are beginning to consider it worth while to strive for clean water pay not the least attention to the sanitary qualities of food; the palatable and æsthetic aspects only appeal to them.

Steam-power is produced by the combustion of coal or oil. Human force is derived by releasing the stored energy of the food in the body. The delicately balanced mechanism of the human body suffers even more from friction than the most sensitive machine, and the greatest loss of potential human energy occurs through ignorance, carelessness, and reckless disregard of nature's laws in regard to food.

It is necessary to know, first, what is the normal composition of a given food-material. This is found by analyses of many typical samples. Second, is the sample under consideration normal? To answer this requires an analysis of it, and a comparison of the results with standards. If it is not normal, in what way does it depart from the standard both in healthfulness and in quality? Third, if a food-substance is normal, what are its valuable ingredients and in what proportions are they to be used in the daily diet?

In regard to meat, milk, and fish, the sanitary aspect for the chemist resolves itself into two questions: Is the substance so changed as to become a possible source of poisonous products? Or has anything in the nature of a preservative been added to it? If so, is it of a nature injurious to man?

There is, however, a great range of quality in some of the most abundant foodstuffs, such as the cereals, especially in the nitrogen content. This is most important to the vegetarian and to institutions where economy must be practised. The following variations in the composition of leading cereals will illustrate:

One sample of wheat flour may contain 14 per cent. of nitrogenous substance, another may yield only 9. A day's ration, 500 grams, will give 70 grams of gluten, etc., in the one case and only 45 in the other. This difference of 25 grams would be a serious factor in the dietary of an institution where little additional proteid is given, and it alone might be the cause of dangerous under-nutrition.

The next step would naturally be to determine how definitely these varying percentages mean varying nutrition. To this end a study of vegetable nitrogenous products in their combination or contact with cellulose, starch, and mineral matter is needed. Much work remains to be done before these questions can be even approximately answered.

At the low cost of one cent a pound, common vegetables yield only about one-fifth as much nutriment as one cent's worth of flour, yet they contain essential elements and deserve to be carefully studied.

Dried fruits and nuts are much undervalued as articles of food, as are rice and lentils. (See table, page 150.)

The discussion of food values will be found in Chapter VIII.

Probably the widest field for the sanitary chemist to-day is the study of the so-called predigested foods, infant foods, "hygienic "preparations, two-minute cereals, and the countless proprietary packages, which, designed to meet the demand for quick results, prove traps for the unwary.

Therefore the sanitary aspect of food demands a study

of normal food and food value even more than of adulterants or of poisonous food, ptomaines and toxines. The cultivation of intelligent public opinion is most important, and each student should go out from a sanitary laboratory a missionary to his fellow men. That is, the office of a laboratory of sanitary chemistry should be so to diffuse knowledge as to make it impossible for educated people to be deluded by the representations of unprincipled dealers. Freedom from superstition is just as important in this as in the domain of astronomy or physics. So long as chemists are employed by manufacturing concerns in making adulterated and fraudulent foodstuffs, so long must other chemists be employed in protecting the people until the public in general becomes wiser. A part of the common knowledge of the race should be the essentials of healthful living, in order that the full measure of human progress may be enjoyed.

There is needed a greater respect for food and its functions in the human body, a better knowledge of its effect on the daily output of energy, its absolute relations to health and life, and the enjoyment of the same. The familiarity with these facts which is given by a few hours' work in the laboratory will make a lasting impression and will enable the student to benefit his whole life, even if he never uses it professionally. It is purely scientific knowledge, just as much as that derived from a study of the phases of the moon or the formulæ of integration.

The variety of operations in such work, calling for great diversity of apparatus and methods, is an educational factor not to be overlooked in laboratory training.

For all detailed discussions and methods the reader is referred to such works as those of Wiley, Allen, Leach, etc., but for the student who needs to study, as a part of general education, only typical substances, and such methods as can

be carried out within the limits of laboratory exercises in a college curriculum, the following pages are written. Not enough is given to frighten or discourage the student, but enough, it is hoped, to arouse an interest which will impel him at every subsequent opportunity to seek for more and wider knowledge.

Food is too generally regarded as a private, individual matter rather than as a branch of social economy; it is, however, too fundamental to the welfare of the race to be neglected. Society, in order to protect itself, must take cognizance of the questions relative to food and nutrition.

Formerly each race adapted itself to its environment and trained its digestion in accordance with the available food supply. In America to-day the question is not how to get food enough, but how to choose from the bewildering variety offered that which shall best promote the health and develop the powers of the human being, and, what is of equal importance, how to avoid over-indulgence, which weakens the moral fibre and lessens mental and physical efficiency. In spite of all preaching, few really believe that plain living goes with high thinking. Professor Patten says that the ideal of health is to obtain complete nutrition. Over-nutrition as well as under-nutrition weakens the body and subjects it to evils that make it incapable of survival.

No other form of social service will give so full a return for effort expended as the help given toward better diet for children and students. Fortunately help is coming fast. The United States Government is giving much study to food problems, and by publications is making available the work of other countries. The later bulletins listed in the bibliography at the end of this volume are especially valuable. What is now needed is a general recognition of the importance of the subject.

CHAPTER II.

AIR: COMPOSITION;

IMPURITIES;

RELATION TO HUMAN

LIFE.

THE average adult human being makes about eighteen involuntary respirations per minute. The tidal volume of air is from 300 to 500 cubic centimeters (30 cu. in.), about 2800 cubic centimeters (170 cu. in.) remaining in the lungs unless voluntarily expelled by deep breathing. The total volume expelled is often called the vital capacity, and is about 3400 cubic centimeters for men and 2500 for women. Even when at rest a volume of 7000 to 12,000 liters (250 to 420 cu. ft.) of air passes through the lungs of each individual in twenty-four hours. Under conditions of exercise more or less prolonged or violent this volume may be doubled. The composition of the normal inspired air by volume is approximately: nitrogen and argon 79 per cent., oxygen 20.9 per cent., other constituents o.1 per cent. The air as it leaves the lungs contains nitrogen 79.5 per cent., oxygen 16.0 per cent., carbon dioxide 4.4 per cent., and is saturated with water-vapor. There has therefore taken place an interchange of gases (called the respiratory exchange), by which oxygen has passed into the fluids of the body, and carbon dioxide into the air contained within the lung-cells. Only about one-fifth of the total oxygen is abstracted during each tide.

If the composition of the inspired air varies from the

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