THE SURFACE TENSION OF LIQUIDS. 597 



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 ; 27 for spirits of turpentine ; 2'6 for petroleum ; 2*5 for abso- 

 lute 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 ^o.&w °f a millime- 

 tre thick. 



Curious effects appear when liquids having different superfi- 

 cial 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 Mens- 



Pig. 5.— A Contour of Silken Thread expanding into a Circle when the Film on which 



it has been laid is broken. 



brugghe, 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, no repulsion ; but the tension astern has been 



