A margin of 1 per cent of error is certainly very small in view of the difficulties of such complicated experiments. It is possible that some of the energy was permanently stored within the body, and another physiological source of slight error lies in possible differences in temperature of the whole body between entering and leaving the calorimeter. The other important use of the respiration calorimeter is to determine the fuel value of different foods, alcohol, etc., in furnishing heat and motion for the body. The complete calorimeter experiment comprises: (1) weighing and analysis of food, feces, and urine; (2) determination of the C02 and water eliminated by the patient; (3) estimation of the energy produced by the body in the form of heat, both when at rest and in motion, the motion being converted into heat by means of the bicycle and dynamo above described. The results obtained by Professor Atwater in two of his calorimeter experiments are tabulated by him as follows:

Comparison Of Daily Income And Outgo Of Protein And Of Energy In The Rest And Work Experiments (Nos. 9 And 6)

PROTEIN.

ENERGY.

Experiment.

Of food.

Actually oxidised.

Of food.

Of material actually oxidised.

Measured.

Difference in per cent of heat of material actually oxidised.

Grammes.

Grammes.

Calories.

Calories.

Calories.

Per cent.

Rest (No. 9)

119.4

115.0

2,717

2,275

2,31O

+ 1.5

Work (No. 6)........

119.4

103.1

3,678

3,830

3.726

-2.7

Average Daily Income And Outgo Of Nitrogen And Carbon In The Rest And Work Experiments (Nos. 9 And 6), With The Estimated Gain Or Loss Of Protein And Of Fat

NITROGEN.

CARBON.

CALCULATED GAIN OR LOSS.

Experiment.

In food.

In feces.

In urine.

Gain

( + )or loss

In food.

In feces.

In urine.

In respiratory products.

Gain

(+)or loss

Of protein.

Of fat.

Gms.

Gms.

Gms.

Gms.

Gms.

Gms.

Gms.

Gms.

Gms.

Gms.

Gms.

Rest (No. 9)..

19.1

1.2

18.4

-O.6

261.5

13.3

12.6

223.6

+ I2.0

-3.6

+ I8.2

Work (No. 6).

19.1

1.5

16.5

+ I.I

336.7

12.4

12.5

345 2

-33.3

+ 6.9

-48.3

An ingenious device was used to differentiate the feces belonging to the period of experimentation before entering the calorimeter from those following ingestion of food within the apparatus. To do this, just before entering the calorimeter the man is required to take a few grains of lampblack with his food. This harmless substance colours the stool and marks the dividing line.

The unit of measurement used in calorimeter experiments is the calorie, which is the amount of heat required to raise 1 kilogramme of water from o° to 1°C; this equals 3,100 foot pounds, or, approximately, it is the heat required to raise one pound of water 40 Fahrenheit. Fuel value is a term denoting the total calories derivable from a pound of any given food substance if it be completely combusted within the body. The fuel values are calculated for a given food by the factors of Rubner as follows: 4.1 calories per gramme of either protein or carbohydrate, and 9.3 calories per gramme of fat. This corresponds with 18.6 calories of energy for each hundredth of a pound of protein or carbohydrate and 42.2 calories for the same quantity of fat in any food (W. O. Atwater). Or, as stated by C. F. Langworthy, the fuel value of the three chief classes of nutrients is as follows:

One pound of protein yields......................... 1,860 calories.

" "fats " ......................... 4,220

" " "carbohydrates......................... 1,860 "

"In other words, when we compare the nutrients in respect to their fuel values, their capacities for yielding heat and mechanical power, a pound of protein of lean meat or albumin of egg is just about equivalent to a pound of sugar or starch, and a little over two pounds of either would be required to equal a pound of the fat of meat or butter or the body fat".

The work of the average man is calculated to be about 2,000,000 foot pounds per diem (R. H. Thurston). This may exceptionally be increased to 3,000,000 foot pounds. According to the study of R. C. Carpenter of a phenomenal athlete named Miller, the latter developed work amounting to over 15,000,000 foot pounds (7,500 foot tons) on the first day of a six-days' bicycle contest, and 5,500,000 foot pounds (2,750 foot tons) on the last day.

Elaborate investigations have been made with all the principal classes of foods in order to estimate their nutrient, their heat-producing, and their force-producing value, and many statistical tables have been compiled. It should be remembered that all this work is merely approximate, and that the liability to error in the various factors is considerable, but in a general way the results are instructive, and they are certainly interesting and not altogether without practical application. " The fuel value of a pound of protein as it is ordinarily burned in the body is very nearly the same as that of a pound of carbohydrates, but fats have a fuel value of two and one quarter times that of protein and carbohydrates, or 4,220 calories per pound " (A. C. True). A day labourer requires 0.28 pound of protein per diem plus enough fat and carbohydrate to yield a total fuel value of 3,500 calories. A professional man requires 0.22 to 0.25 pound of protein to yield 2,700 to 3,000 calories of energy, but more than that is often consumed. Study at the Storrs experiment station in Connecticut (1896) of nine families of professional men showed an actual consumption of 107 grammes of protein, and a diet fuel value of 3,430 calories.

The standard for a man at light muscular labour demands 112 grammes of protein and a fuel value of 3,000 calories.

The mere calculation of the nitrogen and carbon in a food does not at all show its force value in the body, unless it can at the same time be demonstrated that it is assimilable. Many substances appear to contain abundant food energy which in reality are not economical foods at all from the point of view of supplying all the needs of the body; thus beef fat is wholesome, but it contains no nitrogen for tissue building, and peas contain a large proportion of flesh formers as compared with heat givers. Wood pulp can be made to furnish cellulose and yield much energy, but it is worse than useless in the stomach of man, although some of the lower animals, like rodents, can digest it and make it available for nutrition. Sugar can be made from old rags in the laboratory, yet no stomach can deal with such material.

Tables are now available for the calculation of the force value of rations for large bodies of men under different conditions, as, for example, soldiers in barracks or on the march, which are based upon the principle of careful comparisons between the income and output of energy of the body. That is to say, a study of the force-producing value of different classes of foods, as obtained by chemical research in the laboratory, is carefully compared with the amount of waste matter which is eliminated by the system while a man is being fed upon a measured quantity of food and kept under uniform conditions as regards the amount of work performed. In this manner a check is established upon the theoretical calculations of food values as compared with their practical uses in maintaining the equilibrium of the body. Obviously these experiments require great care and system, and if they are to be made of intrinsic value they can only be conducted by expert physiological chemists upon persons who are willing to subject themselves from periods varying from several days to several weeks to conditions involving monotony of diet and existence.

For this reason the number of actual experiments of this kind which have been made is comparatively small, and the conclusions drawn from them must be accepted with considerable allowance for possible error. A diet system to which a man may be willing to submit for a few weeks is by no means always that which will prove best for him through a longer period, and a too rigid application of the rules established for the computation of the force value of foods yields much less practical results than the experience derived by those who actually control the commissary department with due regard to proper economy and variation in food, but entirely without resort to calculations of grammes of carbon, nitrogen, etc.

A criticism reached me from the inmates of a large girls' college, where the diet was for some time experimentally regulated by an expert in such matters, that "if one half the time were bestowed upon properly serving and selecting the food that is given to computation of its force-producing value, the girls would have very much better appetites and digestion." On the other hand, it is easy to err if the dictates of hunger and capriciousness of appetite are allowed to wholly control diet. In the lower animals the instincts which these factors develop are very much safer guides than in the case of man.

Nevertheless, the knowledge derived from experiments of the kind under discussion is certainly useful and instructive, when allowance is made for its relative value by taking into consideration the wide range of circumstances that will modify its application which occur in the organisation of different persons, in their varying capacity for work, and in the condition of their external surroundings. With this word of explanation the following tables from different authors are reproduced. It will be observed that there are some few discrepancies among them, but the cause has been explained above.