154 PASSAGE OF GASES THROUGH POROUS FILMS. 



to the air, the film is promptly depressed into a deep concavity, and 

 bursts. By these methods the passage of all kinds of vapors and gases 

 may be demonstrated, oxygen, hydrogen, carbonic acid, protoxide of ni- 

 trogen, the vapors of peppermint, lavender, and various essential oils. 



By many experiments on such different substances, I found that the 

 Law of e uilib- ^ aw ^ &|fc$*l#iaHi f r gases and vapors is the same as for 

 rium, and ex- liquids. No matter what the thickness or thinness of a po- 

 f lfc ' rous barrier may be, movement takes place through it, un- 

 til the media on its opposite sides have the same chemical composition. 

 The observed action, in particular cases, will therefore altogether depend 

 on the circumstances untler which the experiment is made. A soap-bub- 

 ble full of carbonic acid, exposed to the air in a closed bottle, collapses 

 only to a certain extent, when the percentage constitution of the air it 

 contains is the same as that of the air in the bottle, contaminated with 

 the carbonic acid which the bubble has yielded it. But if the bubble be 

 exposed to the free atmosphere, it collapses almost completely, for now 

 the carbonic acid escapes finally away. 



One of the most interesting facts connected with these results is the 

 Action throu h P er ^ ect manner in which a film of excessive tenuity will dis- 

 fiims of ex- charge these mechanical functions. With a little care, a 

 Ult7 ' film may be obtained so thin as to be invisible except in 

 certain lights, when it presents a velvety black aspect. In this condi- 

 tion, as Newton has proved, it is not thicker than three eighths of a 

 millionth of an inch, yet endosmosis takes place perfectly through it : it 

 expands and collapses, rises up into a dome, or is depressed into a con- 

 cavity, as the circumstances of its exposure may be. And this should 

 prepare us to admit that in organic tissues of the utmost degree of tenu- 

 ity these physical phenomena may occur, and that even under these most 

 unlikely circumstances such tissues may give origin to mechanical forces 

 of the greatest intensity, as we shall now prove. 



Graham's law of the diffusion of gases has but a very limited physio- 

 logical application. The introduction of it in cases to which 

 of Graham's it does not properly apply has led to several errors. There 

 js nothing common in the result of the movement of gases 

 exposed freely to one another, and exposed with the intervention of a 

 close-pored tissue. The tissue itself gives origin to mechanical force of 

 such intensity as not only to modify the diffusion rate, but, in a great 

 many of the most important cases, absolutely to invert the direction of 

 the motion. Thus, through a stucco plug, in which the pores are of 

 sensible size, atmospheric air passes more rapidly to carbonic acid than 

 carbonic acid does to it, but through the thinnest film of water just the 

 reverse takes place. A bubble full of that acid, exposed to the air, lets 

 it escape with so much rapidity that in a few moments a complete col- 



