76 A RESPIRATION CALORIMETER. 



per cent. If we assume for an average percentage of air inside the 

 chamber 19 per cent of oxygen, i per cent of carbon dioxide, and 80 

 per cent of nitrogen, it follows that every time the food aperture is 

 open there is an admission of oxygen to the system and a loss of carbon 

 dioxide, with no very great change in the amount of nitrogen. The 

 actual variations in the amounts thus admitted and removed have as yet 

 not been taken into consideration, though during heavy- work experi- 

 ments, when as much as 0.04 gram of carbon dioxide is collected in the 

 lo-liter air sample used for residual analysis, as much as 0.02 gram of 

 carbon dioxide may be lost from the system every time the food aperture 

 is open. In experiments it is sometimes opened as often as 20 times 

 per day, and it is thus seen that under these circumstances a not incon- 

 siderable amount of carbon dioxide may be lost from the system. The 

 amount of oxygen thus admitted is of less consequence, though the 

 desirability of certainty as to its amount would suggest that more atten- 

 tion might be paid to this correction. 



Another important correction in connection with the opening and 

 closing of the food aperture is the displacement of air by the various 

 articles of food, excreta, and dishes passed into and out of the chamber. 

 If we consider the volume of air in the respiration chamber as 5,000 

 liters, then obviously, if a liter of water or metal or glass is passed into 

 the food aperture, the total volume of air is reduced to 4,999 liters. On 

 the contrary, if a liter of urine or of drip water or a volume of glass 

 and metal equivalent to i liter is passed out of the chamber, the volume 

 of air is increased to 5,001 liters. 



It is therefore of considerable importance that the volume of this in- 

 terchange through the food aperture be known, not merely from day 

 to day, but from period to period. To aid in determining this inter- 

 change, a schedule has been prepared, the so-called " food aperture 

 sheet," in which entries are made of all material entering and leaving 

 the chamber. On this sheet are recorded the time at which the food 

 aperture is opened, the nature and weight of the containers and their 

 contents, and the temperature when not that of the chamber. 



The temperature records are more numerous for materials entering 

 the chamber than for those leaving it, since the attempt is made to have 

 all articles in the chamber remain there until they have acquired the 

 chamber temperature. 



In calculating the volume of air displaced by the different materials 

 entering and leaving the food aperture, it is necessary to take into 

 account not only the weight but also the specific gravity. For the 

 materials entering the chamber, i. e., the food, drink, and containers, 

 the following specific gravities are used : For glass, porcelain, etc., 2.6 ; 



