Papers from the Marine Biological Laboratory at Tortugas. 



35 



tionate concentration of Na has decreased with respect to the other cations, 

 and as Na is the most potent stimulant while Mg is the depressant, we 

 might expect the stimulating power of this concentrated sea-water to have 

 increased less rapidly than the ratio of concentration, for in natural sea- 

 water the ratio of the Mg to Na is as i to 8, and in sea-water concentrated 

 to 1.69 it is as I to 7.75. 



If, on the other hand, the sea-water be diluted to 50 per cent of its 

 natural concentration by mixing it with an equal volume of distilled water, 

 a calculation, similar to the one given in detail above, indicates that while 

 the molecular concentration of the dissolved salts has declined to 0.5 (that 

 of natural sea-water being i), the concentration of the cations is 0.528; 

 and for every 100 Na cations there are 124 Mg, 26 Ca, and 25 K cations, 

 and 1,376 CI' anions. The results of these several calculations are sum- 

 marized in table 2. 



Table 2. 



As the osmotic pressure of these solutions is proportional to the respec- 

 tive sums of their fixed molecules and dissociated ions, we find that if the 

 osmotic pressure of Tortugas sea-water be i , that of 50 per cent sea-water 

 is 0.5004, and that of 1.69 sea-water is 1.64. Thus, knowing that the 

 osmotic pressure of Tortugas sea-water is about 24.8 atmospheres, that of 

 sea-water diluted with an equal volume of distilled water is 12.4, and that 

 of 1.69 sea-water is 40.7 atmospheres. Table 3 shows the relations between 

 the osmotic pressures and concentrations of the various solutions experi- 

 mented with in this research, and also the molecular concentration of a 

 solution of NaCl which is practically isotonic with each solution. We must 

 remember, however, that pure NaCl solutions, such as those mentioned in thisi 

 table, are not, strictly speaking, comparable with those found in sea-water. 

 Thus Osterhout (191 1) shows that a pure NaCl solution penetrates the mem- 

 branes of plant-cells more rapidly than if the NaCl is mixed with CaCl2 

 in the proportions found in sea-water. There is also other evidence of a 

 chemical combination involving the NaCl and CaCl2 of sea-water in their 

 mutual influence upon animals. For example, Mayer (1906, p. 49) states 



