In recent years several different calorimeters, based on the principles of the apparatus just described but adapted in size and shape to different types of experimentation, have come into use. Notable among these are the "chair" and the "bed" calorimeters, which are so constructed as to accommodate a subject in the sitting or reclining position in comfort but in a minimum of space; for only by making the calorimeter chamber small is it practicable to obtain a high degree of accuracy in experiments of a few hours' duration. Figures 9, 10, and 11 show sectional diagrams of the original calorimeter chamber and of the more recent chair and bed calorimeters respectively.

Fig. 9. - Horizontal section of the original Atwater-Rosa-Benedict respiration calorimeter. Courtesy of the United States Department of Agriculture.

Detailed and illustrated descriptions of the chief forms of apparatus now in use may be found in the publications by Benedict and Carpenter, by Langworthy and Milner, and by Lusk, Riche, and Soderstrom, full references to which are given at the end of this chapter.

Fig. 10.- Horizontal cross-section of chair calorimeter, showing cross-section of copper wall at A, zinc wall at B, hair-felt at E, and asbestos outer wall at F; also cross-section of all upright channels in the steel construction. At the right is the location of the ingoing and outgoing water and the thermometers. At C is shown the food aperture, and D is a gasket separating the two parts. The ingoing and outcoming air-pipes are shown at the right inside the copper wall. The telephone is shown at the left, and in the center of the drawing is the chair with its foot-rest, G. In dotted line is shown the opening where the man enters. Courtesy of Dr. F. G. Benedict and the Carnegie Institution of Washington.

Fig. 11. - Vertical cross-section of bed calorimeter, showing part of steel construction, also copper and zinc walls, food aperture, and wall and air-resistance thermometers, and heat absorbers. Courtesy of Dr. F. G. Benedict and the Carnegie Institution of Washington.

By means of the Atwater-Rosa-Benedict apparatus and its various modifications, it has been possible to measure the heat production or energy expenditure of a man for a day or for a period of days very accurately. In the original Atwater-Bene-dict series it was found that the difference in results determined by direct and indirect calorimetry was rarely as much as 2 per cent, and in the average of 45 experiments covering a total of 143 days the difference was only 0.01 per cent. The results obtained by direct energy measurements are therefore the same as those computed from respiration and metabolism experiments when the technique is of the best and the experiments are sufficiently prolonged. This agreement is in general less exact in individual experiments in proportion as the experimental periods are shortened; but the methods are now so highly developed that the results of direct and indirect calorimetry are considered practically interchangeable even for experiments of a few hours' duration. In 1913 Armsby compiled the following summary of experiments both upon men, dogs, and cattle which had been published up to that time. It will be seen that the difference between the total heat production as computed and as directly measured is only one fourth of one per cent, or quite within the limits of accuracy of experimental methods of this sort.

As Armsby points out: "These results may be taken as demonstrating that the animal heat arises exclusively from the combustions in the body, but they have a much broader significance. They show that the transformations of chemical energy into heat and work in the animal body take place according to the same general laws and with the same equivalencies as in our artificial motors and in lifeless matter generally. The great law of the conservation of energy rules in the animal mechanism, whether in man, carnivora, or herbivora, just as in the engine. The body neither manufactures nor destroys energy. All that it gives out it gets from its food, and all that is supplied in its food is sooner or later recovered in some form."

Since the time of Armsby's compilation the agreement between the observed and computed heat production has been confirmed in many additional experiments, and both by the same and different experimenters.

Working with the original Atwater calorimeter, Atwater and Benedict conducted "rest" experiments upon six different men who lived in the calorimeter as quietly as was feasible for days at a time, taking as a rule but little more exercise than was involved in dressing and undressing, folding and unfolding the bed, table, and chair, taking samples and observations pertaining to the experiment, writing, etc., in short, the life of a healthy man, confined to one small room.

The average daily metabolism of each of the subjects was as follows:

Extreme deviations from the mean, + 184 to - 309 Calories, or + 8.4 to - 14 per cent.

Omitting the results obtained with the one subject who was considerably older than the others, the figures become as follows:

Mean of individual averages, 2277 Calories. Extreme deviations from mean, + 120 to - 144. Calories, or + 5.2 to - 6.3 per cent.

Deviations in body weight, + 8.7 to - 7.1 per cent.

The subject "H. F.," aged fifty-four, who believed that he consumed only half the usual amount of food, had a food requirement about 15 per cent less than that of the younger men averaging about the same weight. The five younger men varied in age from twenty one to thirty-four years, were natives of three different countries, and had been accustomed to very different dietary habits and modes of life, yet they differed less in energy requirements than in body weight.