PHYSIOLOGY OF CHROMATOPHORES OF FISHES 553 



30 



tracting time of MgCl2, that is, ^ = 1.4. The average quotient for 

 these fifty trials was 1.8. 



From these figures it is clear that in this case the contraction 

 is not so simple a matter as it seems to be in the potassium salts. 

 Furthermore (1) the cation does not cause the first contraction 

 by its independent action as may be the case with the potassium 

 salts; (2) the nature of the anion has a distinct effect upon the 

 cation; (3) the speed of the reaction is almost (1.8 : 1.7) the recip- 

 rocal of the theoretical expectation if the assumption is made that 

 the effect is a function of the number of ions (cations in this case) 

 in solution. 



The solutions 0.1 M MgClz and 0.2 M MgS04 are isionic as 

 regards the number of Mg++ ions. In fifteen successive trials 

 with matched pairs of scales immersed simultaneously in these 

 solutions the melanophores in MgS04 invariably contracted be- 

 fore those in MgCL. Here again if the cation action were inde- 

 pendent of the .nature of the anion, and if it were the cause of 

 this initial contraction (cf. potassium salts) the melanophores 

 should have shown approximately the same time of contraction 

 in the two solutions. This was clearly not the case. 



3. Effects of mixed electrolytes 



Since sodium and potassium salts have opposite effects upon the 

 melanophores, experiments were made with mixtures of these 

 salts to determine, if possible, their relative specific strengths. 



It was easil}^ shown that equal quantities of potassium and 

 sodium salts do not balance each other in solution. The potas- 

 sium effect is prepotent. 



The proportions and concentrations of both salts were varied 

 in a series of experiments in an attempt to determine the strength 

 at which potassium and sodium would approximately balance 

 each other. It was found that the effect varied according to the 

 previous treatment of the melanophores. Thus, in a solution of 

 7 cc. of 0.125 M NaCl + 1 cc. of 0.125 M KCl, contracted mel- 

 anophores expanded. The NaCl seems to predominate. In a 

 solution of 7 cc. 0.125 M NaCl + 2 cc. of 0.125 M KCl no such 



