For the study of many questions in nutrition, it becomes necessary to determine accurately the transformations of energy within the body as contrasted with the transformation of matter; the total income and outgo of energy, measured in terms of heat, are to be compared one with the other and a balance struck. Further, in studying the metabolism of carbohydrate and fat it is necessary to determine the output of gaseous products through the lungs and skin; to estimate the excretion of carbon dioxide and water, and the intake of oxygen. For these purposes, a special form of apparatus known as a respiration calorimeter is employed. The double name is indicative of the twofold character of the apparatus, viz., a suitably constructed chamber so arranged as to permit of measuring at the same time the respiratory products and the energy given off from the body. The form of apparatus best known to-day, and with which exceedingly satisfactory work has been done, is the Atwater-Rosa apparatus, as modified by Benedict. It consists essentially of a respiration chamber, in reality an air-tight, constant-temperature room (with walls of sheet metal, outside of which are two concentric coverings of wood completely surrounding it, with generous air spaces between), sufficiently large to admit of a man living in it for a week or more at a time. Connected with the chamber is a great variety of complex apparatus for maintaining and analyzing the supply of oxygen, determining the amount of carbon dioxide and of water, etc., etc. As an apparatus for measuring heat, the chamber may be described as “a constant-temperature, continuous-flow water calorimeter, so devised and manipulated that gain or loss of heat through the walls of the chamber is prevented, and the heat generated within the chamber cannot escape in any other way than that provided for carrying it away and measuring it.”[33] [33] For an account of the respiration calorimeter and the great diversity of apparatus accessory thereto, together with a description of the methods of measurement, analysis, etc., see Publication No. 42, Carnegie Institution of Washington, “A Respiration Calorimeter with Appliances for the Direct Determination of Oxygen.” By W. O. Atwater and F. G. Benedict. In illustration of the efficiency of an apparatus of this description, and of the close agreement obtainable by direct calorimetric measurement with the estimated energy, as figured from the materials oxidized in the body, we may quote the following data from Dr. Benedict’s report, referred to in the footnote. The subject was a young man who had been fasting for five days. The experiment deals with the metabolism on the first day after the fast, when a diet composed mainly of milk was made use of, containing 53.31 grams of proteid, 211.87 grams of fat, and 75.41 grams of carbohydrate. The following table shows the results of the experiment: