74 CALORIMETERS FOR STUDYING RESPIRATORY EXCHANGE, ETC. 



water-vapor, for while there are a large number of methods for determin- 

 ing small amounts of carbon dioxide with great accuracy, the method for 

 determining water-vapor to be accurate calls for the use of rather large 

 quantities of air. From preliminary experiments with a sling psychrom- 

 eter it was found that its use was precluded by the space required to suc- 

 cessfully use this instrument, the addition of an unknown amount of water 

 to the chamber from the wet bulb, and the difficulties of reading the in- 

 strument from without the chamber. Recourse was had to the determina- 

 tion of moisture by the absolute method, in that a definite amount of air 

 is caused to pass over pumice-stone saturated with sulphuric acid. It is of 

 interest here to record that at the moment of writing a series of experiments 

 are in progress in which an attempt is being made to use a hair hygrometer 

 for this purpose. 



The method of determining the water-vapor and carbon dioxide in the 

 residual air is extremely simple, in that a definite volume of air is caused 

 to pass over sulphuric acid and soda-lime contained in U -tubes. In other 

 words, a small amount of air is caused to pass through a small absorbing- 

 system constructed of U-tubes rather than of porcelain vessels and silver- 

 plated cans. Formerly a very elaborate apparatus was employed for aspi- 

 rating the air from the chamber through U-tubes and then returning the 

 aspirated air to the chamber. This involved the use of a suction-pump and 

 called for a special installation for maintaining the pressure of water con- 

 stant. More recently a much simpler device has been employed, in that 

 we have taken advantage of the pressure in the ventilating air-system 

 developed by the passage of air through the blower. After forcing a definite 

 quantity of air through the reagents in the U-tubes, it is then conducted 

 back to the system after having been measured in a gas-meter. 



This procedure is best noted from fig. 30. The connected series of three 

 U-tubes on the rack on the table is joined on one end by well-fitting rubber 

 connections to the tube leading from the mercurial manometer and on the 

 other end to the rubber tube A leading to the gas-meter. On lowering the 

 mercury reservoir E, the mercury is drained out of the tube D and air 

 passes through both arms of the tube and then through the three U-tubes. 

 In the first of these it is deprived of moisture, and in the last two of 

 carbon dioxide. The air then enters the meter, where it is measured and 

 leaves the meter through the tube B, saturated with water-vapor at the 

 room temperature. To remove this water-vapor the air is passed through 

 a tower filled with pumice-stone drenched with sulphuric acid. It leaves 

 the tower through the tube C and enters the ventilating air-pipe on its 

 way to the calorimeter. 



The method of manipulation is very simple. After connecting the 

 U-tubes the pet-cock connecting the tube C with the pipe is opened, the 

 mercury reservoir E is lowered, and air is allowed to pass through until 



