1918] TRUE—MARINE ALGAE 75 
It appeared from these experiments that the osmotic presure 
of a 30 per cent sea water solution was approximately equal to about 
7.0 atmospheres. By plain calculation the osmotic value of undi- 
luted sea water would be about 23.3 atmospheres. 
Since, however, it is well known that salts in aqueous solutions 
diispciate electrolytically in greater proportion in dilute solution 
than in greater concentrations, a given number of molecules might 
through their ionization be expected to cause a proportionally 
greater osmotic pressure at 30 per cent dilution than in a solution 
having three times that concentration. In order to get an idea of © 
the general order of magnitude of the change here concerned, it is 
assumed that the behavior of the sea water approximates that of 
a half-normal NaCl solution. In this solution, corresponding to 
the undiluted sea water, about 73 per cent of the molecules would 
be dissociated at 18° C. (KoHLRauscH and HoLporn, 12), while a 
30 per cent sea water solution corresponding roughly to N/6 concen- 
tration of NaCl would be dissociated about 81 percent. This would 
increase the relative osmotic value from 173 to 181. This difference 
amounts to about 5 per cent of the osmotic value of the N/2 solu- 
tion. To correct for this overestimate would require the subtrac- 
tion of about 1.0 atmosphere from the first calculation. This 
would give an osmotic value of about 22.3 atmospheres for the 
sample of sea water here used. 
In this connection it is of interest to compare this approximation 
with other determinations of this value. The salt content of the 
sample of sea water used may be calculated from the specific gravity 
reading t.oz10at 71° F._ This reading, reduced to a basis of specific 
gravity F = by means of Lipsey’s (14) table, becomes 1.0216. 
This value reduced to terms of salt content by the use of PETTER- 
Son’s (17) comparison of specific gravity with results obtained by 
titration of Cl content indicates a total salt content of about 2.93 
per cent. Assuming this result to have been approximately cor- 
rect, it is possible by use of the “Challenger” (7) analyses to ascertain 
? A discussion of the methods of calculating specific gravity and salt content with 
a diagram for the ready handling of these data is found in Science N.S. 42:732-735- 
IIs. 
