PHARMACODYNAMICS OF SALTS AND DRUGS 105 



' ~ ~ log ~ == K . (6) 



That is, the logarithm o) the ratios o) the dilution oj the minimum 

 fatal doses oj two salts, divided by the difference oj the sums oj the ionic 

 potentials oj the two salts, is a constant. 



This formula is very similar to that derived by me empirically 

 from a study of the dilutions of the minimum fatal doses of salts 

 toward the eggs of Fundulus heteroclitus. The empirical formula was 



V a =- 



. 15+0. 02 E a 



In this formula E a and E were the decomposition tensions of the 

 salts. 



If we take instead of 2 the base of the Naperian logarithms 2 . 718, 



and instead of - ; - ^- we write K, this goes over into the form 



Taking natural logarithms 



log F a =log V -K(E a -E ) 



This, in other words, is the same expression as that already derived, 

 using the decomposition tension; i. e., the sum of the solution tensions 

 of the ions, in place of the sum of the ionic potentials. 



The formula may also be derived in another way. If V is the 

 dilution of the minimum fatal dose, and if we let X represent|the 

 difference between the sum of the ionic potentials of protoplasmic 



di) 



ions and salt ions, obviously from the form of the curve j- varies 

 with its position on the curve, that is with V 



--KV 

 ~ ' 



An application of this formula to the results of McGuigan and 

 myself give the following values for K. In each case it is assumed 



