GENERAL FEATURES OF WATER EXCHANGES 203 



titative relations. Therefore the four may at this point be charac- 

 terized briefly. They are : 



(1) Time (t). Events are followed, beginning either with the 

 time at which water balance is first disturbed or the time at which 

 recovery is permitted. Completion of recovery is marked by 

 restoration of water balance, in which state the rates of gain of 

 water equal the rates of loss of it. Time intervals (At) are diverse 

 periods of time selected for study, differing not merely in clock 

 units, but also in physiological significance. Initial periods of 

 recovery are particularly distinguished from indifferent (steady) 

 periods and from periods of maximal rates. 



(2) Velocity quotients (1/At) are the reciprocals of time inter- 

 vals. They characterize processes of water exchange, being ob- 

 tained either by dividing rates of exchange by load, or from the 

 analysis of exponential curves of load during recovery. 



(3) Water increments (AW). Excesses and deficits of water 

 contained in living unit are measured in diverse ways, often as 

 increments of body weight. Definitions of control content are set 

 up, both in relation to water exchanges and in relation to non- 

 aqueous materials of the body. Diverse means of establishing 

 increments are distinguished. Extreme increments are barely 

 tolerated loads of water, as judged by survival or other criterion. 

 Only moderate increments prevail in ordinary fluctuations of con- 

 tent measured at successive equal intervals of time. 



(4) Water exchanges (SW/At). Gains and losses of water are 

 total, partitional, or net. The economy quotient is the ratio of 

 gains to losses, departing extremely from 1 whenever water bal- 

 ance is lost. The ratios of maximal to minimal rates indicate the 

 modifiability of total exchanges and of their separable paths. 



Equilibration diagrams, by comparing rates of exchanges with 

 water contents, indicate the relative responses to diverse displace- 

 ments from water balance. At balance the turnover rates are 

 characteristic for the species and its physiological state. 



During recoveries of water content, rates of exchange are 

 traced in respect to time. For several species and several types 

 of disturbance of water content, numerical comparisons, in each 

 of the diverse dimensions, indicate the activities concerned in 

 recovery. 



Time intervals (At) and the corresponding velocity quotients 



