Age of the Oceans

The temperature of maximum density of sea water of any specific gravity was determined by Knudsen, the temperature of maximum density is lower as the concentration of the sea water is greater, as is shown in the following table:— In this table density signifies the specific gravity of the sea water in question at 0 temperature.

Further Physical Properties of Sea Water.

The laws of physical chemistry relating to complex dilute solutions apply to sea water, and hence there is a definite relation between the osmotic pressure, freezing-point, vapour tension and boiling point by which when one of these constants is given the others can be calculated.

The salt of salt lakes which have been formed in the areas of internal drainage in the hearts of the continents by the evapora tion of river water are entirely different in composition from those of the sea, as the existence of the numerous natron and bitter lakes shows. Magnesium sulphate amounts to 4.7% of the total salts of sea water according to Dittmar, but to 23.6% of the salts of the Caspian according to Lebedinzeff ; in the ocean magnesium chloride amounts to 10•9% of the total salts, in the Caspian only to 4.5%; on the other hand calcium sulphate in the ocean amounts to 3.6%; in the Caspian to 6.9%. These two disparities make it extremely difficult, if not impossible, to view ocean water as merely an extract of the salts in the rocks.

The determination of salinity was formerly carried out by evaporating a weighed quantity of sea water to dryness and weigh ing the residue. This method was inexact. Sorensen and Knudsen after a careful investigation abandoned the old definition of salinity as the sum of all the dissolved solids in sea water and substituted the weight of the dissolved solids in i,000 parts by weight of sea water on the assumption that all the bromine is replaced by its equivalent of chlorine, all the carbonate converted into oxide and the organic matter burnt. The advantage of the new definition lies in the fact that the estimation of the chlorine (or rather of the total halogen expressed as chlorine) is sufficient to determine the salinity. According to Knudsen the salinity is given in weight per thousand parts by the expression S= 0.030+ 1.8050 Cl, where S is the salinity and Cl the amount of total halogen in a sample. This definition was adopted by the Inter national Council for the Study of the Sea in 1902.

Besides the determination of salinity by titration of the chlorides, the method of determination by the specific gravity of the sea water is still used. In the laboratory the specific gravity is determined in a pyknometer by actual weighing, and on board ship by the use of an areometer or hydrometer. In all areometer work it is necessary to ascertain the temperature of the water The biologic importance of the osmotic pressure of sea water will be easily understood from the fact that a frog placed in sea water loses water by exosmosis and soon becomes 20% lighter than its original weight, while a true salt water fish suddenly trans ferred to fresh water gains water by endosmosis, swells up and quickly succumbs. The elevation of the boiling-point is of little practical importance, but the reduction of vapour pressure means that sea water evaporates more slowly than fresh water, and the more slowly the higher the salinity. The fact that sea water does evaporate more slowly than fresh water has been proved by the observations of Mazelle at Trieste and of Okada in Azino (Japan). Their experiments show that in similar conditions the evaporation of sea water amounts to from 7o to 91% of the evaporation of fresh water. G. Wiist calculates that in one year on an average 0.82 metre of water is evaporated over all the seas of the world; in the trade wind zone something like I•I o metres and in high latitudes only 0.5 metre or less.

Transparency and Colour of Sea Water.

The optical prop erties of sea water are of direct importance in biology, as they affect the penetration of sunlight into the depths. The transpar ency of sea water has frequently been measured at sea by the simple experiment of sinking white-painted discs and noting the depth at which they become invisible. In the North sea north of the Dogger bank, for instance, the disc is visible in calm weather to a depth of from 20 to 3o metres, but in rough weather only to metres. The greatest transparency hitherto reported is in the Sargasso sea, where Kriimmel observed a 2 metre disc to depths of 66 metres, but in the cold currents of the north Atlantic and also in the equatorial current the depth of visibility was only from zo to 30 metres. In the tropical Indian and Pacific oceans the depth of visibility increases to from 4o to 5o metres.