85 



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 in- 

 flatino- with cold air, and also when the bubble is held perpendicular- 

 ly 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 the letter U, and partially filled with water; the con- 

 tractile 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 colour 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 

 general results of which were the same : the numerical details of 

 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 required the greater force. 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. 



Several other experiments with films of soap water were also 

 described, which afford striking illustrations of the principles of capil- 

 larity, and which apparently have an important bearing on the whole 

 subject of cohesion. 



Professor Henry's communication gave rise to observations 

 from Dr. Ludlow, Professor Frazer, and Mr. Trautwine. 



On motion, the Librarian was authorized to present a 

 copy of the Catalogue of the Library to the Academy of Natu- 

 ral Sciences of Philadelphia. 



VOL. IV. — M 



