400 ME. T. GRAHAM ON THE ABSOEPTION AND 



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 " generally highly soluble in water or other liquids." 

 The memoir of Dr. Mitchell was ably commented upon, shortly after its publication, 

 by Dr. Draper of New York, who also added many new observations on the passage of 

 both gases and liquids through membranous septa f. These early speculations, how- 

 ever, lose much of their fitness from not taking into account the two considerations 

 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 

 membrane 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 tobe expected from the porosity of the 

 solid mass, then assumed in explanation of the penetrativeness of gaseous fluids. But 

 on placing 50 grms. of thin sheet rubber, 0-6 millim. in thickness, in carbonic acid over 

 mercury, it was seen that the rubber gradually absorbed 0-78 volume of gas in twenty- 

 four hours at 15°, of which 0-7 volume was taken up in the first hour. The mass of 

 rubber was previously measured with care by the displacement of mercury in a specific- 

 gravity bottle, and again when the rubber was charged with carbonic acid ; it gave the 

 same displacement of mercury within a hundredth of a gramme. No measurable 

 change in the bulk of the rubber, therefore, had occurred. It may be added that the 

 absorbent power of vulcanized rubber for carbonic acid appears to be less than that of 

 rubber in its natural state, being found only 0*57 volume in a comparative experiment. 



The penetration of rubber by gases may be illustrated by their passage into a vacuum, 

 as well as into an atmosphere of another gas in Dr. Mitchell's experiments. The dif- 

 fusiometer, consisting of a plain glass tube of about 22 millims. in diameter and nearly 

 a whole metre in length, closed at the upper end by a thin plate of stucco and open 

 below, is taken advantage of in such experiments. A thin film of rubber from a small 

 balloon is stretched over the upper end of the tube, where it is supported by the stucco 

 plate, bound with copper wire, and cemented at the edges in contact vrith the glass with 

 gutta percha softened by heat. If the tube be now filled with mercury and in- 

 verted, a Torricellian vacuum is obtained above, into which the air of the atmosphere 

 gradually penetrates, passing through the film of rubber and depressing the mercurial 



• " On the Penetrativeness of Fluids," by J. K. Mitchell, M.D. — Philadelphia Journal of Medical Sciences, 

 vol. xiii. p. 36; or Journal of the Royal Institution, vol. ii. pages 101 and 307: London, 1831. 



t A Treatise on the forces which produce the organization of Plants, with an Appendix containing several 

 Memoirs on Capillary Attraction, Electricity, and the Chemical Action of light, by John Wuuam Drajer, M.D. 



