44 GASEOUS METABOLISM OF INFANTS. 



use during the period. The oxygen supply is then shut off and the bell 

 gradually sinks. It is highly desirable that at the end of each period the 

 bell should always be sinking, thus in part compensating for the slight 

 alteration in tension. More recently we have found it advantageous 

 to move the counterpoise rod, g, g, g, up or down by hand at the exact end 

 of the period until the very delicate petroleum manometer indicates that 

 there is no pressure. At this point the reading is taken. 



Manometer. — The small oxygen consumption and the large volume 

 of the respiration chamber with its accessory parts make the influence 

 of slight changes in temperature and pressure of great moment in 

 measuring the total oxygen consumption. Consequently it is essential 

 to note the exact pressure inside the chamber. This is assumed to be 

 atmospheric, but it is possible that the spirometer does not respond 

 instantly to slight changes in pressure; accordingly it is more efficacious 

 to use a very delicate manometer. This manometer (Af ) is of the type 

 employed by Pettersson and Sonden in their gas-analysis apparatus 

 and indicates the slightest alteration in atmospheric pressure. It con- 

 sists of a glass tube bent in the form of an arc and containing a few 

 drops of petroleum oil. 



Balance. — The soda-lime bottles weigh, together with the Williams 

 bottles, approximately 6 kilograms. The necessity for determining the 

 amount of carbon dioxide produced in a half -hour period to within 0.01 

 gram makes it imperative to secure a balance with a large carrying 

 capacity and extreme sensitiveness. Such balances we have as yet 

 been able to obtain from only one manufacturer. 1 Fortunately they 

 are quite inexpensive. These balances — of which there are many sizes, 

 all of which have been tested in this laboratory— give very accurate 

 results, for with a load of 10 kilograms 1 centigram is easily recorded. 

 The balances are substantially mounted and surrounded by a glass 

 case for protection against any disturbing drafts. 



METHODS OF TESTING THE RESPIRATION APPARATUS. 



The respiration apparatus just described, though extremely simple 

 in principle, nevertheless has certain complexities. For experiments 

 with infants, therefore, it is necessary to test completely the feasibility 

 of the apparatus for measuring or indicating the several factors. For 

 this purpose it is of prime importance to know that the apparatus is 

 absolutely air-tight, so that when the cover is properly in place, no air 

 can enter or leave the circulating air-current. Fortunately this is very 

 readily tested in this type of apparatus. 



TESTS FOR TIGHTNESS. 



By a consideration of the diagram given in figure 2, it will be seen 

 that the entire ventilating current is a closed circuit, the tension equal- 

 izer or spirometer allowing it to expand or contract according to the 



1 A. Sauter, Ebingen, Wi'irttemberg, Germany. The specifications are as follows: No. 7 Ha, 

 with aluminium beam and iron support, black enameled, in glass case, with carrying power 10 

 kilograms. 



