Chapter 1: Early Determination of Salinity: from Ancient Concepts to Challenger Results
Chapter 2: Introduction of the International Standard Seawater – by Martin Knudsen
Chapter 3: Implementation of the electrical conductivity of seawater –partly M.Knudsen
Chapter 4: Development of the Practical Salinity Scale
1. Early Determination of Salinity: from Ancient Concepts to Challenger Results
Empedocles
The saltiness of seawater has been recognized throughout recorded history. Theories about origin of the seawater and nature of saltiness of the sea have first been formulated by ancientphilosophers. In the 6th century BC Greek scientists abandoned mythological interpretations of the universe in favor of explanation relying on natural causes. Famouspre-Socratic philosopher and poet Empedocles (490-430BC) is known as a founder of the cosmogony theory of the four classical elements. But hewas also the first philosopher, who left such poetical definition of the seawater origin:"The Sea is the Sweat of the Earth " (The Fragments, Book1, p.179) Another philosopher said that all the earth was at first surrounded with moisture, some of which later formed the sea in a process of drying which would ultimately end in the loss of the sea (attributed to Anaximander and Diogenes of Apollonia). One more philosopher simply attributed the saltiness to the earth that the water picked up as it came in contact with the earth as it ran over it, just as water strained through ashes is known to be salty. The sea itself was explained by the accumulation of the run-off (attributed to Metrodorus of Chios and Anaxagoras).
Aristotle
Aristotle (384-322BC) summarizedGreek philosopher´sviews by saying: “At first the Earth was surrounded by moisture. Then the sun began to dry it up, part of it evaporated, and is the cause of winds while the remainder formed the seas. So the seas are being dried up. Others say that the sea is a kind of sweat exuded by the earth when the sun heats it, and that this explains its saltiness, for all sweat is salt. Others say that the saltiness is due to the earth. Just as water strained through ashes becomes salt, so the sea owes its saltiness to the mixture of earth with similar properties.” Aristotle mentioned that salt water was heavier and more dens than fresh water, and salt water would seek a lower level. Aristotle was aware that the sea contained other than just salt and commented on both its salt and bitter taste. Aristotle apparently was the first person to have noticed and attempted to explain the bitter quality of sea water - a point which did not occur in the literature for at least two thousand years later. More specifically he was the first to mention something other than salt and water in seawater. The salt from the sea (prepared by evaporation) was weaker in saltiness and was generally not as white, and less lumpy than "normal" salt. He added: "Why is the sea salty and bitter? Is it because the juices in the sea are numerous? For saltiness and bitterness appear at the same time." One of Aristotle's experimental proofs that saltiness is due to the admixture of some substance was with the use of a completely closed wax vessel. This container was lowered into the sea and: "then the water that percolates through the wax sides of the vessel is sweet, the earthy stuff, the admixture of which makes the water salt, being separated of as it were by a filter." Aristotle's attempts to explain the saltiness of the sea were hardly altogether clear and the wax container experiment is just one simple example. Later confusion andmisconceptionsdid ariseconcerningthis subject area. From a compositional standpoint, Aristotle tried to answer the questions: why the sea water was salty, why water which is naturally fresh became salt, and what was the nature of the material that caused the bitter taste present in sea water.
Pliny the Elder
Romannatural philosopher and naval commander Pliny the Elder (25-76AC) in his fundamental work "Naturalis Historia" in Chapter 104 "Why the sea is salt", presents Aristotele's discoveriesandgivesqualitative description of salinity distribution with depth: "Hence it is that the widely-diffused sea is impregnated with the flavor of salt, in consequence of what is sweet and mild being evaporated from it, which the force of fire easily accomplishes; while all the more acrid and thick matter is left behind; on which account the water of the sea is less salt at some depth than at the surface." In his explanation Pliny followed Aristotle, and helped to open up magnificent arena for the scholastics of the Middle Ages to dispute in. Pliny appears to have been the first person to give an early quantitative estimate for the amount of salt in sea water by which one could make sea water: "If more than a sextarius of salt is dropped into four sextari of water, the water is overpowered, and the salt does not dissolve. However, a sextarius of salt and four sextari of water give the strength and properties of the saltest sea. But it is thought that the most reasonable proportion is to compound the measure of water given above with eight cyathi of salt. This mixture warms the sinews without chafing the skin." As in a case of Aristotle, Pliny thought the salt content should be greater at the surface due to the loss of water here. Yet along with Aristotle Pliny knew that salt water was more dense than fresh water, and he indicates that patches of fresh water can be found floating on the surface of the sea.
Seneca
Another Roman and contemporary of Pliny was the philosopher LuciusSeneca (3BC-65AD). Seneca's view as to the nature of the world appeared in his Quaestiones Naturales. Seneca was a keen observer, and much of the Quaestiones Naturales present his own observations with more originality than, for example, Pliny. Seneca had noticed that the water level and the salinity of the sea remained constant even though water was constantly being added by rivers and rain. The constancy was, he believed, due to the evaporation of the sea's waters. He believed that saline waters could be filtered by earth and attributed the formation of calcareous tuffs to this action. Seneca thought that the world in the beginning was characterized by a primordial ocean, and the substances dissolved therein separated out over some space of time. Although he know that solubility of a substance was in some way related to the water's temperature and that the temperature of the sea varied, he seems to have believed that the ocean's saltiness was a constant.
Bishop Watson says: "There are few questions respecting the natural history of the globe which have been discussed with more attention, or decided with less satisfaction, than that concerning the primary cause of the saltiness of the sea. The solution of it had perplexed the philosophers before time of Aristotle; it surpassed his own great genius, and those of his followers who have attempted to support his arguments have been betrayed into very ill grounded conclusions concerning it. Father Kircher, after having consulted three and thirty authors upon the subject, could not help remarking, that the fluctuations of the ocean itself were scarily more various then opinions ofmen concerning the origin of its saline impregnation".
Leonardo da Vinci
In the Renaissance period Leonardo da Vinci (1452-1519) in his famousThe Notebooks in Note 946 "Refutation of the Pliny's theory of the saltiness of the sea" polemicizewith Pliny: "Pliny says, that the water of the sea is salt because the heat of the sun dries up the moisture and drinks it up; and this gives to the wide stretching Sea the savor of salt. But this cannot be admitted, because if the saltiness of the sea were caused by the heat of the sun, there can be no doubt that lakes, pools and marshes would be so much the more salt, as their waters have less motion and are of less depth; but experience shows us, on the contrary, that these lakes have their waters quite free from salt. Again it is stated by Plinythat this saltiness might originate, because all the sweet and subtle portions which the heat attracts easily being taken away, the more bitter and coarser part will remain, and thus the water on the surface is fresher than at the bottom; but this is contradicted by the same reason given above. Again, it has been said that the saltiness of the sea is the sweat of the earth; to this it may be answered that all the springs of water which penetrate through the earth, would then be salt. But the conclusion is, that the saltiness of the sea must proceed from the many springs of water which, as they penetrate into the earth, find mines of salt and these they dissolve in part, and carry with them to the ocean and the other seas, whence the clouds, the begetters of rivers, never carry it up. And the sea would be saltier in our times than ever it was at any time; and if the adversary were to say that in infinite time the sea would dry up or congeal into salt, to this I answer that this salt is restored to the earth by the setting free of that part of the earth which rises out of the sea with the salt it has acquired, and the rivers return it to the earth under the sea."
Robert Boyle
Scientific work on ocean salts was first done by the celebrated British natural philosopher Robert Boyle in 1674 with his publication of "Observations and Experiments in the Saltiness of the Sea": "The Cause of the Saltiness of the Sea appears by Aristotle's Writings to have busied the Curiosity of Naturalists before his time; since which, his Authority, perhaps much more than his Reasons, did for divers Ages make the Schools and the generality of Naturalists of his Opinion, till towards the end of the last Century, and the beginning of ours, some Learned Men took the boldness to question the common Opinion; since when the Controversy has been kept on foot, and, for ought I know, will be so, as long as this argued on both sides but by Dialectical Arguments, which may be probable on both sides, but are not convincing on either. Wherefore I shall here briefly deliver some particulars about the Saltiness of the Sea, obtained by my own trials, where I was able; and where I was not, by the best Relations I could procure, especially from Navigators."
“After all, (says he,) it may be observed, that we are inquiring into the cause of a phenomenon, which it may be said had no secondary cause at all. It is taken for granted, in this disquisition, that the water which covered the globe in its chaotic state, was not impregnated with salt as at present, but quite fresh: now this is an opinion concerning a matter of fact, which can never be proved either way; and surely we extend our speculations very far, when we attempt to explain a phenomenon, primeval to, or coeval with, the formation of the earth."
Saltness of the Sea
This sensible writer then states the different experiments which have been made to discover the saltiness of the sea, round the shores of Britain; and proposes the following simple method of ascertaining it with tolerable certainty:"As it is not every person who can make himself expert in the use of common means of estimating the quantity of salt has g contained in sea water, I will mention a method of doing it, which is so easy and simple, that every common sailor may understand and practice it; and which, at the same time, from the trials I have made of it, seems to be as exact a method as any that has yet been thought of. - Take a clean towel, or any other piece of cloth; dry it well before the sun or before the fire, then weigh it accurately, and note down its weight; dip it in the sea water, and, when taken out, wring it a little till it will not drip when hung up to dry; weigh it in this wet state, then dry it in the sun or at the fire, and when it is perfectly dry, weigh it again: the excess of the weight of the wetted cloth above its original weight, is the weight of the sea water imbibed by the cloth; and the excess of the weight of the cloth after being dried, above its original weight, is the specific gravity of the salt retained by the cloth; and by comparing this weight with the weight of the sea water imbibed by the cloth, we obtain the proportion of salt contained in that species of sea water." Whoever undertakes to ascertain the quantity of salt contained in sea water, either by this or any other method, would do well to observe the state of the weather preceding the time when the sea water is taken out of the sea; for the quantity of salt contained in the water near the surface, may be influenced, both by the antecedent moisture, and the antecedent heat of the atmosphere. And this leads to the consideration of a question proposed by Aristotle, - Why are the upper parts of the sea Salter and warmer than the lower? Some philosophers, admitting the fact, have followed him in attempting to explain it; whilst others have thought themselves authorized by experiment to deny the truth of the position; and those, perhaps, will argue with the greatest justness, who shall affirm that it is neither generally to be admitted, nor generally to be rejected, but that the sea in some places, and under certain circumstances, is saltier and warmer at the surface, than at any considerable depth beneath it, while in many others the reverse is true. The question consists of two parts, between which, though there probably is a connection, yet it is not so necessary a one as to hinder us from considering each part by itself. With regard to the use of this salt property of sea water, it is observed, that the saltiness of the sea preserves its waters pure and sweet, which otherwise would corrupt, and emit a stench like a filthylake, and consequently that none of the myriads of creatures which now live therein could exist. From thence also the sea water becomes much heavier, and therefore ships of greater size and burden are safely borne thereon. Salt water also does not freeze so soon as fresh water, hence the Seas are morepossiblefor navigation.