THE CELL 



53 



very obvious reactions by which it is easy to detect 

 slight traces of them in a solution. I have, for instance, 

 some water in this glass. I add to it a few drops of an 

 iron salt and then some of another liquid (a solution of 

 tannin), and the solution previously as colourless as 

 water turns as black as ink ; in fact it is not quite 

 correct to say as ink, because it actually is ink. We 

 put into the vessel which contains water a bladder of 

 collodion also filled with water (i). Then we pour some 



FIG. 19. 



iron salt into the vessel and some tannin into the 

 bladder. Almost immediately a greyish tint appears 

 near the inner wall of the bladder, and in a few minutes 

 all the liquid in the bladder is turned into ink (2) . We 

 notice therefore that the iron salt spontaneously 

 penetrates into our cell ; and we know that this process 

 will continue as long as the solution of the salt in the 

 cell is weaker than that in the vessel outside it, for only 

 then will as many particles enter the cell as pass out of 

 it in a word, until equilibrium is established. But 

 here a question arises : can an equilibrium of that kind 



