216 Miscellanies. 



neutralize the polarity of the molecules so as to give them perfect free- 

 dom of motion around every imaginable axis. The small amount of 

 cohesion (53 grains to the square inch) exhibited in the foregoing ex- 

 periment, is due, according to the theory of capillarity of Young and 

 Poisson, to the tension of the exterior film of the surface of w^ater 

 drawn up by the elevation of the plate. This film gives way first, and 

 the strain is thrown on an inner film, which, in turn, is ruptured ; and 

 so on until the plate is entirely separated ; the whole effect being simi- 

 lar to that of tearing the water apart atom by atom. 



" Reflecting on this subject, Prof. H. had thought that a more cor- 

 rect idea of the magiiitude of the molecular attraction might be obtain- 

 ed by studying the tenacity of a more viscid liquid than water. For 

 this purpose he had recourse to soap water, and attempted to measure 

 the tenacity of this liquid by means of weighing the quantity of water 

 which adhered to a bubble of this 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 re- 

 sults, yet they showed that the molecular attraction of water for water, 

 instead of being only about 53 grains to the square inch, is really sev- 

 eral 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 dimin- 

 ish the mobility of the molecules, and thus render the liquid more 

 viscid. 



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

 nomena of liquid cohesion, and all those of capillarity, are due to a 

 contractile force existing at the free surface of the liquid, and which' i 

 tends in all cases to urge the liquid in the direction of the radius of cur- 

 vature towards the centre, with a force inversely as this radius. Ac- 

 cording to this theory the spherical form of a dew-drop is not the effect : 

 of the attraction of each molecule of the water on every other, as in 

 the action of gravitation in producing the globular form of the planets, 

 (since the attraction of cohesion only extends to an unappreciable dis- 

 tance,) but it is due to the contractile force which tends constantly to 

 enclose the given quantity of water within the smallest surface, name- 

 ly, that of a sphere. Prof. H. finds a contractile force perfectly similar 

 to that assumed by this theory in the surface of the soap bubble ; in- 

 deed, the bubble may be considered a drop of water with the internal 

 liquid removed, and its place supplied by air. The spherical force in 

 the two cases is produced by the operation of the same cause. The 

 contractile force in the surface of the bubble is easily shown by blow- 

 ing a large bubble on the end of a wide tube, say an inch in diameter ; 



