GENERAL DESCRIPTION OF RESPIRATION APPARATUS. 61 



only is taken out and replaced with new potash-lime. Thus, three-quarters 

 of the contents of the can is used over and over, while the first quarter is 

 freshly renewed every day. Potash-lime has not been found practicable for 

 the U-tubes because one can not, as in the case of soda-lime, see the whiten- 

 ing of the reagent where the carbon dioxide is absorbed. 



The importance of having the soda-lime or potash-lime somewhat moist, 

 to secure the highest efficiency for the absorption of the carbon dioxide, 

 makes it necessary to absorb the moisture taken up by the dry air in passing 

 through the potash-lime can. Consequently a second vessel containing 

 sulphuric acid is placed in the system to receive the air immediately after 

 it leaves the potash-lime can. Obviously the amount of water absorbed 

 here is very much less than in the first acid absorber and hence the same 

 absorber can be used for a greater number of experiments. 



BALANCE FOE WEIGHING ABSORBERS. 



The complete removal of water-vapor and carbon dioxide from a current 

 of air moving at the rate of 75 liters per minute calls for large and some- 

 what unwieldly vessels in which is placed the absorbing material. This is 

 particularly the case with the vessels containing the rather large amounts 

 of sulphuric acid required to dry the air. In the course of an hour there 

 is ordinarily removed from the chamber not far from 25 grams of water- 

 vapor and 20 to 30 grams of carbon dioxide. This necessitates weighing 

 the absorbers to within 0.25 gram if an accuracy of 1 per cent is desired. 

 The sulphuric-acid absorbers weigh about 18 kilograms when filled with 

 acid. In order to weigh this receptacle so as to measure accurately the 

 increase in weight due to the absorption of water to within less than 1 per 

 cent, we use the balance shown in fig. 29. This balance has been employed 

 in a number of other manipulations in connection with the respiration calo- 

 rimeter and accessory apparatus and the general type of balance leaves 

 nothing to be desired as a balance capable of carrying a heavy load with 

 remarkable sensitiveness. 



The balance is rigidly mounted on a frame consisting of four upright 

 structural-steel angle-irons, fastened at the top to a substantial wooden bed. 

 Two heavy wooden pieces run the length of the table and furnish a sub- 

 stantial base to which the standard of the balance is bolted. The balance 

 is surrounded by a glass case to prevent errors due to air-currents (see 

 fig. 2). The pan of the balance is not large enough to permit the weigh- 

 ing of an absorber, hence provision is made for suspending it on a steel or 

 brass rod from one of the hanger arms. This rod passes through a hole in 

 the bottom of the balance case, and its lower end is provided with a piece 

 of pipe having hooks at either end. Since the increase in weight rather 

 than the absolute weight of the absorber is used, the greater part of the 

 weight is taken up by lead counterpoises suspended above the pan on the 



