1844] WRITINGS OP JOSEPH HENRY. 219 



other, as in the action of gravitation in producing the glob- 

 ular form of the planets, (since the attraction of cohesion 

 extends only to an unappreciable 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. Professor Henry finds a contractile force per- 

 fectly similar to that assumed by this theory in the surface 

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

 a drop of water with the internal liquid removed, and its 

 place supplied by air. The spherical form in the two cases 

 is produced by the operation of the same cause. The con- 

 tractile force in the surface 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 bub- 

 ble 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 in- 

 flated, 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. 



Many experiments were made to determine the amount 

 of this force, by blowing a bubble on the larger end of a 

 glass tube in the form of the letter U, and partially filled 

 with water; the contractile force of the bubble, transmitted 

 through the enclosed air, forced down the water in the 

 larger leg of the tube, and caused it to rise in the smaller. 

 The difference of level observed by means of a microscope, 

 gave the force in grains per square inch, derived from the 

 known pressure of a given height of water. The thickness 

 of the film of soap water which formed the envelope of the 

 bubble, was estimated as before by the color exhibited just 

 before bursting. The results of these experiments agree 

 with those of w'eighing the bubble, in giving a great inten- 

 sity to the molecular attraction of the liquid, equal at least 

 to several hundred pounds to the square inch. Several 

 other methods were employed to measure the tenacity of 

 the film, the general results of which were the same; the 



