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 most easily observed is the freezing-point, and according to the very careful determinations of H. T. Hansen the freezing-point τ° C. varies with the degree of concentration according to the formula
τ = −0.0086 − 0.0064633σ0 − 0.0001055σ02.
According to the investigations of Svante Arrhenius the osmotic pressure in atmospheres may be obtained by simply multiplying the temperature of freezing (τ) by the factor −12.08, and it varies with temperature (t) according to the law which holds good for gaseous pressure.
Pt = P0(1 + 0.00367t)
and can thus be reduced to its value at 0° C. Sigurd Stenius has calculated tables of osmotic pressure for sea-water of different degrees of concentration. The relation of the elevation of the boiling-point (t°) to the osmotic pressure (P) is very simply derived from the formula t = 0.02407P0, while the reduction of vapour pressure proportional to the concentration can be very easily obtained from the elevation of the boiling-point, or it may be obtained directly from tables of vapour tension.
Physical Properties of Sea-Water.
| Salinity per mille | 10 | 20 | 30 | 35 | 40 |
| Freezing-point (C.) | −0.53 | −1.07 | −1.63 | −1.91 | −2.20 |
| Osmotic pressure P0 atmospheres | 6.4 | 13.0 | 19.7 | 23.1 | 26.6 |
| Elevation of boiling point (C.) | 0.16 | 0.31 | 0.47 | 0.56 | 0.64 |
| Reduction of vapour pressure (mm.) | 4.2 | 8.5 | 13.0 | 15.2 | 17.6 |
The importance of the osmotic pressure of sea-water in biology 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 transferred 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. Unfortunately no observations of evaporation from the surface of the open sea have been made and very few comparisons of the evaporation of salt and fresh water are on record. The fact that sea-water does evaporate more slowly than fresh water has been proved by the observations of Mazelle at Triest and of Okado in Azino (Japan). Their experiments show that in similar conditions the evaporation of sea-water amounts to from 70 to 91% of the evaporation of fresh water, a fact of some importance in geophysics on account of the vast expanses of ocean the evaporation from which determines the rainfall and to a large extent the heat-transference in the atmosphere.
The optical properties of sea-water are of immediate importance in biology, as they affect the penetration of sunlight into the depths. The refraction of light passing through sea-water is dependent on the salinity to the extent that the index of refraction is greater as the salinity increases. From isolated observations of J. Soret and E. Sarasin and longer series of experiments by Tornöe and Krümmel this relation is shown to be so close that the salinity of a sample can be ascertained by determining the index of refraction. According to Krümmel the following relations hold good at 18° C. for the monochromatic light of the D line of the sodium spectrum in units of the fifth decimal place.
Relation of Refractive Index and Salinity.
| For water of salinity (per mille) | 0 | 10 | 20 | 30 | 35 | 40 |
| Refractive index 1.33000 + units of 5th decimal place | 308 | 502 | 694 | 885 | 981 | 1077 |
The refractometer constructed by C. Pulfrich (of the firm of Zeiss, in Jena) has been successfully used by G. Schott and E. von Drygalski for the measurement of salinity at sea, and was found to have the same degree of accuracy as an areometer with the great advantage of being quite unaffected by the motion of the ship in a sea-way.
The transparency of sea-water has frequently been measured at sea by the simple expedient of sinking white-painted disks and noting the depth at which they become invisible as the measure of the transparency of the water. For the north European seas disks of about 18 or 20 in. in diameter are sufficient for this purpose, but in the tropics, where the transparency is much greater, disks 3 ft. in diameter at least must be used or the angle of vision for the reflected light is too small. In shallow seas the transparency is always reduced in rough weather. In the North Sea north of the Dogger Bank, for instance, the disk is visible in calm weather to a depth of from 10 to 16 fathoms, but in rough weather only to 6½ fathoms. Knipovitch occasionally observed great transparency in the cold waters of the Murman Sea, where he could see the disk in as much as 25 fathoms, and a similar phenomenon has often been reported from Icelandic waters. The greatest transparency hitherto reported is in the eastern basin of the Mediterranean, where J. Luksch found the disk visible as a rule to from 22 to 27 fathoms, and off the Syrian coast even to 33 fathoms. In the open Atlantic there are great differences in transparency; Krümmel observed a 6 ft. disk to depths of 31 and 36 fathoms in the Sargasso Sea, but in the cold currents of the north and also in the equatorial current the depth of visibility was only from 11 to 16½ fathoms. In the tropical parts of the Indian and the Pacific Oceans the depth of visibility increases again to from 20 to 27 fathoms. Some allowance should be made for the elevation of the sun at the time of observation. Mill has shown that in the North Sea off the Firth of Forth the average depth of visibility of a disk in the winter half-year was 4½ fathoms and in the summer half-year 6½ fathoms, and, although the greater frequency of rough weather in winter might tend to obscure the effect, individual observations made it plain that the angle of the sun was the main factor in increasing the depth to which the disk remained visible.
There are some observations on the transparency of sea-water of an entirely different character. Such, for instance, were those of Spindler and Wrangell in the Black Sea by sinking an electric lamp, those of Paul Regnard by measuring the change of electric resistance in a selenium cell or the chemical action of the light on a mixture of chlorine and hydrogen, by which he found a very rapid diminution in the intensity of light even in the surface layers of water. Many experiments have also been made by the use of photographic plates in order to find the greatest depth to which light penetrates. Fol and Sarasin detected the last traces of sunlight in the western Mediterranean at a depth of 254 to 260 fathoms, and Luksch in the eastern Mediterranean at 328 fathoms and in the Red Sea at 273 fathoms. The chief cause of the different depths to which light penetrates in sea-water is the varying turbidity due to the presence of mineral particles in suspension or to plankton. Schott gives the following as the result of measurements of transparency by means of a white disk at 23 stations in the open ocean, where quantitative observations of the plankton under 1 square metre of surface were made at the same time.
| Volume of Plankton. |
Depth of Visibility. | |
| Mean of 11 stations poor in plankton | 85 cc. | 14¼ fathoms |
| Mean of 12 stations rich in plankton | 530 cc. | 8¾ fathoms |
Any influence on transparency which may be exercised by the temperature or salinity of the water is quite insignificant.
The colour of ocean water far from land is an almost pure
