198 Elementary Biology. 



bottom made of parchment or bladder instead of glass. In 

 the smaller vessel is put a solution of common salt (NaCl). 

 If the apparatus be left for some time, and the two liquids 

 be then examined, it will be found that, although none of the 

 salt solution would have passed through the membrane had 

 the smaller vessel been suspended in air instead of in w r ater, 

 under the latter circumstances some of the salt solution has 

 permeated the membrane, and can be detected in the sur- 

 rounding water of the larger vessel by adding a small quantity 

 of a solution of nitrate of silver, which gives with sodium 

 chloride a white precipitate not obtainable from the dis- 

 tilled water before the experiment was begun. Moreover, 

 if the contents of the inner vessel be examined, it will be 

 found to contain a much weaker percentage of salt than 

 at the beginning of the experiment, showing that the solution 

 has been diluted by the addition of waiter. The quantity of 

 the solution is, moreover, greater absolutely, showing that 

 the passage of the salt alone, without water, will not account 

 for the weakening of the solution (fig. 104). Both water 

 and sodium chloride are crystalline substances, the former 

 at a temperature of o C., the latter at all ordinary tempera- 

 tures. Now if for the sodium chloride a solution of gum, 

 or of white of egg, or other non-crystallisable substance, be 

 substituted, it will be found that very little, if any ; of the 

 gum will pass through into the outer solution, though the 

 density of the gum solution will become less in consequence 

 of the passage of water into the inner vessel. Further, if 

 sodium chloride solutions of different density be placed on 

 either side of the membrane, a current is established between 

 the two solutions, which continues until the density of both 

 is identical. Lastly, if the membrane separate two solutions, 

 both uncrystallisable, neither substance will pass over into 

 the other, though the water of the solution may, until the 

 density of the two fluids be the same. 



From these experiments we may formulate a law to the 

 effect that if a vegetal or animal membrane separate two 



