of Liquids to each other. 307 



tive spider-like insects, with long legs, run about over 

 the water without their feet sinking into the liquid, or 

 even being wet by it ; when I have seen several minute 

 bodies at a distance from each other, resting upon the 

 surface of the water contained in a small vessel, tremble 

 every time that the surface of the water was touched 

 with the point of a needle, I have been unable to 

 doubt the existence of a resisting surface, a sort of pel- 

 licle on the surface of the liquid. 



There is another phenomenon which seems to me to 

 furnish a demonstrative proof of the existence of this re- 

 sisting surface. When water is heated in any vessel, 

 as soon as the liquid begins to become warm a consider- 

 able quantity of air is disengaged in the form of spheri- 

 cal bubbles, larger or smaller, which, passing through the 

 liquid from below upwards, escape into the air. Now 

 it very often happens that these little bubbles, after 

 having traversed the liquid with great rapidity, are 

 stopped all of a sudden when they have nearly reached 

 the surface. 



What is it that stops these bubbles if not a resisting 

 pellicle at the surface of the liquid ? 



I endeavoured, but in vain, to explain these facts, by 

 calling to my aid the atmospheric air. I saw clearly, 

 as I .observed the little globule of mercury situated in 

 its little pocket, which sank sensibly lower than the 

 level surface of the water, and which was scarcely large 

 enough to hold the globule, I saw, I say, that the film 

 of air, which we might suppose still attached to the sur- 

 face of the globule (if such a film really existed), could 

 not be thick enough to buoy up this heavy body and 

 make it float, hydrostatically, on the surface of the water. 

 But when to the testimony of these experiments and to 



