Page:Popular Science Monthly Volume 43.djvu/516

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THE POPULAR SCIENCE MONTHLY.

tensions is greater than the sum of the other two, the three fluids can not be in equilibrium in contact.

If, therefore, the tension of the surface separating air from water is greater than the sum of the tensions of the surfaces separating air from oil and oil from water, then a drop of oil can not be in equilibrium on the surface of water. The edge of the drop where the air meets the oil and the water becomes thinner and thinner, till it covers a vast expanse of water.

M. Quinke has determined the superficial tensions of different liquids in contact with one another and with air, and the following is an extract from his table of results. The tension is measured in grammes per linear centimetre at twenty degrees centigrade:

Liquid. Specific gravity. Tension of surface
separating liquid
from air.
Tension of surface
separating liquid
from water.
Water 1·0000 ·08235 ·00000
Olive oil 0·9136 ·03760 ·02096

Although olive oil is here taken as the representative of oils, it is not considered so well adapted for use at sea as some of the others. Whale oil has given the best results, but its surface tensions do not seem to have been determined. It may be presumed that they do not differ greatly from the values given for olive oil.

An inspection of the above table will show that the tension of the surface separating air from water is greater than the sum of the tensions of the surfaces separating air from oil and oil from water, which explains why a film of oil will spread over the surface of a body of water.

Through the operation of surface tensions much of the force which breakers have is lost. Let us imagine a "break" to occur after the surface of the water is covered by the oily film.

[1] For


  1. Above it has been assumed that the superficial tension per unit of length has the same numerical value as the superficial energy per unit of area, which can be proved as follows:
    PSM V43 D516 Graph coordinate relating superficial surface tension.jpg

    Let the equation to the curve B C A be y = f(x). Take any ordinate, as C D, whose length is y, and let the whole tension exerted across the line be represented by Φ, then the superficial tension is measured by the tension across a unit length of y, or, since Φ is the tension across the whole ordinate y, if T, which is constant, is the superficial tension per unit of length, Φ = Ty = T.f (x). Suppose that the variable ordinate y is originally in contact with the axis O B, and that the surface included