*Surface Tension.*—Before proceeding further, it will be advisable to introduce and explain the term “surface tension.” We frequently use it, without attaching to it any numerical value, to express the fact that the free surface of a liquid is subjected to stretching forces, or is in a state of tension; and thus we say that certain phenomena are “due to surface tension.” But the physicist does not content himself with merely observing occurrences; he tries also to measure, in definite units, the quantities involved in the phenomena. And hence surface tension is defined as the force tending to pull apart the two portions of the surface on either side of a line 1 centimetre in length. That is, we imagine a line 1 centimetre long on the surface of the liquid, dividing the surface into two portions on opposite sides of the line, and we call the force tending to pull these two portions away from each other the surface tension. Experiments show that this force, in the case of cold water, is equal to about 75 dynes, or nearly 8/100 of a gramme. If we choose a line 1 inch long on the surface of water, the surface tension is represented by about 3 1/6 grains. It is always necessary to specify the length when assigning a value to the surface tension; and unless otherwise stated a length of 1 centimetre is implied. The values for different liquids vary considerably; and it is also necessary to note that the figure for a given liquid depends upon the nature of the medium by which it is bounded—whether, for example, the surface is in contact with air or another liquid. The following table gives the values for several liquids when the surfaces are in contact with air:— ────────────────────────────────────────────────────────────────── Liquid. Tension at 15° C. (59° F.), dynes per cm. ────────────────────────────────────────────────────────────────── Water 75 Aniline 43 Olive Oil 32 Chloroform 27 Alcohol 25 ────────────────────────────────────────────────────────────────── When one liquid is bounded by another, the _interfacial_ tension, as it is called, is generally less than when in contact with air. Thus the value for water and olive oil is about 21 dynes per centimetre at 15° C. We are now in a position to speak of surface tension _quantitatively_, and shall frequently find it necessary to do so in order to explain matters which will come under our notice later. [Illustration: __Figs._ 10, 11 and 12.—The Diving Drop. Three stages._]