metre of superficial length, at 60° Fahr., as 7·5 for distilled water; 49 for mercury; 4 for glycerin; 3·6 for olive-oil; 2·8 for soap-suds; 2·7 for spirits of turpentine; 2·6 for petroleum; 2·5 for absolute alcohol; and 1·88 for ether. It is diminished when the liquid is warmed, and is weakened and even destroyed by impurity. M. Terquem has determined, from observations on the interference of luminous rays, that the envelope is less than 1/20,000 millimetre thick.
Curious effects appear when liquids having different superficial tensions are brought together, and when solids containing volatile properties are thrown upon a liquid. With two liquids that will mix, as water and alcohol or ether, the tension at the point of contact becomes null, and the lighter fluid spreads out over the other. This is followed, according to M. Van der Mensbrugghe,
Fig. 5.—A Contour of Silken Thread expanding into a Circle when the Film on which it has been laid is broken.
by a retreat of this fluid toward the point where it was dropped, in consequence of an increased tension given to that point by the cooling that follows the evaporation of the dropped liquid. If the liquids will not mingle, as when oil or turpentine is dropped on water, the drop spreads over the surface, forming a thin layer upon it which is marked by beautiful plays of colors.
M. Devaux exemplifies one of these effects by an experiment (Fig. 6) in which a tin boat, having a notch cut in the stern, is launched upon the water. On letting a drop of alcohol fall at the notch, the boat moves away as if driven by some repulsion. There is, however, ho repulsion; but the tension astern has been
