304 Scientific Intelligence. 



SCIENTIFIC INTELLIGENCE. 

 I. Physics. 



1. The Growth of Air Bubbles at the Walls of a Benker con- 

 taining a Liquid, when the Gas at the free Surface is not air ; by 

 C. Barus (communicated). — When water is poured into a beaker 

 in air and an artificial atmosphere of hydrogen is then allowed to 

 rest on the surface, it is surprising that the invisible air bubbles, 

 which abound at the surface of all solid parts under the liquid, 

 gradually become inflated to a relatively enormous size. They 

 may be finally lifted off by their buoyancy. All submerged 

 objects, notably wire gauze, become coarsely jeweled in the 

 lapse of time ; but the bubbles do not reappear when shaken off. 

 This, however, is the second stage of the phenomenon. If the 

 bubbles are not forcibly removed, they will of their own accord 

 ultimately vanish by contraction. 



Clearly the phenomenon as a whole is a case of the diffusion of 

 hydrogen, through water, into the space indicated by the original 

 microscopic air adhesions ; but it is not at once evident why 

 hydrogen should diffuse from top to bottom ; i. e., against the 

 hydrostatic pressure gradient of the liquid. Nor does it appear 

 why it should afterwards reverse the process ; for the phenom- 

 enon is quite as evident for columns of water a foot or more high, 

 or for large bulks of air under hydrogen. The explanation, 

 which I have given, is as follows : Let h be the head of water 

 above a given small air bubble, and B the barometric pressure. 

 At the outset, therefore, the pressure of the hydrogen is B and 

 that of the air underneath, B + hp g, in the usual notation. The 

 pressure urging diffusion is thus initially from hydrogen into 

 pure air B, and for pure air into hydrogen B 4- hpg. But as 

 the diffusion proceeds, the gas within the bubble is no longer 

 pure air, but a mixture of hydrogen and air, corresponding to the 

 partial pressures/? and//; so that throughout the experiment 

 B + hpg =p + p' . Now p' continually diminishes from above 

 atmospheric pressure, as the 'air escapes from the bubble, while 

 p continually increases from the influx of hydrogen into it. The 

 time must, therefore, come when p' =. hpg or B = p, at which 

 the influx of hydrogen must cease altogether. When p' dimin- 

 ishes further, owing to the escape of air, it follows that since 

 p'<Ji pg,p^> B. In other words, both hydrogen and air after- 

 wards escape from the enormously inflated bubble, until it van- 

 ishes. 



In this way the very curious result is brought about of the 

 apparent diffusion of a gas against hydrostatic pressure into an 

 infinitesimal. air bubble, until the latter is inflated to a bead, and 

 thereafter of the reversal of diffusion and deflation — all in a con- 

 tinuous sequence of phenomena. 



Brown University, Providence, R. I. 



