402 Mr. T. Graham on the Absorption and 



vered a power in gases to penetrate india-rubber in a thin sheet, 

 or in the form of the little transparent balloons which Dr. Mit- 

 chell was the first to prepare from that substance. He remarked 

 in particular that such balloons collapse sooner when inflated with 

 hydrogen than with atmospheric air, and still sooner when filled 

 with carbonic acid ; and he connected the latter fact with the 

 observation that a solid piece of india-rubber is capable of absorb- 

 ing its own volume of carbonic acid when left long enough in the 

 pure gas. By means of a proper arrangement, Dr. Mitchell 

 found that various gases passed spontaneously through the caout- 

 chouc membrane when there was air on the other side } with differ- 

 ent degrees of velocity. " Ammonia transmitted in 1 minute as 

 much as sulphuretted hydrogen in 2 \ minutes, cyanogen in 3^ 

 minutes, carbonic acid in 5^ minutes, nitrous oxide in 6J mi- 

 nutes, arsenietted hydrogen in 27J minutes, olefiant gas in 28 

 minutes, hydrogen in 37-| minutes, oxygen in 1 hour and 53 

 minutes, carbonic oxide in 2 hours and 40 minutes." The rate 

 of penetration of nitrogen appeared to be even, slower than that 

 of carbonic oxide*. 



It will be observed that those gases penetrate most readily 

 which are easily liquefied by pressure, and which are also " gene- 

 rally highly soluble in water or other liquids/'' The memoir of 

 Dr. Mitchell was ably commented upon, shortly after its publi- 

 cation, by Dr. Draper of New York, who also added many new 

 observations on the passage of both gases and liquids through 

 membranous septaf. These early speculations, however, lose 

 much of their fitness from not taking into account the two con- 

 siderations already alluded to, which appear to be essential to 

 the full comprehension of the phenomena — namely, that gases 

 undergo liquefaction when absorbed by liquids and such colloid 

 substances as india-rubber, and that their transmission through 

 liquid and colloid septa is then effected by the agency of liquid 

 and not gaseous diffusion. Indeed the complete suspension of 

 the gaseous function during the transit through colloid mem- 

 brane cannot be kept too much in view. 



Dr. Mitchell was led to infer, from a single casual observation, 

 that rubber expands in volume when carbonic acid is absorbed — 

 a result to be expected from the porosity of the solid mass, then 

 assumed in explanation of the penetrativeness of gaseous fluids. 

 But on placing 50 grins, of thin sheet rubber, 0*6 millim. in 



* "On the Penetrativeness of Fluids," by J. K. Mitchell, M.D.— Phila- 

 delphia Journal of Medical Sciences, vol. xiii. p. 36 ; or Journal of the Royal 

 Institution, vol. ii. pp. 101 & 307 (London, 1831). 



t A Treatise on the forces which produce the organization of Plants, with 

 an Appendix containing several Memoirs on Capillary Attraction, Electri- 

 city, and the Chemical Action of Light, by John William Draper, M.D. 



