Who with fine films, suspended o'er the deep, Of Oil effusive lull the waves to sleep.

CANTO III. l. 87.

There is reason to believe that when oil is poured upon water, the two surfaces do not touch each other, but that the oil is suspended over the water by their mutual repulsion. This seems to be rendered probable by the following experiment: if one drop of oil be droped on a bason of water, it will immediately diffuse itself over the whole, for there being no friction between the two surfaces, there is nothing to prevent its spreading itself by the gravity of the upper part of it, except its own tenacity, into a pellicle of the greatest tenuity. But if a second drop of oil be put upon the former, it does not spread itself, but remains in the form of a drop, as the other already occupied the whole surface of the bason, and there is friction in oil passing over oil, though none in oil passing over water.

Hence when oil is diffused on the surface of water gentle breezes have no influence in raising waves upon it; for a small quantity of oil will cover a very great surface of water, (I suppose a spoonful will diffuse itself over some acres) and the wind blowing upon this carries it gradually forwards; and there being no friction between the two surfaces the water is not affected. On which account oil has no effect in stilling the agitation of the water after the wind ceases, as was found by the experiments of Dr. Franklin.

This circumstance lately brought into notice by Dr. Franklin had been mentioned by Pliny, and is said to be in use by the divers for pearls, who in windy weather take down with them a little oil in their mouths, which they occasionally give out when the inequality of the supernatant waves prevents them from seeing sufficiently distinctly for their purpose.

The wonderful tenuity with which oil can be spread upon water is evinced by a few drops projected from a bridge, where the eye is properly placed over it, passing through all the prismatic colours as it diffuses itself. And also from another curious experiment of Dr. Franklin's: he cut a piece of cork to about the size of a letter-wafer, leaving a point standing off like a tangent at one edge of the circle. This piece of cork was then dipped in oil and thrown into a large pond of water, and as the oil flowed off at the point, the cork-wafer continued to revolve in a contrary direction for several minutes. The oil flowing off all that time at the pointed tangent in coloured streams. In a small pond of water this experiment does not so well succeed, as the circulation of the cork stops as soon as the water becomes covered with the pellicle of oil. See Additional Note, No. XIII. and Note on Fucus, Vol. II.

The ease with which oil and water slide over each other is agreeably seen if a phial be about half filled with equal parts of oil and water, and made to oscillate suspended by a string, the upper surface of the oil and the lower one of the water will always keep smooth; but the agitation of the surfaces where the oil and water meet, is curious; for their specific gravities being not very different, and their friction on each other nothing, the highest side of the water, as the phial descends in its oscillation, having acquired a greater momentum than the lowest side (from its having descended further) would rise the highest on the ascending side of the oscillation, and thence pushes the then uppermost part of the water amongst the oil.