The phrase to pour oil on troubled waters means to try to settle a disagreement or dispute with words intended to placate or pacify those involved.
It alludes to the calming effect of oil on the agitated surface of water, which the Roman statesman and scholar Pliny the Elder (23-79) mentioned in his vast encyclopaedia of the natural and human worlds, Naturalis Historia (The Natural History – 77):
Everything is soothed by oil, and that this is the reason why divers send out small quantities of it from their mouths, because it smoothes any part which is rough and transmits the light to them.
(translation: John Bostock & H.T. Riley – London, 1855)
omne oleo tranquillari, et ob id urinantes ore spargere, quoniam mitiget naturam asperam lucemque deportet.
The phrase is also associated with a story about the Irish missionary St. Aidan (died 651), as told by the English monk, theologian and historian St. Bede (circa 673-735) in Historia ecclesiastica gentis Anglorum (The Ecclesiastical History of the English People – 731):
Book III, Chapter XV. How Bishop Aidan foretold to certain seamen that a storm would arise, and gave them some holy oil to calm it.
[…] A certain priest, whose name was Utta, a man of great weight and sincerity, and on that account honoured by all men, even the princes of the world, was sent to Kent, to bring thence, as wife for King Oswy, Eanfled, the daughter of King Edwin, who had been carried thither when her father was killed. Intending to go thither by land, but to return with the maiden by sea, he went to Bishop Aidan, and entreated him to offer up his prayers to the Lord for him and his company, who were then to set out on so long a journey. He, blessing them, and commending them to the Lord, at the same time gave them some holy oil, saying, “I know that when you go on board ship, you will meet with a storm and contrary wind; but be mindful to cast this oil I give you into the sea, and the wind will cease immediately; you will have pleasant calm weather to attend you and send you home by the way that you desire.”
All these things fell out in order, even as the bishop had foretold. For first, the waves of the sea raged, and the sailors endeavoured to ride it out at anchor, but all to no purpose; for the sea sweeping over the ship on all sides and beginning to fill it with water, they all perceived that death was at hand and about to overtake them. The priest at last, remembering the bishop’s words, laid hold of the phial and cast some of the oil into the sea, which at once, as had been foretold, ceased from its uproar. Thus it came to pass that the man of God, by the spirit of prophecy, foretold the storm that was to come to pass, and by virtue of the same spirit, though absent in the body, calmed it when it had arisen. The story of this miracle was not told me by a person of little credit, but by Cynimund, a most faithful priest of our church, who declared that it was related to him by Utta, the priest, in whose case and through whom the same was wrought.
(from: Bede’s Ecclesiastical History of England – London, 1907)
Presbyter quidam, nomine Utta, multae grauitatis ac ueritatis uir, et ob id omnibus, etiam ipsis principibus saeculi honorabilis, cum mitteretur Cantiam ob adducendam inde coniugem regi Osuio, filiam uidelicet Aeduini regis Eanfledam, quae occiso patre illuc fuerat adducta; qui terrestri quidem itinere illo uenire, sed nauigio cum uirgine redire disponebat, accessit ad episcopum Aidanum, obsecrans eum pro se suisque, qui tantum iter erant adgressuri, Domino supplicare. Qui benedicens illos ac Domino commendans, dedit etiam oleum sanctificatum: ‘Scio,’ inquiens, ‘quia, ubi nauem ascenderitis, tempestas uobis, et uentus contrarius superueniet; sed tu memento, ut hoc oleum, quod tibi do, mittas in mare; et statim quiescentibus uentis, serenitas maris uos laeta prosequetur, ac cupito itinere domum remittet.’ Quae cuncta, ut praedixerat antistes, ex ordine conpleta sunt; et quidem inprimis furentibus undis pelagi, temtabant nautae anchoris in mare missis nauem retinere, neque hoc agentes aliquid proficiebant. Cumque uerrentibus undique et inplere incipientibus nauem fluctibus, mortem sibi omnes inminere, et iamiamque adesse uiderent, tandem presbyter reminiscens uerba antistitis, adsumta ampulla misit de oleo in pontum, et statim, ut praedictum erat, suo quieuit a feruore. Sicque factum est, ut uir Dei et per prophetiae spiritum tempestatem praedixerit futuram, et per uirtutem eiusdem spiritus hanc exortam, quamuis corporaliter absens, sopiuerit. Cuius ordinem miraculi non quilibet dubius relator, sed fidelissimus mihi nostrae ecclesiae presbyter, Cynimund uocabulo, narrauit, qui se hoc ab ipso Utta presbytero, in quo et per quem conpletum est, audisse perhibebat.
The calming effect of oil on the sea has therefore been common knowledge since ancient times (see footnote), but it remained little more than an observation until the experiment carried out by the American statesman, inventor and scientist Benjamin Franklin (1706-90). In a letter to the British doctor and scientist William Brownrigg (1711-1800), written in London on 7th November 1773, Franklin first explained what led him to conduct this experiment (incidentally, it is interesting that the method used by divers in the Mediterranean had not changed since Pliny’s time, one thousand and seventy hundred years earlier):
In 1757, being at sea in a fleet of 96 sail bound against Louisbourg, I observed the wakes of two of the ships to be remarkably smooth, while all the others were ruffled by the wind, which blew fresh. Being puzzled with the differing appearance, I at last pointed it out to our captain, and asked him the meaning of it? “The cooks, says he, have, I suppose, been just emptying their greasy water through the scuppers, which has greased the sides of those ships a little;” and this answer he gave me with an air of some little contempt, as to a person ignorant of what every body else knew. In my own mind I at first slighted his solution, tho’ I was not able to think of another. But recollecting what I had formerly read in Pliny, I resolved to make some experiment of the effect of oil on water, when I should have opportunity.
Afterwards being again at sea in 1762, I first observed the wonderful quietness of oil on agitated water, in the swinging glass lamp I made to hang up in the cabin, as described in my printed papers, page 438 of the fourth edition.—This I was continually looking at and considering, as an appearance to me inexplicable. An old sea captain, then a passenger with me, thought little of it, supposing it an effect of the same kind with that of oil put on water to smooth it, which he said was a practice of the Bermudians when they would strike fish, which they could not see, if the surface of the water was ruffled by the wind. This practice I had never before heard of, and was obliged to him for the information; tho’ I thought him mistaken as to the sameness of the experiment, the operations being different; as well as the effects. In one case, the water is smooth till the oil is put on, and then becomes agitated. In the other it is agitated before the oil is applied, and then becomes smooth.—The same gentleman told me, he had heard it was a practice with the fishermen of Lisbon when about to return into the river, (if they saw before them too great a surf upon the bar, which they apprehended might fill their boats in passing) to empty a bottle or two of oil into the sea, which would suppress the breakers, and allow them to pass safely: a confirmation of this I have not since had an opportunity of obtaining. But discoursing of it with another person, who had often been in the Mediterranean, I was informed that the divers there, who, when under water in their business, need light, which the curling of the surface interrupts by the refractions of so many little waves, let a small quantity of oil now and then out of their mouths, which rising to the surface smooths it, and permits the light to come down to them.—All these informations I at times revolved in my mind, and wondered to find no mention of them in our books of experimental philosophy.
(from: Of the stilling of Waves by means of Oil. Extracted from sundry Letters between Benjamin Franklin, LL. D. F. R. S. William Brownrigg, M. D. F. R. S. and the Reverend Mr. Farish – published by the Royal Society of London in Philosophical Transactions – London, 1774)
Franklin then described to William Brownrigg the experiment itself:
At length being at Clapham where there is, on the common, a large pond, which I observed to be one day very rough with the wind, I fetched out a cruet of oil, and dropt a little of it on the water. I saw it spread itself with surprising swiftness upon the surface; but the effect of smoothing the waves was not produced; for I had applied it first on the leeward side of the pond, where the waves were largest, and the wind drove my oil back upon the shore. I then went to the windward side, where they began to form; and there the oil, though not more than a tea spoonful, produced an instant calm over a space several yards square, which spread amazingly, and extending itself gradually till it reached the lee side, making all that quarter of the pond, perhaps half an acre, as smooth as a looking-glass.
After this, I contrived to take with me, whenever I went into the country, a little oil in the upper hollow joint of my bamboo cane, with which I might repeat the experiment as opportunity should offer; and I found it constantly to succeed.
In these experiments, one circumstance struck me with particular surprize. This was the sudden, wide, and forcible spreading of a drop of oil on the face of the water, which I do not know that any body has hitherto considered. If a drop of oil is put on a polished marble table, or on a looking-glass that lies horizontally; the drop remains in its place, spreading very little. But when put on water it spreads instantly many feet round, becoming so thin as to produce the prismatic colours, for a considerable space, and beyond them so much thinner as to be invisible, except in its effect of smoothing waves at a much greater distance.
Two hundred years after Franklin’s experiment the historical researches of another surface chemist, Professor Charles Giles, enabled him to identify the pond, which still existed on Clapham Common. He repeated Franklin’s experiment taking photographs of the effect:
Clapham pond before and after a drop of oil is added
source: Franklin’s teaspoon of oil – the Royal Society of Chemistry
The earliest known figurative use of oil on troubled waters refers explicitly to Benjamin Franklin’s experiment; it is from a letter that the physician, politician, social reformer and educator Benjamin Rush (1745-1813) wrote on 25th May 1786 to Richard Price (1723-91), a Welsh Nonconformist minister, moral philosopher, and political and economic theorist:
Our venerable friend Dr. Franklin continues to enjoy as much health and spirits as are compatible with his time of his life. I dined with him a few days ago in a most agreeable circle where he appeared as chearful [sic] and gay as a young man of five and twenty. But his conversation was full of the wisdom and experience of mellow old age. He has destroyed party rage in our State, or to borrow an allusion from one of his discoveries, his presence and advice, like oil upon troubled waters, have composed the contending waves of faction which for so many years agitated the State of Pennsylvania.
(from: Franklin in His Own Time, edited by Kevin J. Hayes and Isabelle Bour – University of Iowa Press, 2011)
The second-earliest figurative use that I have found is from The Globe (London) of 8th December 1809 (puffer denotes a person who praises the merits of someone in an exaggerated manner):
The puffers of Marquis Wellesley have commenced their operations in a truly Oriental style.—No common flattery, they conceive, will suit his palate, or please an epicure of such exquisite taste. The Courier, one of the most industrious, although not the most skilful of cooks, last night, served up a large dish to his Lordship, in which, it says, “Marquis Wellesley’s departure from Spain was considered a public calamity, and it was at first intended, by the populace, to prevent his embarking for England, so anxious were they to keep him in Spain.”—In another part of the same paper we are told, that his Lordship’s accession to the Ministry, under Mr. Perceval, has gratified all parties, and that it has no doubt his Lordship will amply gratify their good opinion.—This prophecy, we fear, will not be verified; and the cook in The Courier will probably find, that, instead of operating like oil on the troubled waters, it is more likely to operate like oil in the fire.
Note: The earliest recorded observations of adding oil to water seem to date back to the 18th century BC in Babylon. The phenomena observed when oil was poured into a bowl of water were seen as omens for the future and descriptions of the phenomena and the events they foretold were inscribed in cuneiform on clay tablets. (source: Franklin’s teaspoon of oil – the Royal Society of Chemistry)