even the most remote of the western ifles, St. Kilda.

About fifteen years before the publication of Dr. Franklin's Memoirs, the following paragraph, perhaps copied from fome London Newfpaper, was inferted in the Annual Regifter. "It has been remarked, it is faid, that the oil spilt into the river to prevent the fpreading of the late dreadful fire in Thames ftreet, vifibly quieted the waves thereof. This efficacy of oil, in fmoothing the furface of water, feems to have been long known. By an ancient law, when goods were to be thrown overboard to lighten the ship in ftormy weather, if there happened to be any oil on board, and it could be come at, it was to go first; and the Ragufians at this day, when they go a fish-fpearing, throw oil upon the water with a sprinkling brush, and thereby obtain a clear profpect of the bottom. The openings thus formed by the drops they exprefsly call windows."

By all thefe obfervations, it will appear, that Dr. Franklin cannot be called the discoverer of this fact: but ftill the philofophical world is greatly indebted to the ardour and zeal, with which he profecuted his inquiry and experiments, which enabled him to give fo ingenious an illustration of the phænomenon.

FACTS

FACTS and QUERIES relative to ATTRACTION and REPULSION. BY THOMAS PERCIVAL, M.D. &c.

I

To the LITERARY and PHILOSOPHICAL SOCIETY.

MANCHESTER, Dec. 5, 1784.

COMMUNICATED to you, a few weeks ago, fome curious and valuable observations, on the phenomena which take place between oil and water, tranfmitted to me by my learned and very ingenious friend Dr. Wall, of Oxford. My engagements deprived me of the pleasure and inftruction, of attending their difcuffion in the fociety: And, folicitous to recover what I have loft, I trust you will indulge me with permiffion, to recall your attention to the fubject, by the recital of a few mifcellaneous facts and enquiries, which the perufal of that paper fuggefted to my mind.

I. If a glafs tumbler, containing equal parts of water and of oil, in fuch quantity as to occupy two thirds of it,, be fufpended by a cord, and fwung backwards and forwards, the oil will remain perfectly smooth and undisturbed, whilft the water, below, is in violent commotion. But if the oil be poured out, and its place supplied

with

with water, the fluid will remain perfectly tranquil, throughout the whole veffel, although the fame motion be given to it as before. I have frequently repeated this experiment, and have fometimes varied it, by fubftituting rectified fpirit of wine, in the place of water. The oil then being the heavier fluid, becomes agitated, whilft the fpirit remains at reft. Dr. Franklin, who first noticed this fingular phenomenon, informs us, that he fhewed it to a number of ingenious perfons. "Thofe," fays he, "who are but "flightly acquainted with the principles of hydrostatics, &c. are apt to fancy, immedi

ately, that they understand it, and readily attempt to explain it: But their explanations "have been different, and, to me, not very "intelligible. Others more deeply skilled in "thofe principles, feem to wonder at it, and "promise to confider it. And I think it is "worth confidering. For a new appearance, if "it cannot be explained by our old principles,

may afford us new ones, of use perhaps in "explaining fome other obfcure parts of natural "knowledge."* It is with diffidence, I offer as a conjecture, that the fact, in question, may arife from a repulfive power, fubfifting between the particles of oil and water, and depending poffibly on the vibrations of that fubtle æther,

* See Dr. Franklin's Letters and Papers on philofophical fubjects, p. 438.

which Sir Ifaac Newton fuppofes to pervade all bodies. For, when this æther is excited into motion, by percuffion or agitation, its elaftic force is augmented, because it becomes denfer in the pulfes of its vibrations, than in a quiefcent ftate. But in propofing this hypothefis, I may perhaps be chargeable with the paradoxical opinion of a celebrated French philofopher, M. Fontenelle, who afferts, that if there be more than one way of accounting for any appearances in nature, there is a general prefumption, that they proceed from caufes, which are leaft obvious and familiar. I fhall not, therefore, at present, enlarge upon this point, as it would anticipate what may be better urged, in our fubfequent converfation. But the facts, above recited, furnifh a prefumption, that the effect of oily fubstances, on the cryftallization of falt, is, in part, owing to a mechanical caufe. At Droitwich, it is the practice, as appears by Dr. Wall's quotation from the hiftory of Worcestershire, to throw, into the brine pan, a piece of refin, about the fize of a pea, to produce a finer granulation. The more refin they ufe, the fmaller will be the grain of the falt; and if a lump, of the fize of two walnuts, were put into the pan, the particles of falt would be fo minute as not to be capable

On the properties of Ether, confult Dr. Bryan Robinfon's Works, paffim.

of

of fubfiding. Refin, butter, or tallow, when liquefied by the heat of the boiling brine, float upon its furface, and will remain perfectly smooth and undisturbed, whilft the water, beneath, may be put into strong agitation, by the action of the fire. Such agitation must break down the crystals of falt, as they shoot; and consequently, only small granules will be produced.

II. Every one has experienced the fuffocating effects of air, loaded with the effluvia of burnt greafe, or the fnuff of a lamp. When fuch fumes are infpired, there is the fenfation of a conflict in the lungs, which effentially differs from what is felt, on breathing either fixed or inflammable air. And is not the most easy folution of it, to fuppofe, that the air quits the oily, to unite with the watery vapours, which are brought into contiguity, by this action of the animal œconomy; and that a ftrong repulfion fucceeds? "For, as, in algebra, where "affirmative quantities vanish and cease, there "negative ones begin, fo, in mechanics, where "attraction ceases, there a repulfive power ought "to fucceed," according to the doctrine of Sir Ifaac Newton. It is, alfo, an axiom, laid down by this great philofopher, that "to the fame "natural effects, we must always affign, as "far as poffible, the fame caufes." I fhall therefore proceed to illuftrate this fubject, by other more decifive examples of repulsion; after

premifing