While former authors have bestowed but little attention on the question of how the subjects of their investigations regarding metabolism behaved while under observation — whether they were still or moved — this circumstance of increased or decreased activity is of the most vital interest the moment we include respiratory metabolism in our investigation. The decomposition of N-containing matter is only to a slight degree influenced by activity, especially in the case of infants; but the decomposition of nitrogen-free matter, and with it the elimination of CO2 rises immediately, as soon as the subject makes any action, moves or cries. Even in the second experiment which Eubner and Heubner2 carried out on infants in the Pettenkoffer apparatus, these two authors attached very slight value to the performance of the child. They said:
Physical rest and activity on the part of the subjects may be regarded as nearly, if