CORRELATIVES OF WATER CONTENT AND EXCHANGES 259 



ability" to water. This is accomplished by finding the volume of 

 water exchanged per unit of time, the area of surface believed 

 effective in the exchange, and the gradient of a measurable or sup- 

 posed pressure under which the water moves. Very often permea- 

 bility is termed such only when ''osmotic" pressure is believed to 

 be the only force acting. How far does permeability to water 

 represent the exchanges concerned in equilibration of water 

 content ? 



In the investigation here pursued, diverse factors have been 

 correlated with the exchanges of water as measured. The area of 

 surface of exchange, and the gradient of known pressure existing, 

 may be regarded as two such factors taken at random. For water, 

 the rate of exchange SW/At = /i X S X AP. The area of surface 

 exchanging is S, the gradient of effective pressure is AP, and the 

 permeability coefficient is h. Automatically the instances in which 

 h can be computed are those where S and AP are known in addition 

 to the rates of exchange. The accident whereby these two enter 

 into the definition of a familiar physical "constant" does not, I 

 think, make them different from others. I believe that any set of 

 factors may be grouped into equations and coefiicients which when 

 dubbed and investigated will be enlightening. 



Permeabilities, like other coefficients, are useful in that they 

 stand for ratios of two or more measurable factors. They are 

 useful even when they are not constant. A coefficient of any sort 

 comes into "good" standing when it succeeds in cancelling factors 

 or variables ; it is confusing when its implications are taken to 

 represent more than a coefficient. 



Above I found it useful to eliminate body mass and volume of 



water from certain equations by putting SW/At -^ AW = 1/At. 



This bit of algebra facilitates comparisons of rates (SW/At) among 



diverse increments of water (AW) exchanging with diversely sized 



and specific organisms or parts. The permeability coefficient 



represents a similar attempt to eliminate the two further factors, 



surface area, and gradient of at least one of the forces believed 



concerned in the exchange of water. The coefficient happens to 



, J.1 T • mass distance ,, , 



nave the dimensions : r-r-^ ;— tt^ or 5 ry-r- thereby 



area X lorce X time force X time '' 



allowing numerical comparison of diverse sizes and species of 



organisms, but is no " better ' ' than any other ratio relating other 



dimensions. 



