CALORIMETER, The Respiration. A respiration calorimeter is an instrument of precision by means of which gaseous exchange and heat production can be measured. It com prises a chamber in which the interchange of gases and the production of heat occurs, the walls of which are air-tight and heat-proof, with accessory devices by which any gases added to or removed from the air of the cham ber are measured, while the heat imparted to the air of the chamber is also removed and its quantity determined. The apparatus was origi nally designed for use in the study of funda mental problems of physiology with the living organism, it particularly with respect to nutri tion, but t has been found valuable for a great variety of other uses.
Investigations of nutrition commonly com prise what are known as complete metabolism experiments. The subject, for example a man, is given a prescribed diet during an experi mental period which may continue several days. All his food and drink and all solid and liquid excreta are weighed and samples analyzed. From a comparison of the quantities of ele ments and compounds in the materials taken into and given off by the body it is possible to tell whether the store of material has in creased or decreased under the experimental conditions. In order that the balance may be complete, however, it is necessary to include with the data obtained by analysis of the solids and liquids, the quantities of oxygen consumed, and those of water vapor and carbon dioxide given off by the lungs and skin. To this end the subject remains during.some part or all of an experimental period in a device called a respiration apparatus by means of which these quantities may be measured. From all of the data thus obtained it is possible to estimate with considerable accuracy the actual transformation of matter within the body.
The transformations of energy are likewise learned from comparison of the intake and output of energy by the body, the quantities of energy being measured as heat, because other kinds of energy can be converted into heat and whatever the nature of the energy utilized in the body it is ultimately converted into heat, and eliminated from the body as such.
The actual income of energy to the body is the difference between the potential energy of the combustible material in the food and drink and that of the unoxidized residues of food and body material in the solid and liquid excreta, which are determined by burning samples of each in oxygen in a bomb calorimeter. The actual output of energy by the body is the heat resulting from the expenditure of energy in the maintenance of bodily functions and the performance of muscular work. To measure the amount of energy given off from the body and after it leaves is determined, and from these data the quantities of gases imparted to the air by the subject are ascertained. By another method somewhat similar the air leav ing the chamber is passed through purifying devices which remove all the carbon dioxide and water vapor from it, the amount of each i being found from the gain in weight of its absorber. The quantities of gaseous exhalation can be determined with considerable accuracy in such manner, and with small animals as subjects the consumption of oxygen can also be ascertained; but with larger animals, such as man, the determination of the quantity of oxygen consumed is somewhat more difficult. This can be accomplished more readily by the closed circuit method in which air is withdrawn from the chamber, passed through the purify as heat, the respiration apparatus in which the subject spends the experimental period is ar ranged also a calorimeter. The term is intended to signify that the device measures simultaneously the respiratory exchange and the heat output of the body. In recent years a large amount of work has been done with the respiration calori meter in experiments in which measurements of the gaseous exchange and energy produc tion were the chief end, few or none of the other factors of income and outgo being con sidered.