INTRODUCTION. 



DIRECT AND INDIRECT CALORIMETRY. 



The data regarding the heat production in short periods are especially 

 valuable in demonstrating the accuracy of so-called indirect calorimetry as 

 compared with direct calorimetry a demonstration which is of great theo- 

 retical as well as practical importance. Such a comparison has been repeatedly 

 made for periods of 24 hours 1 by Atwater and associates, and while the methods 

 of these investigators differ somewhat from those used by Zuntz, it has been 

 shown conclusively that indirect calorimetry for periods of this length is 

 extremely accurate. This demonstration, however, is of little practical value. 

 In the first place, in relatively few instances is the total carbon dioxide excre- 

 tion for 24 hours determined from which the heat production can be calculated. 

 Secondly, it is extremely rare that the total oxygen consumption for this period 

 is determined. Practically all of the computations made by the Zuntz school 

 have been based upon experiments of 15 to 40 minutes' duration. It becomes, 

 therefore, of fundamental importance to demonstrate the relationship between 

 the gaseous exchange and the heat production not only in periods of 24 hours 

 but in short periods, also. 



The calculation of the total metabolism from the data regarding the nitro- 

 gen excretion, the carbon-dioxide excretion, and the oxygen consumption 

 assumes that the nitrogen and the carbon dioxide excreted and the oxygen 

 consumed during a given period represent direct molecular transformations 

 which resulted in an energy transformation during that period; that is, that 

 there was no material delay in the oxidative processes; that there was no 

 accumulation of either oxygen or carbon dioxide in the system; and that the 

 nitrogen corresponded to the protein broken down during the period under 

 investigation. Considerable criticism has been made of this method of calcu- 

 lation, but in all probability, if proper precautions are taken to secure constant 

 conditions of diet for a sufficiently long period beforehand, there will be a gen- 

 eral uniformity in metabolism; and while the metabolism actually measured 

 in any hour period, as for instance, between 8 and 9 a. m., may not represent 

 the exact transformation during the period, nevertheless it does represent a 

 certain definite average transformation which is approximately accurate. 

 Until, however, it has been clearly demonstrated that indirect and direct 

 calorimetry agree even for short periods, we can place no absolute dependence 

 upon observations and calculations based upon indirect calorimetry. 



PURPOSE OF THE RESEARCH. 



To sum up, then, it is possible for us to measure with great accuracy the 

 carbon-dioxide excretion, oxygen consumption, water vaporization, and nitro- 

 gen excretion during any given experimental period. We can likewise measure 

 with considerable accuracy the heat eliminated by radiation, by conduction 

 and in the latent heat of water vaporized. On the other hand, we find great 



Atwater and Benedict, loc. cit.; also Benedict and Milner, U. S. Dept. Agri., Office of 

 Exper. Sta., Bull. 175, 1907. 



