101 



ON THE PENETRATIVENESS OF FLUIDS*. 



By J. K. MITCHELL, M.D., Lecturer on Medical Chemistry in the 

 Philadelphia Medical Institute. 



[The generality and importance of this paper is such that we think it quite im- 

 possible to convey any idea of it by an abstract, and feel ourselves bound to 

 bring it before our English readers at full length.] 



TN 1829, I read before the Philosophical Society, a short memoir 

 -* on a new method of forming gum elastic into thin plates, sheets, 

 and bags. In some instances the balloons formed by the process 

 then described had, when filled with hydrogen gas, the power of 

 ascending to a considerable height in the atmosphere. Those which 

 were confined to the atmosphere of my lecture-room, at the Medical 

 Institute, descended again after a period of time, varying from an 

 hour to two days. The cause of the descent, which did not seem 

 of easy explanation, became a subject of investigation. 



The gas might have escaped from the balloons at the ligature, or 

 by permeating the dense wall of gum elastic, or by uniting chemi- 

 cally with the internal surface of the latter. To free the gas from 

 the compression to which it is subjected in a balloon, I confined it 

 in a wide-mouthed bottle, over the aperture of which I tied very 

 firmly a thin sheet of the elastic membrane. In a few hours the 

 descent of the cover into the cavity of the bottle gave evidence of a 

 diminution of the contained gas, and finally the cover was burst 

 inwards by the pressure of the atmosphere, so great had been the 

 rarefaction of that which remained in the bottle. On weighing the 

 membranous cover, no gain in weight could be perceived, so that I 

 presumed that the gas had escaped. By repeating the experiment, 

 and covering the bottle with a small bell-glass holding atmospheric 

 air, I found, after a time, in the latter vessel, an explosive mixture, 

 while the contents of the bottle itself were found to be pure or nearly 

 pure hydrogen. Evidence was thus afforded that hydrogen pene- 

 trated the membrane not by any vis a tergo, for no pressure was 

 applied, but by some inherent power of considerable amount. The 

 facility of permeation appeared also much greater in the hydrogen 

 than in the atmospheric air, which, if it entered at all into the bottle, 

 did not penetrate in any appreciable quantity, when fully one-half of 

 the hydrogen had made its escape. 



In the next experiment the arrangement of the gases was altered : 

 common air was inclosed in the bottle, and a bell-glass confined 

 around it in an atmosphere of hydrogen. As was expected, the 

 * Philadelphia Journal of Medical Sciences, xiii. 36. 



