Chap. 10] 



ELECTRICAL METHODS 



637 



constituents in terms of gram (or better milligram) equivalents per liter. 

 From the concentration C thus expressed, the degree of dissociation is 

 obtained. Sundberg^ has given diagrams showing dissociation as a func- 

 tion of concentration of a number of salts. Only the curves for NaCl are 

 of practical importance. If the dissociation of a solution is a, the number 

 of anions per cc is aC gram equivalents. With a charge F per gram 

 equivalent (= 96500 coulombs), the quantity of electricity carried by the 

 anions is F • a • C • v^ , and i^ • a • C • Vc is that carried by the cations, if v 

 is their velocity for a potential gradient of 1 volt per cm. Hence, the 



Table 60 



Rock or Fohmation 



Igneous and metamorphic rocks 



Dense limestones and sandstones 



Clays and sands in general 



Porous clays, sands, sandstones, cellular limestones, and 



dolomites 



Marl, loess, clay, and sandy soil 



Peat, diatomaceous earth. 



POROSITT 



Ratio 



Table 61 



total quantity per second, the current strength, and thus the conductivity 

 per cm^ is o- = F-aC(vx -f Vc). For infinite dilution, a is 1 and the 

 velocities are Vam and Vcoo- If we designate the quantities U = Fva and 

 Ic = Fvcco , as ion mobiUties, the resistivity for any number of salts in 

 solution is p = l/(2aC[lA + Ic])- For very diluted solutions in which 

 all salts are completely dissociated (C less than 0.5 mg per liter), a = 1 

 and the resistivity in terms of concentration expressed in milligram equiv- 

 alents per hter is p = 1000/(2C[1a + ic])- Theoretically, the sum of the 

 concentrations and ion mobilities should be taken for all salts in solution. 



2 K. Sundberg, A.I.M.E. Geophys. Pros., 381 (1932). 



