not supply so much energy; and, in the second
place, they form waste products that are removed with difficulty from the
body.
*Mineral Salts and their Uses.*--Mineral salts are found in small
quantities in all of the more common food materials, and, as a rule, find
their way into the body unnoticed. They supply the elements which are
found in the body in small quantities and serve a variety of purposes.(51)
Calcium phosphate and calcium carbonate are important constituents of the
bones and teeth; and the salts containing iron renew the hemoglobin of the
blood. Others perform important functions in the vital processes. The
mineral compound of greatest importance perhaps is sodium chloride, or
common salt.(52) This is a natural constituent of most of our foods, and
is also added to food in its preparation for the table. When it is
withheld from animals for a considerable length of time, they suffer
intensely and finally die. It is necessary in the blood and lymph to keep
their constituents in solution, and is thought to play an important role
in the chemical changes of the cells. It is constantly leaving the body as
a waste product and must be constantly supplied in small quantities in the
foods.
*Importance of Water.*--Water finds its way into the body as a pure liquid,
as a part of such mixtures as coffee, chocolate, and milk, and as a
constituent of all our solid foods. (See table of foods, page 126.) It is
also formed in the body by the oxidation of hydrogen. It passes through
the body unchanged, and is constantly being removed by all the organs of
excretion. Though water does not liberate energy in the body nor build up
the tissues in the sense that other foods do, it is as necessary to the
maintenance of life as oxygen or proteids. It occurs in all the tissues,
and forms about 70 per cent of the entire weight of the body. Its presence
is necessary for the interchange of materials at the cells and for keeping
the tissues soft and pliable. As it enters the body, it carries digested
food substances with it, and as it leaves it is loaded with wastes. Its
chief physiological work, which is that of a _transporter of material_,
depends upon its ability to dissolve substances and to flow readily from
place to place.
*Relative Quantity of Nutrients Needed.*--Proteids, carbohydrates, and fats
are the nutrients that supply most of the body's nourishment. The most
hygienic diet is the one which supplies the proteids in sufficient
quantity to rebuild the tissues and the carbohydrates and fats in the
right amounts to supply the body with energy. Much experimenting has been
done with a view to determining these proportions, but the results so far
are not entirely satisfactory. According to some of the older estimates, a
person of average size requires for his daily use five ounces of proteid,
two and one half ounces of fat, and fifteen ounces of carbohydrate. Recent
investigations of this problem seem to show that the body is as well, if
not better, nourished by a much smaller amount of proteid--not more than
two and one half ounces (60 grams) daily.(53)
While there is probably no necessity for the healthy individual's taking
his proteid, fat, and carbohydrate in _exact_ proportions (if the
proportions best suited to his body were known), the fact needs to be
emphasized that proteids, although absolutely necessary, should form but a
small part (not over one fifth) of the daily bill of fare. In recognition
of this fact is involved a principle of health and also one of economy.
The proteids, especially those in meats, are the most expensive of the
nutrients, whereas the carbohydrates, which should form the greater bulk
of one's food, are the least expensive.
*Effects of a One-sided Diet.*--The plan of the body is such as to require
a _mixed diet_, and all of the great classes of nutrients are necessary.
If one could subsist on any single class, it would be proteids, for
proteids are able both to rebuild tissue and to supply energy. But if
proteids are eaten much in excess of the body's need for rebuilding the
tissues, and this excess is oxidized for supplying energy, a strain is
thrown upon the organs of excretion, because of the increase in the
wastes. Not only is there danger of overworking certain of these organs
(the liver and kidneys), but the wastes may linger too long in the body,
causing disorder and laying the foundation for disease. On the other hand,
if an insufficient amount of proteid is taken, the tissues are improperly
nourished, and one is unable to exert his usual strength. What is true of
the proteids is true, though in a different way, of the other great
classes of foods. A diet which is lacking in proteid, carbohydrate, or
fat, or which has any one of them in excess, is not adapted to the
requirements of the body.
*Composition of the Food Materials.*--One who intelligently provides the
daily bill of fare must have some knowledge of the nature and quantity of
the nutrients present in the different materials used as food. This
information is supplied by the chemist, who has made extensive analyses
for this purpose. Results of such analyses are shown in Table 1 (page
126), which gives the percentage of proteids, fats, carbohydrates, water,
and mineral salts in the edible portions of the more common of our foods.
[Fig. 61]
Fig. 61--Relative proportions of different nutrients in well-known foods.
*Food Supply to the Table.*--The main problem in supplying the daily bill
of fare is that of securing through the different food materials the
requisite amounts of proteids, carbohydrates, and fats. In this matter a
table showing the composition of foods can be used to great advantage.
Consulting the table on page 126, it is seen that large per cents of
proteids are supplied by lean meat, eggs, cheese, beans, peas, peanuts,
and oatmeal, while fat is in excess in fat meat, butter, and nuts (Fig.
61). Carbohydrates are supplied in abundance by potatoes, rice, corn,
sugar, and molasses. The different cereals also contain a large percentage
of carbohydrates in the form of starch.
TABLE I. THE COMPOSITION OF
FOOD MATERIALS(54)
Food Water Solids Proteid Fat Carbohydrates Mineral Heat
Materials Matter Value of
One
Pound
Animal Per cent Per cent Per cent Per cent Per cent Per cent Calories(55)
foods,
edible
portion
Beef: 63.9 36.1 19.5 15.6 ... 1 1020
Shoulder
Rib 48.1 51.9 15.4 35.6 ... .9 1790
Sirloin 60 40 18.5 20.5 ... 1 1210
Round 68.2 31.8 20.5 10.1 ... 1.2 805
Veal: 68.8 31.2 20.2 9.8 ... ... 790
Shoulder
Mutton: 61.8 38.2 18.3 19 ... .9 1140
Leg
Loin 49.3 50.7 15 35 ... .7 1755
Pork: 50.3 49.7 16 32.8 ... .9 1680
Shoulder
Ham, 41.5 58.5 16.7 39.1 ... 2.7 1960
salted,
smoked
Fat, 12.1 87.9 .9 82.8 ... 4.2 3510
salted
Sausage: 41.5 58.8 13.8 42.8 ... 2.2 2065
Pork
Bologna 62.4 37.6 18.8 42.8 ... 3 1015
Chicken 72.2 27.8 24.4 1 ... 1.4 540
Eggs 73.8 26.2 14.9 10.5 ... .8 721
Milk 87 13 3.6 4 4.7 .7 325
Butter 10.5 89 .6 85 .5 .3 3515
Cheese: 30.2 69.8 28.3 35.5 1.8 4.2 2070
Full
cream
Skim milk 41.3 58.7 38.4 6.8 6.9 4.6 1165
Fish: 82.6 17.4 15.8 .5 ... 1.2 310
Codfish
Salmon 63.6 36.4 21.6 13.4 ... 1.4 965
Oysters 87.1 12.9 6 1.2 3.7 2 230
Vegetable
foods
Wheat 12.5 87.5 11 1.1 74.9 .5 1645
flour
Graham 13.1 86.9 11.7 1.7 71.7 1.8 1635
flour
(wheat)
Rye flour 13.1 86.9 6.7 .8 78.7 .7 1625
Buckwheat 14.6 85.4 6.9 1.4 76.1 1 1605
flour
Oatmeal 7.6 92.4 15.1 7.1 68.2 2 1850
Cornmeal 15 85 9.2 3.8 70.6 1.4 1645
Rice 12.4 87.6 7.4 .4 79.4 .4 1630
Peas 12.3 87.7 26.7 1.7 56.4 2.9 1565
Beans 12.6 87.4 23.1 2 59.2 3.1 1615
Potatoes 78.9 21.1 2.1 .1 17.9 1 375
Tomatoes 95.3 4.7 .8 .4 3.2 .3 80
Apples 83.2 16.8 .2 .4 15.9 .3 315
Sugar, 2 98 ... ... 97.8 .3 1820
granulated
White 32.3 67.7 8.2 1.7 56.3 .0 1280
bread
(wheat)
Peanuts 9.2 90.8 25.8 24.4 38.6 2 2560
Almonds 4.8 95.2 21 17.3 54.9 2 3030
Walnuts 2.5 97.5 16.6 16.1 63.4 1.4 3285
(English)
_Variety_ in the selection of foods for the table is an essential feature,
but this should not increase either the work or the expense of supplying
the meals. Each single meal can, and should, be simple in itself and, at
the same time, differ sufficiently from the meal preceding and the one
following to give the necessary variety in the course of the day. The bill
of fare should, of course, include fruits (for their tonic effects) and
very small amounts perhaps of substances which stimulate the appetite,
such as pepper, mustard, etc., known as condiments.
*Purity of Food.*--The fact that many of the food substances are perishable
makes it possible for them to be eaten in a slightly decayed condition.
Such substances are decidedly unwholesome (some containing poisons) and
should be promptly rejected. Not only do fresh meats, fruits, and
vegetables need careful inspection, but canned and preserved goods as
well. If canned foods are imperfectly sealed or if not thoroughly cooked
in the canning process, they decay and the acids which they generate act
on the metals lining the cans, forming poisonous compounds. The contents
of "tin" cans should for this reason be transferred to other vessels as
soon as opened.
Foods are also rendered impure or weakened through adulteration, the
watering of milk being a familiar example. The manufacture of jellies,
preserves, sirups, and various kinds of pickles and condiments has perhaps
afforded the largest field for adulterations, although it is possible to
adulterate nearly all of the leading articles of food. A long step in the
prevention of food and drug adulteration was taken in this country by the
passage of the _Pure Food Law_. By forcing manufacturers of foods and
medicines to state on printed labels the composition of their products,
this law has made it possible for the consumer to know what he is
purchasing and putting into his body.
*Alcohol not a Food.*--Many people in this and other countries drink in
different beverages, such as whisky, beer, wine, etc., a varying amount of
alcohol. This substance has a temporary stimulating or exciting effect,
and the claim has been made that it serves as a food. Recently it has been
shown that alcohol when introduced into the body in small quantities and
in a greatly diluted form, is nearly all oxidized, yielding energy as does
fat or sugar. If no harmful effects attended the use of alcohol, it might
on this account be classed as a food. But alcohol is known to be harmful
to the body. When used in large quantities, it injures nearly all of the
tissues, and when taken habitually, even in small doses, it leads to the
formation of the alcohol habit which is now recognized and treated as a
disease. This and other facts show that alcohol is not adapted to the body
plan of taking on and using new material (Chapter XI), and no substance
lacking in this respect can properly be classed as a food.(56) Instead of
classing alcohol as a food, it should be placed in that long list of
substances which are introduced into the body for special purposes and
which are known by the general name of
*Drugs.*--Drugs act strongly upon the body and tend to bring about unusual
and unnatural results. Their use should in no way be confused with that of
foods. If taken in health, they tend to disturb the physiological balance
of the body by unduly increasing or diminishing the action of the
different organs. In disease where this balance is already disturbed, they
may be administered for their counteractive effects, but always under the
advice and direction of a physician. Knowing the nature of the disturbance
which the drug produces, the physician can administer it to advantage,
should the body be out of physiological balance, or diseased. Not only are
drugs of no value in health, but their use is liable to do much harm.
NATURE OF DIGESTION
Before the nutrients can be oxidized at the cells, or built into the
protoplasm, they undergo a number of changes. These are necessary for
their entrance into the body, for their distribution by the blood and the
lymph, and for the purposes which they finally serve. The first of these
changes is preparatory to the entrance of the nutrients and is known as
_digestion_. The organs which bring about this change, called digestive
organs, have a special construction which adapts them to their work. It
will assist materially in understanding these organs if we first learn
something of the nature of the work which they have to perform.
*How the Nutrients get into the Body.*--The nature of digestion is
determined by the conditions affecting the entrance of nutrients into the
body. Food in the stomach and air in the lungs, although surrounded by the
body, are still outside of what is called the _body proper_. To gain
entrance into the body proper, a substance must pass through the body
wall. This consists of the skin on the outside and of the mucous linings
of the air passages and other tubes and cavities which are connected with
the external surface.
To get from the digestive organs into the blood, the nutrients must pass
through the mucous membrane lining these organs and also the walls of
blood or lymph vessels. Only _liquid materials_ can make this passage. It
is necessary, therefore, to reduce to the liquid state all nutrients not
already in that condition. _This reduction to the liquid state constitutes
the digestive process_.
*How Substances are Liquefied.*--While the reduction of solids to the
liquid state is accomplished in some instances by heating them until they
melt, they are more frequently reduced to this state by subjecting them to
the action of certain liquids, called _solvents_. Through the action of
the solvent the minute particles of the solid separate from each other and
disappear from view. (Shown in dropping salt in water.) At the same time
they mix with the solvent, forming a _solution_, from which they separate
only with great difficulty. For this reason solids in solution can diffuse
through porous partitions along with the solvents in which they are
dissolved (page 73).
By digestion the nutrients are reduced to the form of a solution. _The
process is_, simply speaking, _one of dissolving_. The liquid employed as
_the digestive solvent is water_. The different nutrients dissolve in
water, mixing with it to form a solution which is then passed into the
body proper.
*Digestion not a Simple Process.*--Digestion is by no means a simple
process, such, for instance, as the dissolving of salt or sugar in water.
These, being soluble in water, dissolve at once on being mixed with a
sufficient amount of this liquid. The majority of the nutrients, however,
are insoluble in water and are unaffected by it when acting alone. Fats,
starch, and most of the proteids do not dissolve in water. Before these
can be dissolved they have to be changed chemically and converted into
substances that are _soluble in water_. This complicates the process and
_prevents the use of water alone_ as the digestive solvent.
*A Similar Case.*--If a piece of limestone be placed in water, it does not
dissolve, because it is insoluble in water. If hydrochloric acid is now
added to the water, the limestone is soon dissolved (Fig. 62). (See
Practical Work.) It seems at first thought that the acid dissolves the
limestone, but this is not the case. The acid produces a chemical change
in the limestone (calcium carbonate) and converts it into a compound
(calcium chloride) that is soluble in water. As fast as this is formed it
is dissolved by the water, which is the real solvent in the case. The acid
simply plays the part of a chemical converter.
[Fig. 62]
Fig. 62--The dissolving of limestone in water containing acid, suggesting
the double action in the digestion of most foods.
*The Digestive Fluids.*--Several fluids--saliva, gastric juice, pancreatic
juice, bile, and intestinal juice--are employed in the digestion of the
food. The composition of these fluids is in keeping with the nature of the
digestive process. While all of them have water for their most abundant
constituent, there are dissolved in the water small amounts of active
chemical agents. It is the work of these agents to convert the insoluble
nutrients into substances that are soluble in water. The digestive fluids
are thus able to act in a _double_ manner on the nutrients--to change them
chemically and to dissolve them. The chemical agents which bring about the
changes in the nutrients are called _enzymes_, or digestive ferments.
*Foods Classed with Reference to Digestive Changes.*--With reference to the
changes which they undergo during digestion, foods may be divided into
three classes as follows:
1. Substances already in the liquid state and requiring no digestive
action. Water and solutions of simple foods in water belong to this class.
Milk and liquid fats, or oils, do not belong to this class.
2. Solid foods soluble in water. This class includes common salt and
sugar. These require no digestive action other than dissolving in water.
3. Foods that are insoluble in water. These have first to be changed into
soluble substances, after which they are dissolved.
*Summary.*--Materials called foods are introduced into the body for
rebuilding the tissues, supplying energy, and aiding in its general work.
Only a few classes of substances, viz., proteids, carbohydrates, fats,
water, and some mineral compounds have all the qualities of foods and are
suitable for introduction into the body. Substances known as drugs, which
may be used as medicines in disease, should be avoided in health. Before
foods can be passed into the body proper, they must be converted into the
liquid form, or dissolved. In this process, known as digestion, water is
the solvent; and certain chemical agents, called enzymes, convert the
insoluble nutrients into substances that are soluble in water.
*Exercises.*--1. How does oxidation at the cells make necessary the
introduction of new materials into the body?
2. What different purposes are served by the foods?
3. What is a nutrient? Name the important classes.
4. What are food materials? From what sources are they obtained?
5. Name the different kinds of proteids; the different kinds of
carbohydrates. Why are proteids called nitrogenous foods and fats and
carbohydrates non-nitrogenous foods?
6. Show why life cannot be carried on without proteids; without water.
7. What per cents of proteid, fat, and carbohydrate are found in wheat
flour, oatmeal, rice, butter, potatoes, round beef, eggs, and peanuts?
8. State the objection to a meal consisting of beef, eggs, beans, bread,
and butter; to one consisting of potatoes, rice, bread, and butter. Which
is the more objectionable of these meals and why?
9. State the general plan of digestion.
10. Show that digestion is not a simple process like that of dissolving
salt in water.
PRACTICAL WORK
*Elements supplied by the Foods.*--The following brief study will enable
the pupil to identify most of the elements present in the body and which
have, therefore, to be supplied by the foods.
_Carbon._--Examine pieces of charred wood, coke, or coal, and also the
"lead" in lead pencils. Show that the charred wood and the coal will burn.
Recall experiment (page 114) showing that carbon in burning forms carbon
dioxide.
_Hydrogen._--Fill a test tube one third full of strong hydrochloric acid
and drop into it several small scraps of zinc. The gas which is evolved is
hydrogen. When the hydrogen is coming off rapidly, bring a lighted
splinter to the mouth of the tube. The gas should burn. Hold a cold piece
of glass over the flame and observe the deposit of moisture. Hydrogen in
burning forms water. Extinguish the flame by covering the top of the tube
with a piece of cardboard. Now let the escaping gas collect in a tumbler
inverted over the tube. After holding the tumbler in this position for two
or three minutes, remove and, keeping inverted, thrust a lighted splinter
into it. (The gas should either burn or explode.) What does this
experiment show relative to the weight of hydrogen as compared with that
of air?
_Nitrogen._--Nitrogen forms about four fifths of the atmosphere, where,
like oxygen, it exists in a free state. It may be separated from the
oxygen of an inclosed portion of air by causing that gas to unite with
phosphorus. Place a piece of phosphorus the size of a pea in a depression
in a flat piece of cork. (Handle phosphorus with wet fingers or with
forceps.) Place the cork on water and have ready a glass fruit jar holding
not more than a quart. Ignite the phosphorus with a hot wire and invert
the jar over it, pushing the mouth below the surface of the water. The
phosphorus uniting with the oxygen fills the jar with white fumes of
phosphoric oxide. These soon dissolve in the water, leaving a clear gas
above. This is nitrogen. Place a cardboard under the mouth of the jar and
turn it right side up, leaving in the water and keeping the top covered.
Light a splinter and, slipping the cover to one side, thrust the flame
into the jar of nitrogen, noting the effect. (Flame is extinguished.)
Compare nitrogen with oxygen in its relation to combustion. What purpose
is served by each in the atmosphere?
_Oxygen._--Review experiments (page 114) showing the properties of oxygen.
_Phosphorus._--Examine a small piece of phosphorus, noting that it has to
be kept under water. Lay a small piece on the table and observe the tiny
stream of white smoke rising from it, formed by slow oxidation. Dissolve a
piece as large as a pea in a teaspoonful of carbon disulphide in a test
tube, pour this on a piece of porous paper, and lay the paper on an iron
support. When the carbon disulphide evaporates the phosphorus takes fire
spontaneously. (The heat from the slow oxidation is sufficient to ignite
the phosphorus in the finely divided condition.) What is the most striking
property of phosphorus? What purpose does it serve in the match?
_Sulphur._--Examine some sulphur, noting its color and the absence of odor
or taste. (Impure sulphur may have an odor and a taste.) Burn a little
sulphur in an iron spoon, noting that the compound which it forms with
oxygen by burning has a decided odor.
_Other Elements._--_Magnesium._ Examine and burn a piece of magnesium
ribbon, noting the white compound of magnesium oxide which is formed.
_Iron._ Examine pieces of the metal and also some of its compounds, as
ferrous sulphate, ferric chloride, and ferric oxide or iron rust.
_Sodium._ Drop a piece of the metal on water and observe results. Sodium
decomposes water. It has to be kept under some liquid, such as kerosene,
which contains no oxygen. (It should not be touched except with the
fingers wet with kerosene.) _Chlorine._ Pour strong hydrochloric acid on a
little manganese dioxide in a test tube, and warm gently over a low flame.
The escaping gas is chlorine. Avoid breathing much of it.
*Composition of the Nutrients.*--The simplest way of determining what
elements make up the different nutrients is by heating them and studying
the products of decomposition, as follows:
_To show that Carbohydrates contain Carbon, Hydrogen, and Oxygen._--Place
one half teaspoonful of powdered starch in a test tube and heat strongly.
Observe that _water_ condenses on the sides of the tube and that a black,
charred mass remains behind. The black mass consists mainly of _carbon_.
The water is composed of hydrogen and oxygen. These three elements are
thus shown to be present in the starch. The experiment may be repeated,
using sugar instead of starch.
_To show that Proteids contain Carbon, Hydrogen, Oxygen, Nitrogen, and
Sulphur._--Place in a test tube some finely divided proteid which has been
thoroughly dried (dried beef or the lean of hard cured bacon). Heat
strongly in the hood of a chemical laboratory or some other place where
the odors do not get into the room. First hold in the escaping gases a wet
strip of red litmus paper. This will be turned blue, showing _ammonia_
(NH3) to be escaping. Next hold in the mouth of the tube a strip of a
paper wet with a solution of lead nitrate. This is turned black or brown
on account of _hydrogen sulphide_(H2S) which is being driven off. Observe
also that _water_ condenses in the upper part of the tube and that a
black, charred mass remains behind. Since the products of decomposition
(H2O, NH3, H2S, and the charred mass) contain hydrogen, oxygen, nitrogen,
sulphur, and carbon, these elements are of course present in the proteid
tested.
_To show the Presence of Mineral Matter._--Burn a piece of dry bread by
holding it in a clear, hot flame, and observe the ash that is left behind.
This is the mineral matter present in the bread.
*Tests for Nutrients.* _Proteids._--Cover the substance to be tested with
strong nitric acid and heat gradually to boiling. If proteid is present it
turns yellow and partly dissolves in the acid, forming a yellow solution.
Let cool and then add ammonia. The yellow solid and the solution are
turned a deep orange color. Apply this test to foods containing proteid
such as white of egg, cheese, lean meat, etc.
_Starch._--_(a)_ Place a small lump of starch in one fourth of a pint of
water and heat gradually to boiling, stirring well. Then add enough water
to form a thin liquid and fill a test tube half full. Add to this a few
drops of a solution of iodine. (Prepare by dissolving a crystal of iodine
in 25 cubic centimeters (1/20 pint) of a solution of potassium iodide in
water and add water to this until it is a light amber color.) The starch
solution is turned blue, _(b)_ Cut with a razor a thin slice from a
potato. Place this in a weak solution of iodine for a few minutes and then
examine with the microscope, using first a low and then a high power.
Numerous starch grains inclosed in cellulose walls will be seen (Fig. 60).
_Dextrose, or Grape Sugar._--Place a solution of the substance supposed to
contain grape sugar in a test tube and add a few drops of a dilute
solution of copper sulphate. Then add sodium hydroxide solution until the
precipitate which first forms is redissolved and a clear blue liquid
obtained. Heat the upper portion of the liquid slowly to near the boiling
point. A little below the boiling point the blue color disappears and a
yellow-red precipitate is formed. If the upper layer of the liquid is now
boiled, the color deepens and this may be contrasted with the blue color
below. Apply this test to the sugar in raisins and in honey.
_Fat._--Fat is recognized by its effect on paper, making a greasy stain
which does not disappear on heating and which renders the paper
translucent. Try butter, lard, or olive oil. Also show the presence of fat
in peanuts by crushing them in a mortar and rubbing the powder on thin
paper. If the substance to be tested contains but little fat, this may be
dissolved out with ether. If a drop of ether containing the fat is placed
on paper, it evaporates, leaving the fat, which then forms the stain.
*To show the Effect of Alcohol upon Proteid.*--Place some of the white of a
raw egg in a glass vessel and cover it with a small amount of alcohol. As
the albumin (proteid) hardens, or coagulates, observe that the quantity of
clear liquid increases. This is due to the _withdrawal_ of water from the
albumin by the alcohol. Since the tissues are made up chiefly of proteids,
a piece of muscle or of liver may be used in the experiment, instead of
the egg, with similar results.
*To illustrate the Digestive Process.*--To a tumbler two thirds full of
water add a little salt. Stir and observe that the salt is dissolved.
Taste the solution to see that the salt has not been changed chemically.
Now add a little powdered limestone to the water and stir as before.
Observe that the limestone does not dissolve. Then add some hydrochloric
acid and observe the result. State the part played by the acid and by the
water in dissolving the limestone. Apply to the digestion of the different
classes of foods.