22 THE BIOLOGY OF THE SEA-SHORE 



the quantities of the various constituents are given corre- 

 sponding to the amount of chlorine determined. Another 

 point to be considered is that in a dilute solution such as 

 sea-water the salts will to a large extent be disassociated as 

 ions, so that the compounds given in analyses represent 

 arbitrary combinations. On the other hand, an analysis, 

 such as we append below, which is that of Dittmar, implies 

 that the solution obtained by dissolving the different salts 

 in the proportions indicated in the proper amount of water 

 would be similar in composition to sea-water (Lake, 191 5). 

 On an average 1000 grams of sea- water contain — 



Sodium chloride (NaCl) 

 Magnesium chloride (MgClg) . . 

 Magnesium sulphate (MgS04) 

 Calcium sulphate (CaS04) 

 Potassium sulphate (K2SO4) . . 

 Calcium carbonate (CaCOa) . . 

 Magnesium bromide (MgBra) 



35*000 

 The terms density and specific gravity have different 

 meanings and should not be confused. Density is used 

 when temperature is regarded as a variable factor, and is 

 then usually considered as the relation betwen a given mass 

 of sea-water at the particular temperature and the mass of 

 an equal volume of distilled water at 4° C. Specific 

 gravity, on the other hand, is used when both terms of the 

 relation are understood to be reduced to a constant tempera- 

 ture. The most usual method is to relate sea-water at 0° C. 

 to distilled water at 4° C. It is assumed for purposes 

 of comparison that the composition of sea-water taken 

 from the open ocean is the same everywhere ; in fact, 

 standard sea-water. It is obvious, however, that the sea- 

 margin will generally show much variation from this mean 

 in the vital matter of composition expressed as salinity, and 

 that the distribution of the fauna will be correspondingly 

 influenced thereby. Near large rivers and in estuaries the 

 amount of fresh water will lower the density of the sea 

 surface, often for miles, the effect varying, naturally, with the 



