Several methods of measuring the respira tory exchange are employed. One of the simplest is that in which a constant current of air is passed into and out of the respiration chamber by means of pipes through the 'walls, which are otherwise air-tight. The volume of air passed through the chamber is measured, and the composition of the air before it enters ing devices and returned again to the chamber, and oxygen is supplied to it to replace that used by the subject. The gain in weight of the air purifiers shows how much carbon dioxide and water vapor were brought out of the chamber, and the loss in weight of the oxygen container shows how much of this gas was admitted to it; from these data, with al lowance for any change in the composition of the air of the chamber, the actual respiratory exchange by the subject is determined.
Various methods are also employed to de termine the amount of heat produced by the subject in the respiration calorimeter. One method in common use at the present time is to take up the heat as fast as it escapes from the body by a current of cold water flowing through a coil of pipe in the chamber. From the weight of water flowing through the coil and the difference between the temperature of the water entering and that of the water leaving the coil, the quantity of heat carried out is com puted. This constitutes the major part of the heat given off from the body of the subject. A small part, however, leaves the chamber as latent heat of water vapor in the air, the amount thus carried out being computed by factor from the quantity of water caught in the water absorber. With due allowance for change of temperature of any object in the chamber, the sum of these two quantities rep resents the quantity of heat produced by the subject. The temperature of the walls of the chamber is controlled so that no heat will be transmitted through them from within or without.
The illustration gives a general view of a respiration calorimeter of this type as employed in the laboratories of the United States De partment of Agriculture in investigations with men and women. The respiration chamber of this apparatus is 6% feet long, 4 feet wide and 6y, feet high. Though it is rather small, a person can remain within it very com fortably during a period of even several days' duration. The subject enters the chamber through the large opening in the side in which a pane of glass is sealed during an experiment, thus serving as a window, though there is a small electric lamp inside to provide further light if needed. There is also a small electrc fan to keep the air stirred, and a telephone by which the sub ject may communicate with those on the out side. On the walls are hooks for clothing and shelves for books, food receptacles and the like. A tubular opening in one wall, called the °food has a tightly closing door on each end by means of which receptacles for food and excreta and other objects may be passed into or out of the chamber. The
furniture, which varies with the character of the experiment, comprises a chair, a table and a cot, which may all be folded into small bulk, and devices with which definite amounts of muscular work may be performed.
That this apparatus is sufficiently accurate for the purpose for which it is used is demon strated by the fact that it measures at least 99 per cent of the quantities of oxygen used and of water vapor, carbon dioxide and heat produced when known amounts of alcohol are burned in test experiments within the chamber.
The first respiration calorimeter employed in America was devised and constructed in the laboratory of Wesleyan University by the late W. O. Atwater, then director of the Office of Experiment Stations of the Department of Agriculture, and professor of chemistry, and Dr. E. B. Rosa, of the United States Bureau of Standards, then professor of physics at Wesleyan University, who began work on it in 1892. The original device combined an open circuit respiration apparatus similar in prin ciple to that of Pettenkofer of Munich, but was altered in detail in accordance with modifi cations in method of investigation; and a calorimeter that was quite original, the principal features of which were suggested by Professor Rosa. The completion of this apparatus and conducting experiments with it were made part of the investigations on the nutrition of man which were begun by the United States De partment of Agriculture in 1894, and were put in charge of Professor Atwater. During the 12 years in which it was in use at Wesleyan University the respiration calorimeter was considerably modified, particularly with respect to the experiments of respiratory exchange, being changed from an open to .a closed circuit type to afford better means of determining the oxygen consumption. In 1907 it was trans ferred to Washington and completely recon structed in the laboratory of the Department of Agriculture, where it continues in use at the present time. In the same laboratory a smaller respiration calorimeter of similar nature but with modifications that make it to a con siderable extent automatic in operation, has been constructed and employed for use in the study of metabolic activity of small magnitude, as, for example, the ripening of fruits, the wintering of bets and other problems.
Respiration calorimeters are important aids in research and are in use in many laboratories. They have been found of great value for clin ical purposes and for the study of pathological and other medical problems as well as general problems of health and hygiene. Their appli cation to botanical research opens up a large field which is very promising from a theoret ical as well as from a practical standpoint.