ABSORBENT POWER OF MEMBRANES. 11 



water can be pressed out which is not retained by chemical attraction.* It is in 

 the highest degree worthy of notice, that this water, not chemically combined, seems 

 to have the greatest share in the properties which these animal substances possess 

 in the fresh state, for the pressed tendons and yellow ligaments become transparent ; 

 the former lose their flexibilty, the latter their elasticity ; and if laid in water, they 

 recover these properties perfectly. In the pores of a porous substance, the fluid 

 molecules are retained by two kinds of attraction, namely, by the affinity which is 

 exerted between the walls of the pores and the molecules of the fluid, and by the 

 cohesion which acts between the molecules of the fluid itself. It would appear as 

 if the molecules of water were thus brought into different states, and this seems to 

 be the cause of the differences observed in the properties of these animal substances 

 when they contain different proportions of water. fig. i. 



t If the wide opening of the tube, Fig. 1, be tied over with a por- 

 tion of bladder, and water poured into the wide part of the tube, as 

 far as the mark , we shall find that, when mercury is poured into 

 the upright narrow part of the tube, to a certain height, the whole 

 external surface of the bladder becomes covered with minnte drops, 

 which, if the column of mercury be made a few lines higher, unite, 

 so as to form large drops. These continue to flow out uninter- 

 ruptedly, if mprcury be added, so as to keep the column at the same 

 height, till at last the wide part of the tube is emptied of water and 

 filled with mercury. 



Solution of salt, fat oil, alcohol, &c., behave exactly as water does ; 

 under a certain pressure these fluids pass through an animal mem- 

 brane, just as water does through a paper filter. 



The pressure required to cause these liquids to flow through the 

 pores of animal textures depends on the thickness of the membrane, 

 as well as on the chemical nature of the different liquids. 



Through ox-bladder, y^th of a line ( T J^th of an inch) thick, 

 water flows under a pressure of 12 inches of mercury.^ A saturated 

 solution of sea salt requires from 18 to 20 inches ; and oil (mar- 

 row oil) only flows out under a pressure of 34 inches of mercury. 



When the membrane used is the peritoneum of the ox, ^th of a line, (^J-^th of 

 an inch) in thickness, water is forced through it by 8 to 10 inches, brine by 12 to 

 16 inches, oil by 22 to 24 inches, and alcohol by 36 to 40 inches of mercury. 



The same membrane from the calf, g j g th of a line (y^^d of an inch) in thick- 

 ness, allows water to pass through under the pressure of a column of water 4 inches 

 high ; brine passes under a pressure of 8 to 10 inches of brine, and oil under a 

 pressure of 3 inches of mercury. 



In making experiments of this nature, we observe that, after they have continued 

 for some time, the pressure required to force the liquid through the membrane does 

 not continue equal. If during the first 6 hours a pressure of 12 inches of mer- 

 cury were necessary, we often find that after 24 or 36 hours, 8, or even 6 inches 

 will suffice to produce the same effect, obviously because by long-continued con- 

 tact with water, the membrane undergoes an alteration, in consequence of which 

 the pores are widened. 



From these experiments it appears, that the power of a liquid to filter through 

 an animal membrane bears no relation to the mobility of its particles ; for under a 

 pressure which causes water, brine, or oil to pass through, the far more mobile 

 alcohol does not pass. 



The capacity of the animal membrane for being moistened by, and its power of 

 absorbing, the liquid, have a certain share in producing the result of its filtration 

 through the membrane. || 



The following table will show this fact : 



* The portion of water not chemically combined, has the greatest share in the properties of 

 the tissues. 



t Pressure required to cause water and any other liquids to pass through membranes. 

 t The pressure varies with different liquids. 



$ The passage of liquids through membranes not in proportion to their fluidity. 

 H The absorbent power of the membrane for the liquid has a share in the effect. 



