308 IMBIBITION AND EXHALATION. 



the more scanty passage of the salt outward to the water is called 

 exosmosis. 



The mode usually adopted for measuring the rapidity of endos- 

 mosis is to take a glass vessel, shaped somewhat like an inverted 

 funnel, wide at the bottom and narrow at the top. The bottom of 

 the vessel is closed by a thin animal membrane, like the mucous 

 membrane of an ox -bladder, which is stretched tightly over its edge 

 and secured by a ligature. From the top of the vessel there rises 

 a very narrow glass tube, open at its upper extremity. When the 

 instrument is thus prepared, it is filled with a solution of sugar 

 and placed in a vessel of distilled water, so that the animal mem- 

 brane, stretched across its mouth, shall be in contact with pure 

 water on one side and with the saccharine solution on the other. 

 The water then passes in through the membrane, by endosmosis, 

 faster than the saccharine solution passes out. An accumulation 

 therefore takes place inside the vessel, and the level of the fluid 

 rises in the upright tube. The height to which the fluid thus rises 

 in a given time is a measure of the intensity of the endosmosis, and 

 of its excess over exosmosis. By varying the constitution of the 

 two liquids, the arrangement of the membrane, &c., the variation 

 in endosmotic action under different conditions may be easily 

 ascertained. Such an instrument is called an endosmometer. 



If the extremity of the upright tube be bent over, so as to point 

 downward, as endosmosis continues to go on after the tube has 

 become entirely filled by the rising of the fluid, the saccharine solu- 

 tion will be discharged in drops from the end of the tube, and fall 

 back into the vase of water. A steady circulation will thus be 

 kept up for a time by the force of endosmosis. The water still 

 passes through the membrane, and accumulates in the endosmo- 

 meter ; but, as this is already full of fluid, the surplus immediately 

 falls back into the outside vase, and thus a current is established 

 which will go on until the two liquids have become intimately 

 mingled. 



The conditions which influence the rapidity and extent of endos- 

 mosis have been most thoroughly investigated by Dutrochet, who 

 was the first to make a systematic examination of the subject. 



The first of these conditions is the freshness of the membrane itself. 

 This is an indispensable requisite for the success of the experiment. 

 A membrane that has been dried and moistened again, or one that 

 has begun to putrefy, will not produce the desired effect. It has 

 been also found that if the membrane of the endosmometer be 



