WATER EELATIONS OF OTHER SPECIES 129 



Negative increments of water content were tested in one further 

 respect by Verzar ('36). After being deprived of water for 24 

 hours, in which time I suppose 5 to 7% of Bo might have been lost, 

 and the actual deficit of water might be 3.5% of Bq, the rats drink 

 7.3 to 7.8 ml. or perhaps 4% of Bq. Of this amount 48 to 66 per 

 cent is found to be absorbed from the alimentary tract in 0.33 hour, 

 indicating that in deficits the rates of absorption are about equal 

 to those in excesses (fig. 75). 



When rats are depleted of water by exposure to a hot environ- 

 ment, subsequent water by stomach induces almost as large diuresis 

 as usually, according to Heller and Smirk ('32) and Boyd and 

 Garand ('40). From that it might appear as though water by 

 stomach is not in the rat a ' ' physiological equivalent ' ' of water that 

 has been previously lost by evaporation. But rats so depleted also 

 drink very little (Adolph, '42). However, when rats are depleted 

 by water privation as above, subsequent water by stomach induces 

 only the small diuresis that voluntary drinking does (new data). 

 Whenever the rats end at a weight different from that which was 

 formerly Wo, the position of Wo has changed relative to body 

 weight. This illustrates the fact that choice of controls influences 

 the relationships just as readily as does employment of types of 

 water load. 



The turnover of water in rats is higher than in larger mammals 

 per unit of body weight (0.60 to 0.75% of Bo/hour). Most of the 

 loss is evaporative except during water diuresis. The high turn- 

 over does not change the form of the equilibration diagram as com- 

 pared with that found in other species, however. 



Variabilities of water content and of water ingestion (new 

 data) indicate accuracies of maintenance and exchange about equal 

 to those of other mammals (see table 12). Variation of content is 

 not in proportion to the high turnover and small size. 



The behavior toward environmental humidity is such as to 

 stabilize the rat's water content. Single individuals are put into 

 a box 3 meters long, wet air flowing into one end of it, dry air into 

 the other. Where does the rat spend most time? At intervals of 

 about 2 hours the flow of air is reversed, so that during one day the 

 animal has four opportunities to shift its location while still fre- 

 quenting one humidity (A, fig. 78). From 0.4 to 0.8 hour is needed 

 to reverse the gradient of humidity in the box ; the tests are there- 

 fore summarized by taking the positions of the animal at 1.5 hours 



