Page:EB1911 - Volume 19.djvu/1018

 blue, and all the variations of tint towards green are the result of local disturbances, the usual cause being turbidity of some kind, and this in the high seas is almost always due to swarms of plankton. The colour of sea-water as it is seen on board ship is most readily determined by comparison with the tints of Forel’s xanthometer or colour scale, which consists of a series of glass tubes fixed like the rungs of a ladder in a frame and filled with a mixture of blue and yellow liquids in varying proportions. For this purpose the zero or pure blue is represented by a solution of 1 part of copper sulphate and 9 parts of ammonia in 190 parts of water. The yellow solution is made up of 1 part of neutral potassium chromate in 199 parts of water, and to give the various degrees of the scale, 1, 2, 3, 4, &c., % of the yellow solution is mixed with 99, 98, 97, 96, &c., % of the blue in successive tubes. Observations with the xanthometer have not hitherto been numerous, but it appears that the purest blue (0–1 on Forel’s scale) is found in the Sargasso Sea, in the North Atlantic and in similarly situated tropical or subtropical regions in the Indian and Pacific Oceans. The northern seas have an increasing tendency towards green, the Irminger Sea showing 5–9 Forel, while in the North Sea the water is usually a pure green (10–14 Forel), the western Mediterranean shows 5-9 Forel, but the eastern is as blue as the open ocean (0–2 Forel). A pure blue colour has been observed in the cold southern region, where the “Valdivia” found 0–2 Forel in 55° S. between 10° and 31° E., and even the water of the North Sea has been observed at times to be intensely blue. The blue of the sea-water as observed by the Forel scale has of course nothing to do with the blue appearance of any distant water surface due to the reflection of a cloudless sky. Over shallows even the water of the tropical oceans is always green. There is a distinct relationship between colour and transparency in the ocean; the most transparent water which is the most free from plankton is always the purest blue, while an increasing turbidity is usually associated with an increasing tint of green. The natural colour of pure sea-water is blue, and this is emphasized in deep and very clear water, which appears almost black to the eye. When a quantity of a fine white powder is thrown in, the light reflected by the white particles as they sink assumes an intense blue colour, and the experiments of J. Aitken with clear sea-water in long tubes leave no doubt on the subject.

Discoloration of the water is often observed at sea, but that is always due to foreign substances. Brown or even blood-red stripes have been observed in the North Atlantic when swarms of the copepod Calanus finmarchicus were present; the brown alga Trichodesmium erythraeum, as its name suggests, can change the blue of the tropical seas to red; swarms of diatoms may produce olive-green patches in the ocean, while some other forms of minute life have at times been observed to give the colour of milk to large stretches of the ocean surface.

On account of its salinity, sea-water has a smaller capacity for heat than pure water. According to Thoulet and Chevallier the specific heat diminishes as salinity increases, so that for 10 per mille salinity it is 0·968, for 35 per mille it is only 0·932, that of pure Water being taken as unity. The thermal conductivity also diminishes as salinity increases, the conductivity for heat of sea-water of 35 per mille salinity being 4·2% less than that of pure water. This means that sea-water heats and cools somewhat more readily than pure water. The surface tension, on the other hand, is greater than that of pure water and increases with the salinity, according to Krümmel, in the manner shown by the equation ＝77·09 + 0·0221 S at 0° C., where is the coefficient of surface tension and S the salinity in parts per thousand. The internal friction or viscosity of sea-water has also been shown by E. Ruppin to increase with the salinity. Thus at 0° C. the viscosity of sea-water of 35 per mille salinity is 5·2% greater and at 25° C. 4% greater than that of pure water at the same temperatures; in absolute units the viscosity of sea-water at 25° C. is only half as great as it is at 0° C.

The compressibility of sea-water is not yet fully investigated.

It varies not only to a marked degree with temperature, but also with the degree of pressure. Thus J. Y. Buchanan found a mean of 20 experiments made by piezometers sunk in great depths on board the “Challenger” give a coefficient of compressibility ＝491 × 10−7; but six of these experiments made at depths of from 2740 to 3125 fathoms gave ＝480 × 10−7. The value usually adopted is ＝450 × 10−7. The compressibility is in itself very small, but so great in its effect on the density of deep water in situ that the specific gravity (0°/4°) at 2000 fathoms increases by 0·017 and at 3000 fathoms by 0·026. In other words, water which has a specific gravity of 1·0280 at the surface would at the same temperature have a specific gravity of 1·0450 at 2000 and 1·0540 at 3000 fathoms. If the whole mass of water in the ocean were relieved from pressure its volume would expand from 319 million cub. m. to 321·7 million cub. m., which for a surface of 139·5 million sq. m. means an increased depth of 100 ft. The rate of propagation of sound depends on the compressibility, and in ocean water at the tropical temperature of 77° F. the speed is 1482·6 metres (4860 ft.) per second, in Baltic water of 8 per mille salinity and a temperature of 50° F. it is 1448·5 metres (4750 ft.) per second, that is to say, 4 times greater than the velocity of sound in air. This accounts for the great range of submarine sound signals, which can thus be very serviceable to navigation in foggy weather.

The electrical conductivity of sea-water increases with the salinity; at 59° F. it is given according to E. Ruppin’s formula as L＝0·001465 S − 0·00000978 S2 + 0·0000000876 S3 in reciprocal ohms.

The radio-activity of sea-water is extraordinarily small; indeed in samples taken from 50 fathoms in the Bay of Danzig it was imperceptible, and R. T. Strutt found that salt from evaporated sea-water did not contain one-third of the quantity of radium present in the water of the town supply in Cambridge.

Dissolved Gases of Sea-water.—The water of the ocean, like any other liquid, absorbs a certain amount of the gases with which it is in contact, and thus sea-water contains dissolved oxygen, nitrogen and carbonic acid absorbed from the atmosphere. As Gay-Lussac and Humboldt showed in 1805, gases are absorbed in less amount by a saline solution than by pure water. The first useful determinations of the dissolved gases of sea-water were made by Oskar Jacobsen in 1872. Since that time much work has been done, and the methods have been greatly improved. In the method now most generally practised, which was put forward by O. Pettersson in 1894, two portions of sea-water are collected in glass tubes which have been exhausted of air, coated internally with mercuric chloride to prevent the putrefaction of any organisms, and sealed up beforehand. The exhausted tube, when inserted in the water sample and the tip broken off, immediately fills, and is then sealed up so that the contents cannot change after collection. One portion is used for determining the oxygen and nitrogen, the other for the carbonic acid. The former determination is made by driving out the dissolved gases from solution and collecting them in a Torricellian vacuum, where the volume is measured after the carbonic acid has been removed. The oxygen is then absorbed by some appropriate means, and the volume of the nitrogen measured directly, that of the oxygen being given by difference. In the second portion the carbonic acid is driven out by means of a current of hydrogen, collected over mercury and absorbed by caustic potash.

C. T. T. Fox, of the Central Laboratory of the International Council at Christiania, has investigated the relation of the atmospheric gases to sea-water by very exact experimental methods and arrived at the following expressions for the absorption of oxygen and nitrogen by sea-water of different degrees of concentration. The formulae show the number of cubic centimetres of gas absorbed by 1 litre of sea-water; 𝑡 indicates the temperature in degrees centigrade and Cl the salinity as shown by the amount of chlorine per mille:—