soda-lime is a bulb-tube containing a little acid. When the combustion-tube is heated in the furnace, as shown in the larger picture, the nitrogen of the flour is changed to ammonia, which is caught in the acid in the bulbtube. When this is done we have only to find the amount of ammonia and calculate from it the amount of nitrogen. The picture of a chemist sitting by the window shows this latter operation. He has poured the contents of the bulb-tube into a dish called a beaker, added a few drops of litmus, which colors the liquid red, and is carefully drawing another liquid containing ammonia from an upright tube, called a burette, into the beaker. When just enough to neutralize the acid has been drawn into the beaker the color suddenly changes from red to purple. The burette is marked so that he knows just how much of the ammonia is required to neutralize the acid not neutralized by the ammonia from the wheat, and thus the quantity of the latter, and with it the quantity of nitrogen in the wheat, are known.

By such operations as these we are enabled to make analyses of different food materials, of the tissues and fluids of the body, and of other substances as well.

THE CHEMICAL ELEMENTS AND COMPOUNDS OF THE BODY.

BEFORE entering upon our study of foods it will be well to consider with some detail the composition of the human body. For a brief statement of the elements nothing can serve us better than the accompanying reproduction of some of the case-labels of the food collection in the United States National Museum at Washington. The figures are as computed by Messrs. E. A. Welch and R. H. Pomeroy, students in this laboratory, who have been at more pains than any one else, so far as I am aware, to use data collated from all available sources. No one has ever made a complete chemical analysis of a human body, but anatomists have made numerous weighings of the different organs, and chemists have analyzed their constituents. From the figures thus obtained it is possible to make an approximate estimate of the composition of the body of an average man, as is here done.

The diagram on the opposite page will help to a clearer idea of the relative proportions of the elements in the body. In the latter the proportions are expressed in percentages, while in the National Museum labels the estimated weights are stated in pounds.

These thirteen elements are combined with one another in the body, forming a great variety of compounds. Chemists have discovered

DETERMINING THE AMOUNT OF AMMONIA WHICH CAME FROM THE NITROGEN OF THE WHEAT.

more than a hundred different compounds in the bodies of man and other animals. Instead of attempting to enumerate all of them here, it will be more to our purpose to consider some of the principal ones. In doing so we may take advantage of the fact that the compounds. in the body and those in the food are very similar, and discuss them together.

An ox eats grass and meal and transforms the compounds they contain into meat. We eat meat and wheat and change them into the materials of our bodies. Some of the compounds in the food are destroyed, others are only slightly changed in these transformations.

Water, which consists of the two elements hydrogen and oxygen, is a most important constituent of all animal and vegetable tissues. It makes up about seven-eighths of the whole weight of milk and of the flesh of oysters, onefourth that of potatoes and very lean meat (muscle), one-third of bread, a little over half of well-fattened beef or mutton, and one-eighth of the weight of flour and meal. The body of an average man would, by the above calculation, contain about sixty-one per cent, or threefifths water.

Of the materials of our bodies and of our foods the larger part is combustible, as was the case with the grain of wheat; that is to say, it will be burned if put in the fire. A small residue will, however, remain as ashes. This incombustible portion includes the so-called mineral matters. These latter consist of the metals potassium, sodium, magnesium, calcium, and iron, combined with other elements, as oxygen,

The body of a man weighing 148 pounds would contain about 31 pounds of carbon.

The diamond is nearly pure carbon. Graphite (the so-called "black lead" of lead-pencils), anthracite coal, coke, lamp-black, and charcoal are impure forms of carbon.

Carbon exists in combination with other elements in the body, of which it makes about one-fifth the whole weight, and in food. Carbon burns, i. e., combines with oxygen. In this combustion, heat and force are generated and carbonic acid gas formed. The carbon taken into the body in food combines with the oxygen of the inhaled air, yielding heat to keep the body warm and force, muscular strength, for work. The carbonic acid is given out by the lungs and skin. Carbon thus serves as fuel for the body and is the most important fuel element.

PHOSPHORUS-A SOLID.

About I pound and 6 ounces of phosphorus would be found in the body of a man weighing 148 pounds.

Phosphorus is a non-metal, light, very inflammable, and so soft that it is easily cut with a knife. Since it burns so readily in air, it is here kept under water.

United with oxygen, phosphorus forms what is known as phosphoric acid This, with lime, makes phosphate of lime. Most

of the phosphorus of the body occurs in this form in the bones and teeth, though it is also found in the flesh and blood, and especially in the brain and nerves.

The composition of the bodies of different persons varies greatly with age, size, fatness, etc. The amounts of the several elements in the body of an average healthy man, five feet eight inches high, weighing 156 pounds with, and 148 pounds without, clothing, may be roughly estimated to be, in pounds and hundredths of a pound, somewhat as follows:

WEIGHTS OF CHEMICAL ELEMENTS IN THE BODY OF A MAN WEIGHING 148 POUNDS.

The body of a man weighing 148 pounds is estimated to contain about 141⁄2 pounds of hydrogen, which, if set free, would fill about 2600 cubic feet.

Hydrogen, when uncombined, is a gas. It is the lightest substance known. Combined with oxygen it forms water, of which it constitutes one-ninth of the whole weight. Hydrogen occurs in combination with other elements in the body and in food.

Hydrogen, like carbon, unites with oxygen of the inhaled air in the body, thus serving as fuel. The water produced is given off in the respiration through the lungs, and as perspiration through the skin.

CALCIUM-A METAL.

The body of an average man weighing 148 pounds has been estimated to contain some 3 pounds of calcium.

Calcium is a metal somewhat similar in appearance to magnesium or zinc. It is very difficult to obtain free from other elements. United with oxygen it forms lime. This, with phosphoric acid, makes phosphate of lime, the basis of the bones and teeth, in which nearly all the calcium of the body is found. With carbonic acid, it forms carbonate of lime, the chief ingredient of

marble and limestone.

phosphorus, sulphur, and chlorine. Thus, in bone we have phosphate of lime or calcium phosphate, which consists of calcium, phosphorus, and oxygen; in muscle, potassium phosphate and potassium chloride, the latter a compound of potassium and chlorine, and so on. The mineral matters make about thirty per cent. of the weight of bone, one per cent. of the flesh and blood of animals, and from one-half of one to two per cent. of our ordinary vegetable food materials. The mineral matters constitute about six per cent. of the whole weight of the body of an average man.

VOL. XXXIV.— 10.

The combustible portion of the body and of the food that nourishes it consists of socalled organic compounds. Since these are the most important substances we shall have to do with in our study of foods and nutrition, we ought to have a tolerably clear understanding of the nature of at least the principal ones.

If from a piece of meat we remove the bone, gristle, and fat as completely as practicable, and subject the remaining "lean" (muscle) to chemical analysis, we shall find about onefourth, or, to speak more accurately, from twenty-two to thirty per cent., of it to consist of organic compounds, the rest being water with a very little mineral matter. Even if all the visible fat is removed, part of this organic matter will consist of fat in microscopic particles. The fatter the animal from which the meat comes, the more of these minute particles of fat and the less water will there be in the muscle, a fact, by the way, which has the most interesting bearing upon the composition of our own bodies, as we shall see later

on. If, however, we assume that the fat and the mineral matter are both out of the way, some very remarkable compounds will remain. The bulk will consist of substances very similar to the albumen or "white" of eggs, and hence called albuminoid - albumen-like compounds. They are sometimes called proteids, but the name albuminoids is perhaps preferable. Albuminoids in different forms make the basis of blood and muscle. Fresh blood contains blood-albumen and other albuminoids; coagulated blood contains fibrine. Muscle contains muscle-albumen, and other albuminoids called syntonin and myosin. The last is the chief constituent, except water, of muscle. Many persons are surprised to learn that myosin, instead of being the tenacious substance of which muscle is commonly supposed to consist, is in living muscle probably liquid or semi-liquid. How the contractile power of the muscle of an athlete can be exerted by liquid or semi-liquid matter is one of the unsolved problems of chemical physiology.

Albuminoids occur in great variety in plants as well as in animals, but they all consist of the four elements carbon, oxygen, hydrogen, and nitrogen, with perhaps a little sulphur or phosphorus.

Along with muscle, the meat contains what we call gristle, the substance that bothers us so much when we try to carve with a dull knife. This name, however, is applied to several substances, as tendon and cartilage, which, with skin and bone, etc., are called connective tissues. These tissues consist mainly of compounds like the collagen of tendon and the ossein of bone. They are very similar to gelatin (glue) and are changed to gelatin on heating with water. They are hence termed gelatinoids. The gelatinoids are thus the principal ingredients of connective tissue, as albuminoids are the principal ingredients of muscle and blood. The gelatinoids consist of the same elements as the albuminoids; these two classes differ from the other organic compounds in that they contain nitrogen, which most of the others do not. In speaking of the ingredients of foods, it is customary to give to both albuminoids and gelatinoids the generic name of protein. Protein compounds are the most important of all the ingredients of foods.

There is still another class of nitrogenous substances in meat which, though so small in quantity as to be often left out of account, are nevertheless extremely interesting. These are known in the chemical laboratory as creatin, creatinin, carnin, etc., and are designated collectively as "extractives," because they are extracted from flesh by water, as in the case with beef tea and Liebig's Meat Extract.

Chemists find certain analogies between these extractives from flesh and thein and caffein, the active principles of tea and coffee, which they likewise resemble in their stimulating effect. The African traveler Rohlfs tells how invigorating he found a little meat extract spread on a piece of dry bread. The familiar fact that dogs that are quiet and subdued with vegetable food grow fierce on meat is most probably explained as the effect of these same substances. Some people, oftenest those of a fine nervous organization, I presume, find in meat a stimulating effect approaching that of wine. The extractives are similar to alcohol in that they do not form tissue, flesh, or fat. They have, apparently, no effect as fuel. In brief, they are stimulants rather than nutrients.

The extractives give the taste to fresh meat. They impart their savory smell and taste to soups, give roast beef its appetizing odor, and steak its toothsome taste. Our craving for meat is largely due to our fondness for these extractives, as the tastelessness of meat from which they have been removed in making soups bears witness. Indeed, I mistrust that the excessive use of meat, from which the average gourmand — and many of us are veritable gourmands in this respect — suffers so much harm to health, is traceable to the redolence and relish of creatin and other extractives. Though the extractives are different from true protein compounds, they contain nitrogen, and we may follow a common usage and class them as protein.

The body of an average man will contain about eleven per cent. of albuminoids, a little over six of gelatinoids, and about one of extractives, making in all not far from eighteen per cent. of protein.

Among the most important organic compounds of the body and of foods are the fats, of which chemists recognize many different kinds. In the body of man and many other animals, the principal ones are stearin, palmitin, and olein. Stearin, which is obtained in large quantities from beef tallow, is much used for candles, because it does not melt readily. Olein, on the other hand, is an oil at ordinary temperature, and is a chief ingredient of olive oil. A large part of the fat of the human body consists of olein. The fats just named consist of the three elements carbon, oxygen, and hydrogen.

The brain, nerves, and spinal cord contain substances called protagon, lecithin, cerebrin, etc., which, though commonly classed as fats, contain nitrogen and phosphorus, and are therefore known as nitrogenized and phosphorized fats. They have an especial interest because they are believed to be somehow connected with mental activity.

bone, etc., and three-fourths edible flesh. The edible portion was analyzed and found to contain, approximately, sixty per cent. of water and forty per cent. of nutrients. Of the nutrients the protein constituted, in round numbers, twenty, the fats nineteen, and the mineral matters one per cent.

Such numerical statements, however, are not entirely satisfactory, especially when a number are to be studied at once. Diagram III. (pages 70 and 71), in which the proportions of the ingredients are indicated by shaded bands, will doubtless be more acceptable.

Until within the past dozen years very little attention has been given in this country to the chemistry of animal and vegetable products, and most of the work actually done has had reference to their agricultural values. With the exception of analyses of cereals and dairy products we have very few American

to those found to take place in the bodies of other animals. It is by no means urged that the quantities of water and fat which thus mutually replace each other are exactly the same. A striking illustration of

studies of materials used as food for man, aside from those referred to above as executed in behalf of the National Museum, and a series of investigations of the chemistry of food-fishes made for the United States Fish Commission. Much more work in this direction, including the more purely scientific study of the constitution of the materials, is, therefore, most pressingly needed. At the same time the analyses at hand, which have been used in compiling the figures of the diagram, will suffice to give a general and, I think, tolerably correct idea of the average composition of the materials. In some cases where American analyses are lacking, particularly of vegetable foods, I have used European analyses, of which a large number are on record.

I ought to say that different specimens of the same kind of food material may vary the mutual replacement of water and fat may be seen in the case of the lean and the fat mackerel in Part II. of the double-page diagram of composition of food materials beyond.