500 



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 sepa- 

 rating 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 follow- 

 ing is an extract from his table of results. The tension is measured 

 in grammes per linear centimetre at twenty degrees centigrade : 



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 ten- 

 sions 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 sur- 

 face 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.* For 



* 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 : 



Y 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 <J>, then the super- 

 ficial tension is measured by the ten- 

 sion across a unit length of y, or, 

 since < is the tension across the 

 whole ordinate y, if T, which is con- 

 stant, is the superficial tension per 

 unit of length, <f> = Ty = T.f (x). 

 Suppose that the variable ordinate # 

 is originally in contact with the axis 

 ~ x B, and that the surface included 



