WATER BALANCES AND EXCHANGES 283 



content, are rapid enough so that other changes of composition are 

 much smaller than those of water. 



(6) Time (t) is an extremely large factor in determining the 

 state of the organism with respect to water (fig. 106). The curves 

 of change in water content after displacement have in common the 

 trend toward Wo; but the time scales of these changes are very 

 diverse. Initial, steady, maximal, and fractional hours are 

 distinguished. 



"Latent" periods are absent in all water intakes (table 14). 

 They are also absent in water outputs by blood of rabbit, by Plias- 

 colosoma, and by Arhacia eggs, living units in which exchanges 

 occur across the entire surface. Where present, these periods are 

 hypothesized to represent delays (a) of translocation, (b) of arous- 

 ing responses to water load, (c) of paths or processes of exchange, 

 or (d) of development of some unmeasured mediate quantity with 

 which the measured load is correlated. 



(7) Velocity quotients (k) are computed from rates and incre- 

 ments (SW/At-r- AW), or from the exponential curves of water 

 load in time. Often the quotients for net exchanges are nearly 

 uniform over a wide range of water increments, at some selected 

 time after the increment had been imposed. The values of the 

 quotient are various in diverse tissues and species (table 16). All 

 values found at maximal rates are above 0.3/hour, except in water 

 excess of the snake. This means that recovery of water content 

 at balance occurs in all those species in 0.1 to 4 hours ; no load lasts 

 appreciably longer, as would be the case in some organism indiffer- 

 ent to water or unprovided with means of detecting and correcting 

 aberrant water contents. 



(8) Changes of metabolisms (M), or of rates with which proc- 

 esses go on in the organism, furnish numerous correlations with 

 water content. Of these, clearances through vertebrate kidneys 

 have received most study ; next in quantity of information are total 

 rates of oxygen consumption. Studies of neuro-muscular behavior 

 name thirst as an urge that appears in negative water loads, but 

 furnish few measurements of the activities that go with it. Con- 

 vulsions, vomiting, and anorexia have not been quantitatively cor- 

 related. Among some one or few activities of organisms, such as 

 clearance, excretion, chemical transformation, and movement, it is 

 useful to find in what range of water increments, in what types of 

 water load, and in what conditions they attain particular rates. 



