of Edinhurgh, Session 1882-83. 
69 
sion of clean water and oily water is too small to give rise to 
surface currents or film movements of sufficient velocity to produce 
these effects, and that the film would break, that is, that all the oil 
would he blown away before the film behind could be dragged for- 
wards. The ease with which the film glides over the water, and 
the velocity with which one part of the film can drag another is, 
however, much greater than might at first be imagined. Take the 
following examples : — Drop some oil on the surface of water ; note 
the extremely rapid spread of the oil. Now, the rapidity of that 
surface movement is the measure of the power which that particular 
oil confers on the water of dragging forwards surface film. In fact, 
the rapidity of that advance is the same as that with which the 
film can rush forwards in rear of a wind-driven area. That sur- 
prisingly rapid movement seems sufficient to perform the duty the 
theory here propounded lays, upon it. 
Another experimental illustration, better suited to the lecture- 
room, is made by taking a small piece of paper, say 10 cm. long by 
2 cm. broad, and attaching to the opposite sides of the ends of it 
two small pieces of camphor. This is easily done with a little 
bees-wax. If this piece of paper with the attached camphor be 
placed on the surface of clean water, it at once starts into a rapid 
motion of rotation, which will be kept up for hours, till all the’ 
camphor is dissolved if the vessel of water is large enough. The 
rapid movement is here due to the tension of the pure water 
surface on one side of the strip being greater than the tension of 
the weak solution of camphor on the other.. The paper is thus 
dragged round by the unbalanced tension of the clean surface. 
Or we may vary the experiment by placing a number of pieces of 
camphor on one side of the paper. If this paper is placed on the 
surface of a pond, it will be rapidly drawn over the water. 
Take another example of the wonderful effect of this difference 
of surface tension. Till two vessels full of water; put a little oil on 
one of them, then dust some fine powder over the surface of the 
water in both vessels. The powder must be free from greasy 
matter — well-burned ashes sifted through fine wire gauze does well. 
Now direct a jet of air vertically downwards on the surface of the 
water. When this is done on the clean water, it will be observed 
that the very slightest possible current of air causes the surface film 
