114 Mitchell on the Penetrativeness of Fluids. 



glass cylinder, partially fill it with diluted sulphuric acid, and place 

 it in a vessel of clean water, so as to bring the two columns to a 

 level. In a few hours the column holding the acid will rise con- 

 siderably above that of the clean water, proving the greater current 

 to set from water to acid, and not from acid to water. Tests, how 

 ever, show that some acid does pass the membrane *. 



To feel assured of the error of Dutrochet, I repeated the experi- 

 ment in another form. A tube of five-sixteenths of an inch in dia- 

 meter, ending in a funnel-like extremity of an inch and a quarter, 

 was covered at its broad end by animal membrane, then partially 

 filled with diluted acid, and placed, membrane downwards, in clean 

 water, so as to bring both columns to a level. INSTANTLY the rise 

 in the narrow tube was perceptible, and amounted to nearly half an 

 inch in half an hour. Reversing the order, by placing the clean 

 water in the tube, and the diluted acid without, as sudden and pro- 

 gressive a descent of the column of clean water was observable. 

 Tests, after a short time, betrayed the percolation of some acid, and, 

 finally, in every case the liquid became uniformly acidulous through- 

 out, and the two columns fell to a common level an event which 

 may always be expected, unless the combination produced by trans- 

 mission is not penetrant. 



Water may be removed from the surface of a membrane at which 

 it has arrived in many and various methods. Invitation may be 

 given to it by a column of mercury contained in a hollow cylin- 

 der closed above by animal membrane. Water readily passes 

 through, may be seen studding in drops the surface of the mercury, 

 gradually covering the under side of the membrane, causing at 

 length the separation and descent of the mercury, and continuing to 

 enter the cylinder until the mercurial column sinks to the level of 

 the general contents of the trough. There the action ceases, but if 

 the water placed above the membrane be now removed, the mercu- 

 rial column will again rise, and all the water having escaped through 

 the membrane by the process of infiltration into the atmosphere, the 

 mercury will be finally seen in close contact with the membrane 

 from which it had receded. Sometimes before the completion of 

 the process a change takes place in the condition of the animal 

 matter, and some gas being introduced below suspends the ascent 

 of the mercury f. 



* This fact I demonstrated to Dr. Togno. 



f A new hygrometer was suggested by this experiment, of which I purpose 

 giving an account to the Philosophical Society. 



