294 



ABSORPTION, 



FIG. 101. 



described, there appears something like the exercise of choice 

 in the materials admitted into them. But the absorption by 

 bloodvessels presents no such appearance of selection of ma- 

 terials; rather, it appears, that every substance, whether 

 gaseous, liquid, or a soluble or minutely divided solid, may 

 be absorbed by the bloodvessels, provided it is capable of per- 

 meating their walls, and of mixing with the blood ; and that 

 of all such substances, the mode and measure of absorption are 

 determined solely by their physical or chemical properties and 

 conditions, and by those of the blood and the walls of the 

 bloodvessels. 



The phenomena are, indeed, exactly comparable to that 

 passage of fluids through membrane, which occurs quite inde- 

 pendently of vital conditions, and the earliest and best scien- 

 tific investigation of which was made by Dutrochet. The in- 

 strument which he employed in his experiments 

 was named an endosmometer. It may consist of 

 a graduated tube expanded into an open-mouthed 

 bell at one end, over which a portion of mem- 

 brane is tied (Fig. 101). If now the bell be 

 filled with a solution of a salt, say chloride of 

 sodium, and be immersed in water, the water 

 will pass into the solution, and part of the salt 

 will pass out into the water ; the water will pass 

 into the solution much more rapidly than the 

 salt will pass out into the water, and the diluted 

 solution will rise in the tube. To this passage of 

 fluids through membrane the term Osmosis is ap- 

 plied. 



The nature of the membrane used as a septum, 

 and its affinity for the fluids subjected to ex- 

 periment, have an important influence, as might 

 be anticipated, on the rapidity and duration of 

 the osmotic current. Thus, if a piece of ordinary 

 bladder be used as the septum between water and 

 alcohol, the current is almost solely from the 

 water to the alcohol, on account of the much 

 greater affinity of water for this kind of mem- 

 brane ; while, on the other hand, in the case of a membrane of 

 caoutchouc, the alcohol, from its greater affinity for this sub- 

 stance, would pass freely into the water. 



Various opinions have been advanced in regard to the na- 

 ture of the force by which fluids of different chemical compo- 

 sition thus tend to mix through an intervening membrane. 

 According to some, this power is the result of the different de- 

 grees of capillary attraction exerted by the pores of the mem- 



