SEA-SALTS ABOUT ARBACIA EGGS. 177 



the reductions in density may have resulted from one or the other 

 or both of two causes ; either salts are removed from the sea-water 

 directly by the eggs, or the exudate itself affects the sea- water 

 such that the specific gravity must fall. 



The first two assumptions were entirely ruled out by subsequent 

 developments. I shall limit myself therefore to a discussion of 

 the other possibilities. Do Arbacia eggs abstract salts from the 

 sea-water or is the reduction in specific gravity an effect traceable 

 to the materials which the eggs secrete? 



Just how the presence of exudate might reduce specific gravity, 

 is more or less uncertain in detail. Nevertheless this possibility 

 must be reckoned with, both in its thermal, as well as more nar- 

 rowly chemical, aspects. Salts dissolved in water apparently 

 bring about a " contraction " of the solvent. 5 Where ionization is 

 incomplete, this effect, though marked, is not easily calculable ; in 

 dilute solutions, however, the total " contraction " is additively the 

 sum of specific effects or moduli of the individual ions. Thus a 

 gram-molecule of a salt with molecular weight M in m grams of 

 water produces a change of volume A,, such that 



_M m m 



* v ^~s sy 



where 5* is the density of the solution at a given temperature and 

 S the density of pure water at the same temperature. 



Since undissociated molecules also have a " contractile " effect, 

 and since each ion has a specific modulus, it would be quite pos- 

 sible to bring about a reduction in the specific gravity of sea- 

 water by the addition of some agent that disturbs, selectively or 

 otherwise, the ion-salt equilibrium. Our problem then narrows 

 down to this : is the observed decrease in specific gravity as- 

 sociated with a genuine salt-deficit in the solution or is it the 

 outcome, direct or indirect, of a physical-chemical rearrangement 

 among the free solutes? 



If real, and essentially non-selective, a salt-deficit in the solu- 

 tion should be detectable by the titration of the chlorides. For 

 this purpose I used 11/20 AgNO 3 ; and two or three drops of 10 

 per cent. K 2 CrO 4 , in doubly distilled water, as indicator. 



5 Nernst, W., Theoretical Chemistry. MacMillan and Co., London, 1895, 

 P- 33i. 



