490 ABSORPTION. 



eter, may be modified by the concentration of the solution, 

 differences in temperature, and other, conditions. The in- 

 fluence which the membrane exerts upon the relative inten- 

 sity of these currents is dependent to a certain extent upon 

 the diffusion which takes place when the two liquids come 

 together in its substance. 



As a rule to which there are not very many exceptions, 

 pure water will penetrate animal membranes more readily 

 than any other liquid ; and it is consequently from the water 

 to the liquid contained in the endosmometer that the principal 

 current generally takes place. Liquids like alcohol, saline so- 

 lutions, etc., which have this property, are said to be positively 

 osmotic ; while those with which the current takes place in 

 the opposite direction, such as oxalic acid, weak hydrochloric 

 acid, bichloride of platinum, etc., are said to be negatively 

 osmotic. In a series of experiments with different liquids, 

 if the endosmometer be always the same, and all the liquids 

 used be exposed to the action of pure water, in a given time 

 a definite change in the quantity of fluid in the endosmom- 

 eter will be produced, which will be indicated by a certain 

 amount of elevation or depression in its level. 



Endosmotic Equivalents. The term endosmotic equiva- 

 lent is often used in comparing the endosmotic power of 

 different solutions. The degrees by which these equivalents 

 are represented are entirely arbitrary, and are intended to 

 represent the force with which various solutions attract pure 

 water. Among those who have experimented upon this sub- 

 ject, Graham has, perhaps, made the greatest number of com- 

 parative observations. He has avoided some of the sources 

 of error which were not provided against by Dutrochet, and 

 has always been careful to operate with different solutions 

 under conditions as nearly identical as possible. "We can 

 give the best definition of endosmotic equivalents by describ- 

 ing in detail the manner in which the experiments of Gra- 

 ham were performed. 



