A respiration calorimeter is an apparatus designed for the measurement of the gaseous exchange between a living organism and the atmosphere which surrounds it, and the simultaneous measurement of the quantity of heat produced by that organism.

In 1892 Atwater began work upon a calorimeter which could measure the heat production in man, the first description of which appeared in 1897.1 The initiative in the undertaking rested with Atwater, whereas the successful completion of the apparatus was largely due to the physicist Rosa. The original Atwater-Rosa calorimeter was combined with a respiration apparatus of the type designed by Pettenkofer, which measured only the carbon dioxid excretion without determining the oxygen intake.

The apparatus represented technical perfection,2 as was evidenced by the fact that when a measured amount of heat was generated by an electric current within the box it was determined as 100.01 per cent, of the actual value. This test of accuracy is called an electric check. Also, when a known quantity of alcohol was oxidized, the carbon dioxid recovered amounted to 99.8 per cent, and the heat to 99.9 per cent, of the theoretic value. This is an alcohol check. In experiments with men the work frequently lasted during a period of several days. The method of computation was based on that of Voit and Rubner, i. e., the amount of protein carbon excreted was calculated from the nitrogen excreted in the urine and feces, this subtracted from the total carbon excreted in the respiration, urine, and feces gave the total non-protein carbon or that attributable to carbohydrate and fat. It was assumed that all the carbohydrate ingested was oxidized and that after deducting this amount the excess of non-protein carbon was derived from the metabolism of fat. In this way the calories from protein, carbohydrate, and fat were computed. The validity of this method is shown in the work of Atwater and Benedict by the average results per day of forty days of experimentation with three different individuals who took an ordinary mixed diet:

1 Atwater and Rosa: "Report of the Storrs Agricultural Experiment Station," 1897, p. 212.

2 Atwater and Benedict: "Memoirs of the National Academy of Sciences," 1902, viii, 231.

Atwater was not content to omit the determination of oxygen, and turned his attention to this important problem. As already explained (p. 29), the quantity of oxygen required in metabolism depends on the kind of material oxidized in the organism, and the relation between the amount of oxygen absorbed and carbon dioxid eliminated depends on the same factor. The ratio of the volume of carbon dioxid expired to the volume of oxygen inspired during the same interval of time was called by Pfluger the respiratory quotient.

It was known to Lavoisier that any volume of oxygen uniting with carbon produced the same volume of carbon dioxid. Since the volume of oxygen inspired was found in his experiments to be larger than that of the expired carbon dioxid, Lavoisier concluded that a portion of the inspired oxygen must have been used to oxidize hydrogen in the production of water. Under these circumstances the Volume CO2/Volume O2 would be less than unity. The carefully executed experiments of Regnault and Reiset, published in 1849, showed that the value of the respiratory quotient depended on the nature of the food given and not on the species of animal. They found that the respiratory quotient might vary in the same animal from 1.02 to 0.64, and that it varied with the kind of food taken, but was constant with the same food. When fowls were fed with corn or dogs with bread, respiratory quotients of 1.02 and 0.93, respectively, were obtained. The quotients were lower when a meat diet was given and still lower than this when the animal fasted. The low quotients during inanition were obtained alike with herbivorous and carnivorous animals, which indicated to Regnault and Reiset that these animals lived upon their own flesh under conditions not unlike those existing when a meat diet was taken.

Turning now to modern analysis, it is evident that when carbohydrate, in which hydrogen and oxygen are always present in the proportion to form water, is oxidized, the respiratory quotient will be unity. One may express the process thus:

C6H42O6 + 602 = 6CO2 + 6H20

Since equal volumes of gases at the same temperature and pressure contain equal numbers of molecules (Law of Avagadro, 1811) it is evident from the above formula that one volume of oxygen absorbed produces one volume of carbon dioxid during carbohydrate combustion. Hence, for carbohydrate the R. Q. = 1.00.

When fat is oxidized oxygen is utilized not only for the production of carbon dioxid, but also for the oxidation of hydrogen, forming water.

This is evident from the following formula:

C3H5(O2C.CH2.CH2.CH2.CH2.CH2.CH2CH2.

CH2.CH2.CH2.CH2.CH2,.CH2.CH2.CH3)3

Tripalmitin.

If one deducts the intramolecular water from tripalmitin one obtains the following:

This on oxidation yields:

2(C51H86) + 145O2 = I02CO2 + 86H2O

R. Q. = 102 volumes CO2/ 145 volumes O2 = 0.703

Edible fats are usually mixtures of various simple fats, consisting for the most part of tripalmitin, tristearin, and triolein, all of which require nearly the same quantity of oxygen for oxidation. Lehmann, Muller, Munk, Senator, and Zuntz1 analyzed the respiratory quotient which should be obtained from lard as follows:

As the weight of the oxygen molecule is to that of carbon dioxid as 8 is to 11, the respiratory quotient is deduced from the relative weights as follows:

R.Q. =2.805 g. CO2/ 2.876g.O2 x8/11 = 0.710.

Zuntz2 later slightly changed the oxygen value so that the calculated quotient was 0.707. A still more recent computation by Zuntz3 for human fat shows a respiratory quotient of 0.713. The respiratory quotient of fat is, therefore, very constant.