470 Comparative Physiology. 



Mere capillary attraction has nothing to do with absorption, but 

 the remarkable phenomenon to which the term endosmose has 

 been given by its discoverer, Dutrochet, bears so strong a re- 

 semblance to this vital function, that it is scarcely possible to 

 disbelieve its partial concern in it. If a solution of gum or 

 sugar be put into a tube closed at one end with a piece of 

 bladder or other membrane, and the closed end immersed in 

 water, a passage of water will take place through the membra- 

 nous septum, the solution in the tube being greatly increased in 

 quantity and diminished in strength. At the same time there is 

 a counter-current in the opposite direction, a portion of the 

 gunnny or saccharine solution passing through the membrane 

 to the water, but in much less quantity. The first current is 

 termed endosmose, and the counter-current exosmose. It is not 

 universally true that the activity of the process depends on the 

 difference in density of the two fluids ; sometimes the stronger 

 current is from the denser to the lighter ; but with such as are 

 generally contained in the roots of plants it appears to be so. No 

 endosmose takes place between fluids which will not mingle, as 

 oil and water, and very little between such as act chemically on 

 each other. Organic membranes are the best septums, but 

 lamina of pipe-clay will evidently produce the phenomenon 

 also, and it appears that physical laws alone are concerned in it. 

 The passage of fluid through the living membranes has been 

 considered purely vital ; but, if we regard the other vital actions 

 as furnishing the conditions of endosmose, the absorption itself 

 may be due to the phenomenon. After death, the quantity of 

 fluid which first penetrates does not find the conditions changed 

 as in vitality, and saturates the tissues, preventing the admission 

 of more. Wlien the fluid absorbed is drawn off for the purposes 

 of the economy, a demand for a new supply is created, and the 

 action becomes regular and subservient to life. From the dif- 

 ferent times which saline solutions take to pass, as prussiate of 

 potass in 5 hours, sulphuric acid in 6 hours, acetic acid and 

 muriate of soda in 24 hours, it appears that what has been 

 termed the selecting power of absorbent surfaces is not due so 

 much to their peculiar vital properties as to the physical relations 

 between their tissues and the substances brought into contact 

 with them." 



(Zb be continued.^ 



Errata. — In p. 344. line 2. for " albumen " read " alburnum." In p. 352. 

 line 8. from bottom, for " Con." read " edition." 



