Page:Popular Science Monthly Volume 79.djvu/503

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499
THE FOOD SUPPLY

sions are to pass beyond the empirical stage and lead to the establishment of general principles. For this purpose Kellner has used a form of the so-called Pettenkofer respiration apparatus, first devised many years ago by Professor v. Pettenkofer in Munich, while the Pennsylvania institution employs an instrument known as the respiration calorimeter, first devised by the late Professor W. O. Atwater for investigations in human nutrition and which has been enlarged and modified to adapt it for experiments upon domestic animals.

The central feature of both apparatuses consists of an air-tight chamber through which a measured current of pure air passes and within which the animal stands in a comfortable stall, where it can be fed and watered at will. The total energy contained in the feed of the animal is ascertained by determining the amount of heat which a sample of it produces when completely burned, while the energy escaping in the visible excreta is measured in the same way. Furthermore, by analyzing samples of the air-current before and after its passage through the chamber containing the animal the gaseous waste products given off by the latter are determined.

Finally, energy escapes from the animal in the form of heat. In the German experiments the amount of heat produced by the animal is virtually computed from the amount and kind of materials oxidized in the body. This may also be done in the experiments with the respiration calorimeter, but in addition this apparatus is provided with appliances for the direct determination of the heat given off, it being taken up by a current of cold water circulating through copper pipes and its amount measured with the aid of sensitive thermometers. In this way the total income and outgo of the animal can be compared, the difference showing how much of the energy of the food has been stored up as meat or fat, while a comparison of the observed with the computed heat production serves as a check on the accuracy of the experiments.

The method is not unlike that employed in locomotive testing plants like, e. g., that of the Pennsylvania Railroad at Altoona. Just as in the latter, the heat value of the fuel is measured, so in the experiments upon the animal the heat value of the feed, which is the fuel of the animal body, is determined. The losses in the visible excreta of the animal may be compared to unburned coal dropping through the grate, while the gaseous excreta correspond to the flue gases. A large amount of heat is given off in both cases, and the final balance of income and outgo makes it possible to trace exactly the use which the locomotive or the animal makes of the energy supplied to it.

The material or ration to be tested is fed for some three or four weeks with the greatest regularity. During the latter portion of this time, after the effect of the ration has become fully established, the animal spends from two to five days in the respiration apparatus or the