34 Papers from the Marine Biological Laboratory at Tortugas. 



Thus among every 4,870 molecules of NaCI, about 1,217 are at any one 



instant undissociated, while 3,653 are dissociated; or, expressed in tabular 



form for the sea-water as a whole : 



4,870 molecules of NaCl = 1,217 fixed 4- 3.653 dissociated. 

 571 " MgCU =114 + 457 



no " CaCl2 =13 +97 



loi " KCl = 5 4- 96 



The NaCl and KCl dissociate each into 2 ions, while the MgCl2 and 

 CaClg break up into 3 each, thus: 



3,653 dissociated molecules of NaCl give 3,653 Na and 3,653 CI'. 



457 " " MgCl2 457 Mg " 914 CI'. 



97 " " CaCl2 97 Ca " 194 CI'. 



96 " " KCl 96 k " 96 CI'. 



Total 4.303 4.857 



In other words, in Tortugas sea-water for every i ,000 Na cations there 

 are 125 Mg, 26 Ca, 26 K; and the anions may be represented by 1,330 CI'. 



As is well known, at least one-fifth of the CI' anions may be replaced by 

 the SO4" ion without appreciable effect, this being accomplished when the 

 sea-water is made up of sulphates of magnesium, calcium, and potassium 

 instead of chlorides. The cations, on the other hand, play a far more defin- 

 ite r61e in the control of movements; for sustained pulsation is impossible 

 in the absence of even the least concentrated cations, such as Ca or K. 



If the sea-water be concentrated to 1.69, by _ , 1 m /-i 



^' •' 0.823 molecular NaCl. 



evaporatmg 100 volumes of natural sea-water to 59 0.0965 " MgClj. 



volumes, the molecular concentrations of the various 0.0186 " ^^'^* 



' 0.0171 KCl. 



constituents becomes: 



This results in a relative preponderance of the Mg ions, for the degree 



of dissociation of the NaCl declines in a greater ratio than that of the 



MgCl2; for in this concentrated sea-water about 70 per cent of the NaCl is 



dissociated, as are also about 77 per cent of the MgCl2, 86 per cent of the 



CaCl2, and 94 per cent of the KCl. Thus: 



8,230 molecules of NaCl give 2,469 fixed and 5,761 dissociated. 

 965 " " MgClj " 222 " " 743 



186 " " CaClj " 28 " " 158 



171 " " KCl " 10 " " 161 



And of the dissociated molecules : 



5,761 dissociated molecules of NaCl give 5,761 Na and 5,761 CI'. 

 743 " " " MgCU " 743 Mg " 1,486 CI'. 



158 " " " CaClj " 158 Ca " 316 Cr. 



161 " " " KCl " _i6i k " 161 cr. 



Total 6,823 7,724 



Hence, we see that when the molecular concentration of the sodium, mag- 

 nesium, calcium, and potassium is 1.69 times that of natural sea-water, 

 the concentration of the cations of these salts is only 4303 or 1.585, and, 

 therefore, the concentration of the cations has not quite kept pace with 

 that of the sea-water as a whole. 



Moreover, in this concentrated sea-water, for every 1,000 Na cations 

 there are 129 Mg, 27 Ca, 28 K, and 1,340 CI' anions. Thus the propor- 





