Popular Science Monthly/Volume 26/February 1885/Properties and Constitution of Sea-Water
By M. ANTOINE DE SAPORTA.
IT has been said that, without the sea, civilization could not have been developed, and the world would have continued barbarous. That element, from the primitive times of mankind, has brought together the peoples of the most distant countries, and inspired the ancients with the idea of the Infinite. Homer believed in a river Oceanus; the Hindoo mythologians in a liquid expanse, boundless as space. The fishermen who set their rude nets in the creeks of the Cyclades were, perhaps, the first naturalists, and the Phoenician sailors may have been the first marine engineers. In our own time, all the sciences find in the ocean either a limitless field of exploration, or an enemy to be conquered. Zoologists, closeted in their laboratories, endeavor to determine the beings which the dredge has brought up from frightful depths, while hydrographers and constructors study the currents, raise jetties, and excavate ports. The public visit the aquariums, admire the dikes and excavations, and applaud what they see, but do not see all. Our purpose is to explain the researches of the modest investigators who have occupied themselves with the chemical constitution and physical properties of sea-water.
Sea-water, it is well known, when it is not muddy, is one of the clearest of all natural waters. When we walk along the shore at low tide, it is often difficult, unless we are careful, to keep from stepping into the occasional pools on the rocks, the water in the little hollows being so transparent as to be invisible. The question of the color of this water deserves serious examination, and labors on the subject are not wanting. The most notable ones are those of Father Secchi, of Professor Tyndall, and the more recent researches of M. W. Spring and M. Soret.
Father Secchi made his experiments in 1865, on board a Pontifical corvette. A number of disks, formed by stretching variously-colored cloths over iron hoops, the largest twelve feet in diameter, were let down at a time when the conditions of the weather were most favorable for transparency. The largest disk, which was painted white, became invisible at the depth of about forty-two metres, while the smaller disks and a delf plate, distorted by refraction, went out of eight at smaller depths. The disappearance seemed to depend upon the confusion of the image, which was broken up in every direction. The largest disk, the considerable surface of' which offered more resistance to the distortion, finally ceased to be perceived, because its color, turning in succession to light green, blue, and dark blue, became at last as dark as the surrounding medium. Disks, painted yellow or red, were lost to sight still more quickly, or under not more than twenty metres of water. Repetitions of similar experiments gave co-ordinate results; and it may be stated, as a general rule of average, that the practical limit of submarine vision, under favorable circumstances, is at twenty-five metres under the surface.
It was found, by spectroscopic examinations of the light reflected from the differently' colored disks, that the yellow was enfeebled and extinguished first, and next the red, under the increasing thickness of the overlying water. By the gradual disappearance of these two colors, a white object is made to pass through green to blue—the tint which all such objects finally assume when sunk under salt-water. Each of the three simple colors—yellow, red, and blue, or violet—has its distinct part among the solar rays. Yellow is luminous, red is calorific, and violet-blue provokes chemical reactions. Water, in a very thick mass, is neither transparent nor diathermanous; but, being penetrable to the blue, indigo, and violet rays, it is diactinic. These radiations, too, will, of course, gradually lose their energy, and become extinguished at last in a very deep stratum of liquid; but the limit is extremely remote.
According to the theory propounded by Professor Tyndall, the sea-waves present three principal hues—blue, green, and yellow. The indigo-blue waters are the purest, while the yellow ones contain muddy matters in suspension, and the green ones are slightly charged with such substances. The solid particles held in the water constitute a multitude of infinitely little mirrors, from the outside of which is reflected the light that penetrates the mass of the liquid. The rays which are sent out, after having traversed only a thin stratum of water, preserve their yellow parts. If the reflections are attenuated, the water appears green; and if, on account of the absence of solid matter, they do not exist at all, the sea is of a deep blue. In an indigo sea, the crests of the waves will appear green on account of their lack of thickness. The same rules are applicable to fresh water; for the salt is almost without effect on the color of sea-water—not quite without effect, for, according to M. Spring, the clayey particles which make the waves yellow are precipitated with a rapidity proportioned to the salinity of the sea. These general laws are liable to be disturbed by numerous accidental circumstances or local causes. The presence of sea-weed, or of microscopic animalcules, may have great influence on the color of the water. In tolerably shallow basins, the color of the bottom has its effect.
Several seas or gulfs have been given names alluding to their colors. Some of these terms can be explained without difficulty, but others are not so easy to comprehend. The White Sea is so called on account of its ice, the Black Sea from its storms, and the Yellow Sea from the muddy waters poured into it by the Chinese rivers. The waves of the Vermilion Sea, near California, are colored by the Rio Colorado, which itself has a characteristic name. The water that washes the European coasts has no perceptible odor; or, if in single cases it may be odorous, the smell is due to mud, or to decomposing organic matters contained it it. Drinking-water, which is stored for some time may also acquire a smell which it had not at first, through the decay of the impurities in it. The cork of bottles containing salt-water is sometimes eaten by sulphuretted hydrogen formed in the water.
Sea-water owes its characteristic taste to the chloride of sodium held in solution, and to the bitter salts of magnesia which it contains. Sometimes organic remains or weak proportions of fatty matters become mixed with the superficial strata, so as to make them more nauseous than the same water drawn from greater depths. The pleasant taste of the water inclosed in oyster-shells is due to the savory animal juices that are dissolved in it. Mussels, which live and are fished near the shore, sometimes absorb impurities from the drift-matter around them, from which they develop the poisonous alkaloids called ptomaïnes; hence they are unhealthy at some seasons.
By a scientific prejudice that ruled for a considerable time, the bitter taste of sea-water was believed to be caused by traces of bitumen. The chemists who made analyses consoled themselves for not finding a sign of that substance, the existence of which they suspected, by supposing the proportion was too slight to be appreciable. Count Marsigli, who in the reign of Louis XIV tried to make sea-water artificially, took great pains to mix bitumen with the salts he put in solution, in order to make the reproduction perfect. The partisans of the theory cited the Dead Sea, which was near asphalt-beds, and the waters of which were insupportably bitter. But Macquer, assisted by Lavoisier, a hundred years ago, carefully distilled specimens of this water, and found in it no more bitumen than had been found in Mediterranean water—which was none at all. He attributed the bitter taste of this water to the presence of salts of magnesia.
It is not to-day that investigators have sought to make sea-water potable by removing its nauseous taste. The problem was solved long ago, and, as often happens, the usefulness of the invention once so greatly desired has been much depreciated. When fresh water for the provisioning of vessels was stored in wooden casks, it was liable to spoil in a short time. Now it is carried in large iron tanks, in which, instead of spoiling, it is improved by acquiring a ferruginous quality. The ancients did not venture far from the shores, and were contented with a simple coasting-trade; nevertheless, this question interested them, and Pliny describes two means of freshening the water of the Mediterranean, one of which is absurd and the other impracticable: One was to plunge into the sea hollow balls of wax, which, the author affirms, would be filled with pure water; and the other was to expose fleecy sheep-skins on the deck of the vessel, to collect the morning dews.
Whoever examines the series of memoirs published during the seventeenth and eighteenth centuries, on the subject of freshening seawater by distillation, must be struck by the divergence of opinions and the want of concordance in the results, some declaring that distilled water is pure, healthy, and tasteless, others that it is unhealthy and almost as detestable as before the operation. The differences between them are easily explained. Marine salt is not the only substance dissolved in the water, but is accompanied by several other bodies, the principal of which is chloride of magnesium. This salt when dry resists the action of the most violent heat without changing; but in boiling water undergoes a double decomposition, in which the chlorine leaves the magnesium to unite with the hydrogen of the water, while the oxygen thereof unites with the magnesium. There is thereby produced magnesia, which remains in the vessel, and hydrochloric acid, which is distilled over. Now, distilled water is made impotable and unhealthy by any traces of that acid. The difficulty may be obviated by previously removing such salts as can be made to settle, or by adding fresh sea-water. Water boils at a temperature several degrees higher than usual when it is charged with salts. If it is sufficiently diluted it will not disengage hydrochloric acid. Or the acid may be absorbed by substances added to the water for that purpose, and which will not give it up again. Such substances are lime, chalk, potash, soda, and calcined bones, all common and cheap.
The problem of freshening sea-water was formerly regarded as so important that other means of solving it besides that of evaporation were advanced. Even the great Leibnitz lent his name to a proposition which was judged singular, if nothing worse, by his contemporaries. It was to freshen water by forcing it through a filter filled with litharge; but he never tried the experiment. It was believed, on the authority of Pliny, that if an empty bottle, hermetically sealed, were sent down deep into the ocean, it would come back full of pure water. But it was proved that the bottles would either be broken or come back empty. Other naturalists tried filters of earth or sand. But, when Réaumur and the Abbé Nollet constructed a gigantic filter of glass tubes filled with sand, a thousand toises long, they found that the water came out of it as salt as it went in. Lister, in 1684, placed seaweeds with their stems in water, after the fashion of a bunch of flowers, in an alembic which he did not heat, believing that the fresh water would ooze out in drops from the upper part of the plants; but he had to acknowledge that no great result came from his curious process. Samuel Reyer made a discovery of some practical value—that melted sea-ice furnished a potable water. Notwithstanding numerous distilling apparatus were devised by various inventors, ships continued to be furnished until very recently with water stored in casks. The inventions had little practical value, and the management of alembics when the sea was rough was a matter of considerable difficulty.
The sea is in reality an immense and inexhaustible mineral spring. Probably, if it only contained pure water, a fountain as rich in mineral matters as the ocean actually is would attract crowds of drinkers and would be recommended for internal use in all imaginable diseases. But sea-water is abundant and common, and has never been much used internally. On the other hand, the therapeutic employment of sea baths might be made the occasion of long dissertations.
It is generally known that a strong dose of sea-water acts as an emetic; in weaker proportions it is purgative and diuretic. Dioscorides advised diluting it with honey, which might, perhaps, produce an efficacious medicine, but certainly not a savory one. At the beginning of this century it was diluted with wine, but such a mixture could hardly be better than the other one. It was prescribed in Spain against the yellow fever, and in England against worms: in the former case, as an emetic; in the second case, milk was added to it so that the child could drink it without aversion. Sea-baths have been tried as remedies for hydrophobia and insanity, but, it is needless to say, without effect.
Marine water contains a little iodine; it is therefore a resolvent, and adapted to external application for tumors and ulcers, although more energetic and sure remedies are generally employed. Even before iodine was discovered, more than a hundred years ago, Russel had remarked the efficacy of calcined sponges and corals, and of the ashes of sea-weed, substances richer in iodine than sea-water itself.
A general study of the physical properties of sea-water would not be complete if it was limited to that at the surface. It is necessary to obtain specimens drawn from different depths, especially as the density and temperature vary with the depth. Various apparatus have been contrived for bringing to the surface a quantity of water drawn from any desired level. A long-known means, and at the same time a simple and practicable one, is to let down by a rope an empty bottle corked. The increasing pressure upon the bottle becomes strong enough at certain depths to push the cork in and fill the bottle. The rope is then drawn up, and the liquid inside the bottle coming in contact with less dense waters, pushes the cork back into the neck of the bottle and closes it. Thus the water from the deep keeps itself free from mixture with that of the superficial levels. Other more perfect apparatus have been invented, all dependent upon the automatic closing of the vessels.
Salt water is denser than fresh, because of the gravity of the dissolved salts. But wherever large rivers enter the sea, as in the Black Sea and the Baltic, and in cold climates where evaporation is slow, the superficial water is light and of inferior salinity. The water of the Norwegian fiords is brackish, and that of the Gulf of Bothnia, at the upper end of the Baltic, is, in an extremity, potable. The glaciers of Greenland and Spitzbergen pour out in the summer torrents of fresh water which tend to freshen the spaces around their mouths. There is likewise a deficiency of salt in the waters of the White Sea, the Kara Sea, and the Siberian Ocean. Inversely, the Mediterranean, which does not receive, in proportion to its extent, so many nor so large rivers, and is exposed to the ardors of a burning sun, would become indefinitely concentrated by evaporation, were it not that an under-current of less dense water was sent into it by the Atlantic Ocean through the Strait of Gibraltar. Copious rains may play some part in the matter, and that is another reason why Mediterranean waters should preserve their density. Evaporation is very great in the tropics, but the liquid concentrated by it is also expanded by the heat, so that the two effects partly balance one another.
In all the old books on the physics of the globe, and even in some recent ones, no difference was made as to the law of maximum density between salt water and fresh. The latter begins to expand by heat at 4° C. (39° Fahr.), but, between the freezing-point and that temperature, it contracts when it is warmed, so that at 39° Fahr. it is denser than at any other temperature. In temperate countries, the water of the bottom of deep lakes remains at nearly 39° Fahr. by means of its weight, which prevents it from rising to the surface and mixing with either the colder or the warmer parts, and also because water conducts heat very badly.
The phenomena are different in the case of sea-water, and also complicated in other ways. The point of maximum density descends as the weight of the salt-water and its richness in dissolved matter increase. The Swedish chemist and hydrographer, Ekman, after long series of experiments relative to this question, has found that this critical temperature may fall to -4° C. (25° Fahr.) in Atlantic water. The properties of a brackish fluid, such as would be drawn from a fiord, would naturally be intermediate between those of a pure and those of a very salt water. Hence the depths of the ocean can not be at 39° Fahr., as some authors still maintain. A slight excess of salt in solution will weight a stratum of water of mean temperature, whereby a cold zone may be superposed upon another zone which is warmer but more saline. The interior of the ocean, as well as its surface, is plowed by numerous currents, some warm, some cold, which meet, mix, and separate again, so that it is very hard to find out by reasoning what experiment alone can teach. A similar variety is shown in the density of water brought up by soundings. The complication is magnified when we reflect that water is not absolutely incompressible, that each thickness of ten metres exercises a vertical pressure nearly equal to an atmosphere, the action of which added to that of the superior parts weighs upon the inferior liquid, so that at about 4,000 metres the pressure is 400 atmospheres. Water must be extremely dense when it it compressed with so much force, and the influence of salinity and temperature must become very small in these unfathomable abysses. The question of submarine temperatures has given rise to many controversies. Some, with Perron, suppose that the great depths are always cold, like the tops of the highest mountains. On the other extreme, the author of "Epochs of Nature" attributed to the oceanic depths a high temperature on account of their nearness to the central fire. Denis de Montfort and Humboldt are of the opinion that below the superficial parts there prevails a constant temperature, peculiar to each station, and corresponding with the mean annual temperature of the place. This view is correct for regions where the depth is not very great, and in certain bodies of water.
The sea, on account of its great specific heat and its feeble conducting power, plays the part of a moderator of temperature something like that of the fly-wheel of an engine as a moderator of force. In winter it is warmer, in summer it is cooler, than the ambient air, and the difference is emphasized the farther we get away from the shore.
In "The Clouds" of Aristophanes, Strepsiades refuses to pay his creditors who hold that the level of the sea is fixed, believing that, as it receives all the water, it must continue to rise indefinitely. The phenomena of evaporation were not very well understood at that time. Even in the seventeenth century. Father Fournier talked of subterranean fissures or crevasses in which the waters of the Baltic and the Mediterranean, incessantly swelled by the rivers and by the currents of the Sund and of the Strait of Gibraltar, constantly lost themselves. During the last three years, the question of the evaporation of seawater has been much discussed between the partisans of the Saharian sea and their adversaries. The great point was to ascertain whether the proposed sea would not in the end become an enormous marsh. The sub-commission of the French Academy of Sciences was of the opinion that, other things being equal, salt water would evaporate less rapidly than fresh. Experiments by M. Dieulefait, on the other hand, indicated that a nearly equal loss would occur in the case of salt water and of fresh.
Fresh water freezes at 32° Fahr., but a liquid charged with salt congeals at lower temperatures; the rule is about the same as for the maximum of density, except that water slightly salt undergoes its contraction before being converted into ice, while normal sea-water acquires its minimum volume only in a state of surfusion, or when maintained artificially in a fluid state in capillary tubes. In this condition, a number of substances, water among them, are susceptible of being cooled considerably below their point of congelation and still remaining liquid. In the Baltic and White Seas, the waters of which for some depth are but weakly charged with salt, ice forms on the surface when the surrounding temperature has become low enough, while immediately below are strata more dense and relatively warmer. But suppose that below a certain depth of a brackish and warm liquid there is a cold salt current; the latter would produce such a refrigeration in the mixed intermediate strata that a mass of ice would be formed in the interior of the sea at the expense of the less saline zone. The block when formed would rise to the surface by virtue of its specific levity. This is what happens at the mouths of the great Siberian rivers. The Lena, in particular, pours out an enormous mass of warm water which overwhelms the salt waves from the polar regions. Even in the most favorable seasons, the navigator sails in the midst of floating ice-cakes that constitute a constant source of danger, while the thermometer dipped in the sea will indicate a temperature above the freezing-point. The depth of the warm stratum varies with the year, the place, and the prevailing winds; and hence we account for some explorers declaring impracticable tracks which others have easily sailed over. The northeast passage along the Siberian coast can never become a regular commercial route, unless, by repeated soundings accompanied by attentive studies, we can finally discover regular and periodical laws for the phenomena under consideration.
The Swedish physicist, Edlund, having inquired of the Scandinavian fishermen, was assured that they had sometimes, though rarely, seen the sea near the fiords of their country "vomit fragments of ice." The following is a textual reproduction of the story of one of the sailors concerning this curious and still little-known fact: "Not every year, but times enough, out on the open sea, I have seen ice come rapidly up to the surface. If the weather is calm, we can perceive, as far out as we can see, small cakes in the shape of a plate, coming from the bottom, rise to the surface. The edge is in the air, but, when the upper part of the plate gets above the level of the water, the plate turns over and lies flat upon the liquid. It is a dangerous business, for a boat may thus in a few minutes be surrounded by immense masses of new ice."
Aside from this anomaly, the formation of isolated blocks of ice in the open sea is very rare. Water of ordinary salinity becomes denser as it cools, for it freezes at about 28° Fahr., and, as we have explained, attains its maximum density at about 35° only if we keep it artificially in the liquid condition. Water that has lost its caloric in contact with the atmosphere soon sinks; sometimes, as Scoresby attests, ice which is formed at medium depth rises to the surface, while sounding thermometers indicate temperatures near or even below the point of congelation at the bottom. Otto Petterssen is of the opinion that, if water submitted to a cold of a few degrees below its freezing-point does not solidify, it is because immobility favors surfusion, or rather, what is very possible, because we do not know all the laws of nature.
Mr. Petterssen has succeeded by a series of experiments in explaining a variety of phenomena which manifest themselves in the boreal seas, and which Arctic explorers have long been acquainted with, without understanding the reason of them. Sea-water, after its passage to the solid state, has not the same chemical composition as before; but besides this change, which we shall speak of again, it has another interesting peculiarity. If the temperature is very low, the ice of the ocean, like nearly all known bodies, contracts by cold; but at a few degrees below the freezing-point, and before melting, it diminishes in volume when heated, and dilates on cooling. Between 14° Fahr. and -4°, according to the age and source of the block, there is produced a minimum of density, the mass acquiring its maximum volume—that is, the behavior of the solid is the inverse of that of river-water.
While it contracts by heating at about 18° or 23° Fahr., the ice of salt-water loses some of the properties which it possesses at lower temperature, and which are common to it with ordinary ice. It has no longer the vitreous aspect, the fragility, and the homogeneity of solid ice, but becomes softer, more plastic, and less transparent; its fracture is less distinct, and cracks and holes multiply in it. And, when brackish water congeals, it loses its disagreeable taste, but its bad looks and want of limpidity deprive it of commercial value.
Sea-water is a very complex saline solution; chemical analysis discovers in it halogen radicals, simple, as chlorine and bromine, or compound, as sulphuric acid and the four basic principles, soda, magnesia, lime, and potash. Chlorine is by far the most abundant principle, and should be credited with more than half the weight of the saline matter. Open any book on chemistry or the physics of the globe, and you will find that sea-water contains, by the litre, so much chloride of sodium, so much sulphate of magnesia, so much chloride of magnesium, etc. These affirmations are wholly hypothetical, for our acquaintance with chemistry is not sufficient to permit such conclusions. Analysis shows that there are in a litre of sea-water so much chlorine, so much sulphuric acid, and so much magnesia, but does not reveal to us how the radicals are united, for the combinations we get in analysis are not probably precisely the ones that existed in the water. We might say that the numerous simple bodies entering into the composition of seawater are all the time contracting new and incessantly variable alliances, according to the temperature or the concentration of the liquid. It is by intelligently utilizing these laws that they succeed at the salt works in forcing the mother-waters to deposit at one time cooking-salt, at another time some other combination useful in industry, or which it is desirable to get rid of.
In evaporating to dryness a known quantity of sea-water, under certain precautions, we obtain a residue which, well dried and weighed, furnishes the weight of the total quantity of salts originally dissolved. It is then easy, by a simple calculation, to estimate the proportion of solid substances contained in a litre. Salt-water is denser than fresh water of the same volume and temperature, and this excess of density is evidently proportional to its richness in saline matters. This can be obtained by multiplying the excess of density by 1·32. We may thus replace the chemical operation by a determination of density, an easier experiment, and one that can be made on board ships.
The different oceanic regions are not equally rich in salts. What we have said respecting variations of specific weight shows this very clearly. But, if we always draw the water from a sufficient depth, the variations become much less, as Forchharamer has proved. The figures in his tables oscillate between thirty-four and thirty-five grammes per litre. The relative proportion of the different elements is still more invariable, and we can establish a few slight differences only by taking the means of a large number of estimations. This fixity of relation might have been foreseen, because evaporation concentrates, without taking away an atom of salt, while fresh waters dilute without furnishing any. It follows, then, that the composition of a specimen of seawater can be estimated with a fair degree of accuracy by ascertaining the proportion of one of its constituents, the chlorine, for instance, and that element is much used as a standard. The amount of chlorine in a litre of liquid collected along the shore diminishes obviously when the ship is approaching ice, or if it is cruising near the mouth of a great river.
When concentrated by any means, sea-water deposits, first, carbonate of lime, next gypsum, or sulphate of lime, and then salt; and, lastly, the salts of magnesia and the bromides. The phenomena are not quite so simple in practice, and the deposits of salt-marshes are rarely composed of a single substance; but we have only intended to indicate the general course of the operation. The salt of commerce is rich in magnesia, or chloride of magnesium, in proportion to the strength of the concentration. Some have even sought to ascribe the enormous deposits of gypsum found in certain regions to ancient seas which, in drying up, deposited that substance among the first.
Potash and the bromides, substances that are relatively little abundant, accumulate in the mother-waters till they become so condensed as to make the industrial working-out of them remunerative. Bromine is less abundant in the Mediterranean than in the waters of the Dead Sea, which may some day become a source of production. Eighteen centuries ago the Romans, according to Pliny, brought to Italy at great expense the water of the Asphaltine Lake, the curative properties of which were held in high esteem. The excess of bromine in this water, however, corresponds exactly with its greater total saltness, so that, except for a few qualifications to which we shall refer again, the relative composition of the dry residue of the Dead Sea is the same as of that from the ocean. In other words, any marine water evaporated to the same degree of density as that of the Dead Sea would be as deleterious to living beings.
Marine ice was formerly regarded as formed of solidified pure water retaining by mechanical adhesion traces of the saline liquid. These traces could be expelled by energetic pressure, when acids and bases would be found in the residue of desiccation in invariable proportions as in the sea. The question of chemical composition of the ice of the Arctic Ocean is complicated in other ways, but it gains in interest what it loses in simplicity. When salt-water is cooled artificially, a small part escapes solidification. The uncongealed residue is insupportably bitter to the taste, and analysis shows that nearly all the magnesia is concentrated in it. The solid block, if it is homogeneous and is not full of holes, and if previously drained, may furnish a passable drink. The natural ices of the Northern Sea are frequently moistened with a kind of brine, which sometimes embodies crystals of special character, easy to distinguish from the ice around them. According to Otto Petterssen, the relative proportions of chlorine and magnesia are much stronger in these exudations than in the water at the expense of which the ice is formed. The liquid can not then have been mechanically absorbed. On the other hand, there is a deficiency of sulphates; and the conclusion that sea-water ice retains the sulphates more abundantly is confirmed by analysis. With congelation, a sorting of matters takes place; most of the sulphuric acid passes into the part that solidifies, while magnesia and chlorine prevail in the part that remains liquid. Under the influence of variations of temperature, all the chlorides in the block will gradually disappear: some go into the sea and are dissolved; while the rest appear on the surface and form hydrated crystals, or a kind of "salt-snow." The sulphates thus prevail exclusively in old ices, which, according to Mr. Petterssen, constitute mixtures of solidified water and a peculiar chemical compound, the criohydrate of sulphate of soda, a body which, containing five parts of soda to ninety-five of water, is decomposed at a little below the ordinary freezing-point.
By these phenomena of selection, ice, under atmospheric vicissitudes, approaches a limit when its composition would be fixed, without reaching it in reality. Usually, the expulsion of chlorides is not complete, and sudden changes of temperature may liquefy it at once. The Swedish observer compares the ice of salt-water with a kind of granite, each constituent of which should take its turn at decomposing under special circumstances. The warm waters farthest from the pole would bear only the stable constituents brought down by the Arctic current, so that, to continue our comparison, the river, which has gradually eaten away the granite block, finally transports the last remains of the rock in the form of sands and clays, which are destined to accumulate in the sedimentary deposits.
Over and above the substances that exist in considerable proportions, many rarer elements are found in the waters of the ocean; minerals, gases, and organic remains, difficult perhaps to recognize, sometimes impossible to estimate, but which nevertheless play an important part. The phenomena of accumulation which we have considered are absolutely insignificant in comparison with the absorbing power of some of the algæ. In them Courtois discovered iodine in 1812, and Malagutti, after laborious researches, detected copper, lead, silver, and iron, metals which he afterward found in sea-water itself. The quantity of iodine contained is so little appreciable that many doctors have denied the therapeutic virtues which others have attributed to this water. Nevertheless, absorbed and condensed by marine plants, it becomes abundant enough to be extracted with profit. It likewise accumulates in animal organisms; and cod-liver oil owes its beneficent properties to it. The silver was absurdly attributed by Proust to the treasures of shipwrecked vessels. But the quantity, though infinitesimal in a measured quantity of water, is in the aggregate immense. Malagutti more rationally refers its origin to the solution of the lead ores, very abundant all over the globe, with which sulphurets of silver and copper are combined. By the action of salt, the sulphurets are converted into chlorides. As to iron, it would be strange if so universal a substance were not found in the sea; and the same may be said of phosphoric acid.
The researches of M. Dieulefait into the presence of lithium in sea-water have shown that the Dead Sea is an independent body of water, and not an abandoned lagoon of the Red Sea. By chemical and spectral analysis, it contains neither iodine nor silver, nor lithine, while all those substances are found in the Arabian Gulf, a body whose waters differ from those of the oceans only by their greater density consequent on the strong evaporation to which they are subjected.
The determination of the air dissolved in the ocean is attended with many difficulties. We can only indicate a few prominent principles. This air has not the same composition as the air we breathe, although it differs but little in that respect from the air held in springs and rivers. Oxygen, which forms only a fifth of the atmospheric air, being more soluble in water than nitrogen, constitutes about one third of the air which is expelled from water by boiling. The volume of gas absorbable by water diminishes as the temperature rises. Cold water is richer in air than warm. Moreover, the law of decrease being regular for nitrogen, while it is less simple for oxygen, the relative proportions of the two elements are variable in waters of different temperatures. According to Mr. Tornöe, there is a little more oxygen at the surface than theory calls for, while in the zones where animal life is largely developed there is a slight deficiency. The presence of sunlight, or the cutting of it off by clouds, has no important effect; and the same may be said of the enormous pressures to which deep-sea waters are subjected. But little carbonic acid occurs dissolved in a free state, although that gas is very abundant in combination. Mr. Tornöe, who has given this subject careful attention, thinks the older chemists collected carbonic-acid products of the decomposition of carbonates or bicarbonates at the boiling-point. He finds an alkaline reaction in sea-water, which he attributes to a small quantity of free salts of soda. Mr. Hamberg, a Swedish chemist, who has recently studied the waters of the Greenland seas, agrees with Mr. Schloesing that marine water contains neutral carbonates, bicarbonates, and slight traces of free carbonic acid, and that temperature and atmospheric pressure have a complex influence on both the uncombined gas and that which is united to bases.—Translated for the Popular Science Monthly from the Revue des Deux Mondes.