Early Determination of Salinity: from Ancient Concepts to Challenger Results

The saltiness of seawater has been recognized throughout recorded history.  Theories about the origin of the seawater and the nature of saltness of the sea have first been formulated by ancient philosophers.

 In the 6th century BC Greek scientists abandoned mythological interpretations of the universe in favour of explanations relying on natural causes.  Famous pre-Socratic philosopher and poet Empedocles (490-430BC) is known as a founder of the cosmogenic theory of the four classical elements. But he was 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 Appolonia). One more philosopher simply attributed the saltness 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 (384-322BC) summarised Greek philosophers’ views 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 saltness, for all sweat is salt. Others say that the saltness is due to the earth. Just as water strained through ashes becomes salt, so the sea owes its saltness to the mixture of earth with similar properties.” Aristotle mentioned that salt water was heavier and more dense 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 saltness and bitterness appear at the same time.” One of Aristotle’s experimental proofs that saltness 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 off as it were by a filter.” Aristotle’s attempts to explain the saltness of the sea were hardly altogether clear and the wax container experiment is just one simple example. Later confusion and misconceptions did arise concerning this subject area. From a compositional standpoint, Aristotle tried to answer the questions: why the seawater 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 seawater.

Roman natural philosopher and naval commander Pliny the Elder (23-79AD) in his fundamental work “Naturalis Historia” in Chapter 104 “Why the sea is salt”, presents Aristotele’s discoveries and gives qualitative description of salinity distribution with depth: “Hence it is that the widely-diffused sea is impregnated with the flavour 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 dence than fresh water, and he indicates that patches of fresh water can be found floating on the surface of the sea.

Another Roman and contemporary of Pliny was the philosopher  Lucius Seneca (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 calcereous 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 saltness was a constant.

Richard Watson (Bishop of Landaff) said: “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 saltness of the sea. The solution of it had perplexed the philosophers before the 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… The fluctuations of the ocean itself were scarcely more various than opinions of men concerning the origin of its saline impregnation”.