ADDEESS OF PROF. A. M. MAYER. 497 



substance just before it burst, and by determining the thickness of 

 the film from an observation of the color it exhibited in comparison 

 with Newton's scale of thin plates. Although experiments of this 

 kind could only furnish approximate results, yet they show that the 

 molecular attraction of water for water instead of being only about 

 52 grains to the square inch, is really several hundred pounds, and 

 is probably equal to that of the attraction of iCe for ice. The 

 effect of dissolving the soap in the water is not, as might at first 

 appear, to increase the molecular attraction, but to diminish the 

 mobility of the molecules and thus render the liquid more viscid. 



"According to the theory of Young and Poisson, many of the 

 phenomena of liquid cohesion, and all those of capillarity, are due 

 to a contractile force existing at the free surface of the liquid, and 

 which tends in all cases to urge the liquid in the direction of the 

 radius of curvature towards the centre, with a force inversely as the 

 radius. 



"According to this theory the. spherical form of a dew-drop is 

 not the effect of the attraction of each molecule of the water on 

 any other, as in the action of gravitation in producing the globular 

 form of the planets, (since the attraction of cohesion only extends 

 to an inappreciable distance,) but is due to the contractile force 

 which tends constantly to enclose the given quantity of water within 

 the smallest surface, namely that of a sphere. The author finds a 

 contractile force similar to that assumed by this theory, in the sur- 

 face of the soap-bubble; indeed, the bubble may be considered a 

 drop of wa,ter with the internal liquid removed and its place sup- 

 plied by air. The spherical form in the two cases is produced by 

 the operations of the same cause. The contractile force in the sur- 

 face of the bubble is easily shown by blowing a large bubble on 

 the end of a wide tube — say an inch in diameter; as soon as the 

 mouth is removed the bubble will be seen to diminish rapidly, and 

 at the same time quite a forcible current of air will be blown 

 through the tube against the face. This effect is not due to the 

 ascent of the heated air from the lungs with which the bubble was 

 inflated, for the same effect is produced by inflating with cold air, 

 and also when the bubble is held perpendicularly above the face, 

 so that the current is downward. 

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