102 Mitchell on the Penetrati'veness of Fluids. 



hydrogen entered the bottle rapidly, raised up the tense membrane, 

 formed it into a globe, and finally burst through it, and thus made 

 its escape from the Confinement to which it had been spontaneously 

 subjected. 



The minuteness of the atom of hydrogen might readily enough 

 account for the greater facility with which it penetrated the mem- 

 brane, but could not be considered a good reason for the energy 

 \vith which the penetration was accomplished. A gas having a heavy 

 atom was therefore selected for further experiment, and carbonic 

 acid, subjected to the same sort of confinement, was found to 

 permeate the membrane with as great power, and very much 

 greater facility. In succession, most of the gases were submitted to 

 the same ordeal, and all of them found, except nitrogen gas, to ex- 

 ercise the same power, but with very different degrees of rapidity. 

 The power was ascertained by comparison with common air, and the 

 rate of action both in that mode and by comparison with each other. 

 The depression or elevation of the membranous cover clearly indi- 

 cated the escape or entrance of a gas, and when two active gases 

 were placed one on each side of it, its rise or fall expressed the dif- 

 ference of rate, because each was, at the same moment, in the act of 

 permeation, as proved by many examinations of the contents of the 

 bottle and bell-glass. 



Having once ascertained the rate of action of each gas relative to 

 air, a prediction could be made as to their rate in reference to each 

 other. Hence gases which operated on air with nearly equal velo- 

 city, affected the horizontality of the membrane very little when 

 placed on opposite sides of it. Thus carbonic acid and nitrous oxide 

 act with great facility on common air, and in nearly equal degree ; 

 and when placed on opposite sides of the membrane, penetrate it 

 rapidly, but cause a very slow change in its position. The facts 

 here presented warrant the conclusion, that if two gases, equally 

 penetrant exactly, could be found, they would, under the above 

 described arrangement, mix uniformly, without in the slightest 

 degree altering the state of the membrane *. 



The greatest possible degree of effect on the membrane arises, 

 when we place on opposite sides of it the slowest and most speedy 

 penetrator ; for instance, nitrogen and sulphuretted hydrogen. In 

 that case the change is immediately visible. 



* Subsequently having discovered that olefiant gas and arseuuretted hydrogen 

 have, with reference to common air, exactly equal rates, they were placed on 

 opposite sides of a membrane, with a full expectation of sustained horizontality 

 on the part of the membrane j which was confirmed by the result. 



