146 BIOLOGICAL CHEMISTRY 



through absorbers. The air coming out of the chamber is 

 dried by passing it over pumice soaked in strong sulphuric 

 acid. The increase in weight of the absorber gives the amount 

 of water vapour absorbed by the acid and the amount of heat 

 required to vaporise that amount of water can be calculated. 

 The carbon dioxide is absorbed by means of soda lime and 

 the increase in weight of the soda lime absorbers gives the 

 weight of carbon dioxide formed. As the reaction between 

 soda lime and carbon dioxide sets free water this moisture must 

 be retained by concentrated sulphuric acid in an absorber 

 and weighed with the soda lime absorber, 



2NaOH + CO 2 = Na 2 CO 3 + H 2 O 



The air, free from moisture and carbon dioxide and deficient 

 in oxygen, is now supplied with oxygen. The oxygen is run in 

 until the pressure of the gas in the closed space is equal to 

 that of the atmosphere. This volume of gas passed in must be 

 equivalent to the volume of oxygen absorbed by the animal. 

 The amount of oxygen run in can be measured by a meter or 

 run in from a cylinder and the loss in weight of the cylinder 

 used to calculate the amount of oxygen used. 



The external work can be measured in various ways, such as 

 by converting it into heat by friction or by converting it into 

 electricity and measuring this. If it is ultimately converted 

 into heat the heat may be retained in the respiration calori- 

 meter and measured by the water absorbing system. 



The results obtained by complete or partial metabolism 

 experiments cannot all be reviewed here. In connection with 

 nitrogenous metabolism we find that the greater portion of 

 the nitrogen is excreted as urea (see p. 181). 



If an animal is kept without nitrogenous food the excretion 

 of nitrogen soon falls to a low level. In complete starvation 

 this low level is maintained until all the carbohydrate and 

 fat stores are exhausted and the body proteins are required 

 for the energy processes of the body. When all the non- 

 nitrogenous food stores are exhausted protein is used and 

 the nitrogen excretion rises shortly before death occurs. 



In order to obtain nitrogenous equilibrium (i.e. the intake 

 of nitrogen exactly balancing its excretion) it does not suffice 

 to add just enough nitrogen to correspond to the excretion of 

 nitrogen during starvation. Several times as much nitrogen 

 are required, hence we see that an increased intake of nitrogen 

 is followed by an increased output. The amount of nitrogen 

 necessary for the attainment of nitrogenous equilibrium 



