112 Mitchell on the Penetrativeness of Fluids. 



length, if any pressure exist on either side, yield to that and pass to 

 the other side. r 



As some substances have no penetrativeness, such as milk or 

 blood, or at least their solid parts, the water placed on the opposite 

 side of the membrane alone moves, and it is only after the decom- 

 position by putrefaction, and consequent formation of a new fluid 

 having penetrant properties, that any current sets in the direction 

 opposite to that of the water. To prove this, it is only necessary to 

 show that alcohol penetrates gum elastic much more rapidly than 

 water ; and that, therefore, when that kind of membrane is inter- 

 posed between them, the greater current is from alcohol to water, 

 and not from water to alcohol. 



A hollow glass cylinder, open at both ends, was closed com- 

 pletely by two membranes of gum elastic, having been previously 

 perfectly filled with alcohol. It was then sunk in the large pneu- 

 matic trough of my laboratory, where it remained one week. At 

 that time it presented a concavity at each end, of decided depth, 

 proving the escape of a considerable quantity of alcohol. On the 

 other hand, a similarly prepared vessel filled with water and sub- 

 merged in alcohol presented at the end of a week well-marked 

 convexities, demonstrating the insinuation of alcohol. If it be con- 

 tended that the nature of the membrane affects and even reverses 

 the electrical state, it may be well said in reply, that there is no 

 analogy for that, and moreover, the same membrane acts under the 

 movement of gases precisely as an animal membrane. The suppo- 

 sition would invest it with a most Protean character. 



In making experiments for the preparation of gum elastic by 

 ether, that liquid was found to readily infiltrate its tissue. Alcohol 

 has been already shown to penetrate it better than water, and water 

 enters its substance so slowly, that a bag of a thinness productive of 

 almost perfect transparency, and containing four ounces, two drams, 

 and fifty-seven grains, lost by evaporation but eight grains in the 

 first period of twenty-four hours, and fifteen grains during the next 

 three days. Viewing these facts, a prediction was founded on them 

 relative to the effect of placing ether in contact with one surface of 

 such a membrane, while alcohol or water occupied the opposite sur- 

 face. As was expected, the greater quantity accumulated on the 

 side of the less penetrative substance, and the ether always caused, 

 by its transmission, an augmentation of liquid on the side of the 

 alcohol or water. Using animal membranes, facts of a similar kind, 

 previously ascertained, led us to anticipate the opposite result. Ac- 

 cording to expectation, water being most penetrative, passed through 



