mg HJOy 

 Av 



liter air 



White rats; Total water intake and total water output at various atmospheric humidities at 



25°C. 

 Ordinate: Water intake and output in grams per 100 kilocalories of pearled barley metabolized. 

 Abscissa: Humidity in the environmental air. The point of intersection for the two curves 

 should not be taken too seriously in this graph because of inaccuracies in determining the 



evaporation at higher humidities. 



The lowest curve shows the evaporation from the animal. The evaporation at 

 zero humidity is calculated on basis of the determination of evaporation in dry air. 

 From Table 2 it is seen that 20.4 ml of oxygen is used per lOOkcal of barley. Then 

 the evaporation must be 11.0 g HjO. The evaporation decreases with increasing 

 water vapour content in the inspired air as shown by the sloping curve. The mini- 

 mum water loss through the urine is superimposed on the curve for evaporation. It 

 is calculated in the following way: Pearled barley contains 2.31 g protein per 100 

 kcalr which, when metabolized gives 0.79 g urea. The maximum concentration of 

 urea that kangaroo rats can excrete is somewhat above 20%. The minimum amount 

 of water required for the renal excretion of 0.79 g urea is therefore 3.4 g.. The water 



180 



