LECTURES. 209 



the fluid is rarified both in a vertical and horizontal direction_, and by 

 the reaction a tension or contractile force is developed in the whole 

 exterior film. 



Explanation of this by a diagram. 



"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 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 ap- 

 preciable distance, but it 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 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. , TJie contractile 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 bubble will 

 be seen to diminish rapidly, and at the same time quite a forcible cur- 

 rent of air will be blown through the tube against the face. This 

 efiect 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 in- 

 flating with cold air, and also when the bubble is held perpendicularly 

 above the face, so that the current is downwards. 



"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 a 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 weighing the bubble, in giving 

 a great intensity 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 gen- 

 eral results of whicli were the same : the numerical .details o-f these 

 are reserved, however, until the experiments can be repeated with a 

 more delicate balance. 



" The comparative cohesion of pure water and soap- water was de- 

 termined by the weight necessary to detach the same plate from each ; 

 and in all cases the pure water was found to exhibit nearly double the 

 tenacity of the soap-water. The want of permanency in the bubble 

 of pure water is therefore not due to feeble attraction, but to the- 

 perfect mobility of the molecules, which causes the equilibrium, as in 

 the case of the arch, without friction of parts, to be destroyed by the 

 slightest extraneous force." 



(89.) Illustrations of the foregoing ^^'/^inciples. — Great tenacity of a 



14 s 



