73 
of Edinburgh, Session 1882-83. 
powerful will that particular oil he. The rate of movement will 
also probably depend on the viscosity of the oil. The value of the 
oil will depend (2) on the amount of it required to cover unit 
area. With these two properties will require to be taken into con- 
sideration a third point, namely, the market price of the oil. 
Besides these points, attention will require to be given to the 
specific gravity of the oil, its solubility in water, and the rate of its 
evaporation into air. 
I have been able as yet to give attention to only one of these 
points. A few oils have been tested to find the amount to which 
their presence reduced the surface tension of water. Most of the 
methods for ascertaining the surface tension of a liquid are not 
suitable for testing that of an oily film. In the oily surfaced water 
we have not only to measure the, tension of the oil, but also the 
tensions of the bounding surfaces of the oil and the water. As 
these combined tensions vary with the amount of oil on the sur- 
face, the ordinary methods of testing are not suitable, as they do not 
admit of uniform conditions. 
It may be as well to remark here, that there is not, as many 
people suppose, a distinct line of demarcation between the surface 
covered with clean film and that covered with oily film. That is, 
the oil wherever present does not always reduce the surface tension 
to the same amount. On the contrary, there is no line of demarca- 
tion between the clean and the oily film; the one shades imper- 
ceptibly into the other, and the amount to which the oil reduces 
the surface tension depends on the quantity of oil present. Very 
little oil gives a very slight reduction of tension, and as more is 
added the tension becomes less till a minimum is reached, after 
which no reduction takes place. This condition is indicated by 
the oil ceasing to spread rapidly. Its movements after this stage 
is reached depend upon whether the oil is lighter or heavier than 
water. If lighter, it very slowly spreads itself over the water, its 
movements being now due to gravity. If the oil is heavier than 
the water, it collects and forms on the surface depressions, which 
deepen, and ultimately a large drop of oil breaks away and slowly 
sinks in the water. The breaking away of these droj3S forms a very 
beautiful experiment, and the slowness of the process gives an oppor- 
tunity of studying some interesting phenomena. 
