46 A RESPIRATION CALORIMETER. 



that a tube will gain in weight o. 12 gram, it can have already increased 

 in weight 0.88 gram with safety. If, however, the anticipated increase 

 in weight will make a total gain of more than one gram, the tube is 

 refilled before use. 



APPARATUS FOR CARBON-DIOXIDE ABSORPTION. 



For the removal of carbon dioxide, U tubes similar in size to those 

 used for water absorption are employed. Each tube is filled to within 

 10 mm. of the top with soda lime, the particles of which are not so fine 

 as to obstruct the flow of gas. The ends of the tube are then closed 

 with rubber stoppers and glass elbows, as described above. 



Efficiency of absorption. In spite of the remarkable absorptive power 

 of soda lime for carbon dioxide, it is not advisable to use a soda-lime 

 U tube for more than one or two analyses, depending upon the amount 

 of carbon dioxide absorbed. As a rule, it is not safe to use a U tube in 

 which the whitening effect, due to the formation of carbonate, extends 

 more than half the length of the tube. 



In practice, the sample of air is drawn through a sulphuric-acid 

 U tube, a soda-lime U tube, and a second sulphuric-acid U tube. Any 

 moisture escaping from the damp soda lime is retained by this second 

 sulphuric- acid U tube, the quantity thus absorbed being approximately 

 i mg. for every liter of air passing through the soda lime. 



THE EI<STER METER. 



For measuring samples of air for residual analysis, a meter made by 

 S. Elster, of Berlin, has been used. The meter is shown in figure 18, 

 just to the right of the U tubes. 



This meter is very sensitive, and measures volumes up to 10 liters, each 

 liter being graduated to 2 cc. Attached to it is a water manometer. 

 Experiments show that the extreme variation in tension in different 

 parts of the meter when air is freely drawn through it amounts to less 

 than 5 mm. of water. This difference holds regardless of the rate at 

 which the air is drawn through the meter. The meter is provided 

 with a spirit level and leveling screws. The case of the meter is filled 

 with water, the excess being drawn off through an overflow. Since 

 the air enters the meter dry and leaves it moist, water is removed from 

 the meter during the progress of the experiments. As used at present 

 about 4.5 grams of water are evaporated during the course of 24 hours ; 

 consequently water is added from time to time in order to keep a con- 

 stant level. 



In general, ro liters of air are drawn for each residual analysis, the 

 meter being read before drawing the sample and after the manometer 

 has settled to zero at the close. 



